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POWER SYSTEM PROTECTION LAB MANUAL

ASAD NAEEM 2006‐RCET‐EE‐22

EXP# TITLE01 Introduction to MATLAB and Electrical Transients

Analyzer Program ETAP 02 Introduction to Power System Protection 03

IMPACT OF INDUCTION MOTOR STARTING ON POWER SYSTEM

04

SELECTION OF CIRCUIT BREAKER FOR DIFFERENT BRANCHES OF A GIVEN POWER SYSTEM USING ETAP

05 Transient stability analysis of a given power system using ETAP

06 Introduction to Ground Grid Modeling in ETAP07 Ground Grid Modeling of a Given System using ETAP08 Modeling of Single‐Phase Instantaneous Over‐Current

Relay using MATLAB09 Modeling of a Three Phase Instantaneous Over‐Current

Relay using MATLAB10 Modeling of a Differential Relay Using MATLAB11 Comparison between the Step and Touch Potential of a

T‐Model and Square Model of Ground Grids under Tolerable and Intolerable in ETAP

12 Modeling of an Over‐Current Relay using ETAP13 Modeling of a Differential Relay Using ETAP14 Modeling of Single‐Phase Definite Time Over‐Current

Relay using MATLAB



EXPERIMENT NO: 01

Introduction to MATLAB and Electrical Transients Analyzer Program ETAP

MATLAB This is a very important tool used for making long complicated calculations and plotting graphs of different functions depending upon our requirement. Using MATLAB an m‐file is created in which the basic operations are performed which leads to simple short and simple computations of some very complicated problems in no or very short time.

Some very important functions performed by MATLAB are given as follows:

• Matrix computations • Vector Analysis • Differential Equations computations • Integration is possible • Computer language programming • Simulation • Graph Plotation • 2‐D & 3‐D Plotting

Benefits:

Some Benefits of MATLAB are given as follows:

• Simple to use • Fast computations are possible • Wide working range • Solution of matrix of any order • Desired operations are performed in matrices • Different Programming languages can be used • Simulation is possible



Basic Commands:

Some basic MATLAB commands are given as follows:

Addition:

A B

Subtraction:

A‐B

Multiplication:

A*B

Division:

A/B

Power:

A^B

Power Of each Element individually:

A.^B

Range Specification:

A:B

Square‐Root:

A sqrt B

Where A & B are any arbitrary integers

Basic Matrix Operations:

This is a demonstration of some aspects of the MATLAB language.



Creating a Vector:

Let’s create a simple vector with 9 elements called a.

a 1 2 3 4 6 4 3 4 5 a 1 2 3 4 6 4 3 4 5

Now let's add 2 to each element of our vector, a, and store the result in a new vector.

Notice how MATLAB requires no special handling of vector or matrix math.

Adding an element to a Vector: b a 2 b 3 4 5 6 8 6 5 6 7

Plots and Graphs:

Creating graphs in MATLAB is as easy as one command. Let's plot the result of our vector addition with grid lines.

Plot b grid on

MATLAB can make other graph types as well, with axis labels.

bar b xlabel 'Sample #' ylabel 'Pounds'



MATLAB can use symbols in plots as well. Here is an example using stars to mark the points. MATLAB offers a variety of other symbols and line types.

Creating a matrix:

Creating a matrix is as easy as making a vector, using semicolons ; to separate the rows of a matrix.

A 1 2 0; 2 5 ‐1; 4 10 ‐1 A 1 2 0 2 5 ‐1 4 10 ‐1 Adding a new Row: B 4,: 7 8 9 ans 1 2 0 2 5 ‐1 4 10 ‐1 7 8 9 Adding a new Column: C :,4 7 8 9 ans 1 2 0 7 2 5 ‐1 8 4 10 ‐1 9 Transpose:

We can easily find the transpose of the matrix A.

B A' B 1 2 4 2 5 10 0 ‐1 ‐1 Matrix Multiplication:

Now let's multiply these two matrices together.



Note again that MATLAB doesn't require you to deal with matrices as a collection of numbers. MATLAB knows when you are dealing with matrices and adjusts your calculations accordingly.

C A * B C 5 12 24 12 30 59 24 59 117 Matrix Multiplication by corresponding elements: Instead of doing a matrix multiply, we can multiply the corresponding elements of two matrices or vectors using the’.* ‘operator. C A .* B C 1 4 0 4 25 ‐10 0 ‐10 1 Inverse:

Let's find the inverse of a matrix :

X inv A X 5 2 ‐2 ‐2 ‐1 1 0 ‐2 1 And then illustrate the fact that a matrix times its inverse is the identity matrix. I inv A * A I 1 0 0 0 1 0 0 0 1

MATLAB has functions for nearly every type of common matrix calculation.



Eigen Values:

There are functions to obtain Eigen values:

eig A ans 3.7321 0.2679 1.0000 Polynomial coefficients: The "poly" function generates a vector containing the coefficients of the characteristic polynomial.

The characteristic polynomial of a matrix A is

p round poly A

p 1 ‐5 5 ‐1 We can easily find the roots of a polynomial using the roots function.

These are actually the eigenvalues of the original matrix.

roots p

ans 3.7321 1.0000 0.2679 MATLAB has many applications beyond just matrix computation.

Vector Convolution:

To convolve two vectors :

q conv p, p q 1 ‐10 35 ‐52 35 ‐10 1



Or convolve again and plot the result.

r conv p, q plot r ; r 1 ‐15 90 ‐278 480 ‐480 278 ‐90 15 ‐1

Matrix Manipulation:

We start by creating a magic square and assigning it to the variable A.

A magic 3 A 8 1 6 3 5 7 4 9 2

MATLAB IN POWER SYSTEM PROTECTION

The MATLAB System:

The MATLAB system consists of five main parts: Development Environment. This is the set of tools and facilities that help you use MATLAB functions and files. Many of these tools are graphical user interfaces. It includes the MATLAB



desktop and Command Window, a command history, an editor and debugger, and browsers for viewing help, the workspace, files, and the search path.

The MATLAB Mathematical Function Library:

This is a vast collection of computational algorithms ranging from elementary functions, like sum, sine, cosine, and complex arithmetic, to more sophisticated functions like matrix inverse, matrix Eigen values, Bessel functions, and fast Fourier transforms.

The MATLAB Language:

This is a high‐level matrix/array language with control flow statements, functions, data structures, input/output, and object‐oriented programming features. It allows both "programming in the small" to rapidly create quick and dirty throw‐away programs, and "programming in the large" to create large and complex application programs.

Graphics:

MATLAB has extensive facilities for displaying vectors and matrices as graphs, as well as annotating and printing these graphs. It includes high‐level functions for two‐dimensional and three‐dimensional data visualization, image processing, animation, and presentation graphics. It also includes low‐level functions that allow you to fully customize the appearance of graphics as well as to build complete graphical user interfaces on your MATLAB applications.

The MATLAB Application Program Interface API :

This is a library that allows you to write C and FORTRAN programs that interact with MATLAB. It includes facilities for calling routines from MATLAB dynamic linking , calling MATLAB as a computational engine, and for reading and writing MAT‐files.



MATLAB Documentation:

MATLAB provides extensive documentation, in both printed and online format, to help you learn about and use all of its features. If you are a new user, start with this Getting Started book. It covers all the primary MATLAB features at a high level, including many examples. The MATLAB online help provides task‐oriented and reference information about MATLAB features. MATLAB documentation is also available in printed form and in PDF format.

Working with Matrices:

Generate matrices, load matrices, create matrices from M‐files and concatenation, and delete matrix rows and columns.

More About Matrices and Arrays:

Use matrices for linear algebra, work with arrays, multivariate data, scalar expansion, and logical subscripting, and use the find function.

Controlling Command Window Input and Output:

Change output format, suppress output, enter long lines, and edit at the command line.

Bioinformatics Toolbox:

The Bioinformatics Toolbox extends MATLAB to provide an integrated software environment for genome and proteome analysis. Together, MATLAB and the Bioinformatics Toolbox give scientists and engineer a set of computational tools to solve problems and build applications in drug discovery, genetic engineering, and biological research. You can use the basic bioinformatics functions provided with this toolbox to create more complex algorithms and applications. These robust and well tested functions are the functions that you would otherwise have to create yourself.

Connecting to Web accessible databases, Reading and converting between multiple data formats, Determining statistical characteristics of data,



Manipulating and aligning sequences, Modeling patterns in biological sequences using Hidden Markov Model HMM profiles, Reading, normalizing, and visualizing microarray data creating and manipulating phylogenetic tree data interfacing with other bioinformatics software. The field of bioinformatics is rapidly growing and will become increasingly important as biology becomes a more analytical science.

The Bioinformatics Toolbox provides an open environment that you can customize for development and deployment of the analytical tools you and scientists will need. Prototype and develop algorithms Prototype new ideas in an open and extendable environment. Develop algorithms using efficient string processing and statistical functions, view the source code for existing functions, and use the code as a template for improving or creating your own functions. See Prototype and Development Environment.

Visualize data Visualize sequence alignments, gene expression data, phylogenetic trees, and protein structure analyses. See Data Visualization. Share and deploy applications Use an interactive GUI builder to develop a custom graphical front end for your data analysis programs. Create stand‐alone applications that run separate from MATLAB. See Algorithm Sharing and Application Deployment.

Control System Toolbox:

Building Models Describes how to build linear models, interconnect models, determine model characteristics, convert between continuous‐ and discrete‐time models, and how to perform model order reduction on large scale models. This chapter develops a DC motor model from basic laws of physics. Analyzing Models Introduces the LTI Viewer, graphical users interface GUI that simplifies the task of viewing model responses. This chapter also discusses command‐line functions for viewing model responses.

Designing Compensators Introduces the SISO Design Tool, a GUI that allows you to rapidly iterate on compensator designs.



You can use this tool to adjust compensator gains and add dynamics, such as poles, zeros, lead networks, and notch filters. This chapter also discusses command‐line functions for compensator design and includes examples of LQR and Kalman filter design.

Curve Fitting Toolbox:

The Curve Fitting Toolbox is a collection of graphical user interfaces GUIs and M‐file functions built on the MATLAB® technical computing environment. The toolbox provides you with these main features: Data preprocessing such as sectioning and smoothing Parametric and nonparametric data fitting: You can perform a parametric fit using a toolbox library equation or using a custom equation. Library equations include polynomials, exponentials, rationales, sums of Gaussians, and so on. Custom equations are equations that you define to suit your specific curve fitting needs. You can perform a nonparametric fit using a smoothing spine or various interpellants. Standard linear least squares, nonlinear least squares, weighted least squares, constrained least squares, and robust fitting procedures Fit statistics to assist you in determining the goodness of fit Analysis capabilities such as extrapolation, differentiation, and integration A graphical environment that allows you to: Explore and analyze data sets and fits visually and numerically Save your work in various formats including M‐files, binary files, and workspace variables.

Data Acquisition Toolbox:

Introduction to Data Acquisition provides you with general information about making measurements with data acquisition hardware. The topics covered should help you understand the specification sheet associated with your hardware. Getting started with the Data Acquisition Toolbox describes the toolbox components, and shows you how to access your hardware, examine your hardware resources, and get command line help.



Database Toolbox:

Overview of how databases connect to MATLAB, toolbox functions, the Visual Query Builder, major features of the toolbox, and the expected background for users of this product. System Requirements Supported platforms, MATLAB versions, databases, drivers, SQL commands, data types, and related products. Setting Up a Data Source Before connecting to a database, set up the data source for ODBC drivers or for JDBC drivers. Starting the Database Toolbox Start using functions or the Visual Query Builder GUI, and learn how to get help for the product.

Data feed Toolbox:

This document describes the Data feed Toolbox for MATLAB®. The Data feed Toolbox effectively turns your MATLAB workstation into a financial data acquisition terminal. Using the Data feed Toolbox; you can download a wide variety of security data from financial data servers into your MATLAB workspace. Then, you can pass this data to MATLAB or to another toolbox, such as the Financial Time Series Toolbox, for further analysis.

Filter Design Toolbox:

The Filter Design Toolbox is a collection of tools that provides advanced techniques for designing, simulating, and analyzing digital filters. It extends the capabilities of the Signal Processing Toolbox with filter architectures and design methods for complex real‐time DSP applications, including adaptive filtering and MultiMate filtering, as well as filters transformations. Used with the Fixed‐Point Toolbox, the Filter Design Toolbox provides functions that simplify the design of fixed‐point filters and the analysis of quantization effects. When used with the Filter Design HDL Coder, the Filter Design Toolbox lets you generate VHDL and Verilog code for fixed‐point filters.

Key Features:

Advanced FIR filter design methods, including minimum‐order, minimum‐phase, constrained‐ripple, half band, Nyquist, interpolated FIR, and nonlinear



phase Perfect reconstruction and two‐channel FIR filter bank design Advanced IIR design methods, including arbitrary magnitude, group‐delay equalizers, constrained‐pole radius, peaking, notching, and comb filters Analysis and implementation of digital filters in single‐precision floating‐point and fixed‐point arithmetic Support for IIR filters implemented in second‐order sections, including design, scaling, and section reordering Round‐off noise analysis for filters implemented in single‐precision floating point or fixed point FIR and IIR filter transformations, including low pass to low pass, low pass to high pass, and low pass to multiband. Adaptive filter design, analysis, and implementation, including LMS‐based, RLS‐based, lattice‐based, frequency‐domain, fast transversal, and affine projection Multi‐rate filter design, analysis, and implementation, including cascaded integrator‐comb CIC fixed‐point MultiMate filters VHDL and Verilog code generation for fixed‐point filters.

RF Toolbox:

The RF Toolbox enables you to create and combine RF circuits for simulation in the frequency domain with support for both power and noise. You can read, write, analyze, combine, and visualize RF network parameters. Work Directly with Network Parameter Data You can work directly with your own network parameter data or with data from files. Functions enable you to: Read and write RF data in Touchstone® .snp, .ynp, .znp, and .hnp formats, as well as the Math Works .AMP format. Conversion among S, Y, Z, h, T, and ABCD network parameters Plot your data on X‐Y plane and polar plane plots, as well as Smith® charts Calculate cascaded S‐parameters and de‐embed S‐parameters from a cascaded network Calculate input and output reflection coefficients, and voltage standing‐wave ratio VSWR at the reflection coefficient.

Wavelet Toolbox:

Everywhere around us are signals that can be analyzed. For example, there are seismic tremors, human speech, engine vibrations, medical images, financial data, music, and many other types of signals. Wavelet analysis is a new and promising set of tools and techniques for analyzing these signals. The



Wavelet Toolbox is a collection of functions built on the MATLAB® Technical Computing Environment. It provides tools for the analysis and synthesis of signals and images, and tools for statistical applications, using wavelets and wavelet packets within the framework of MATLAB.

The MathWorks provides several products that are relevant to the kinds of tasks you can perform with the Wavelet Toolbox.

The Wavelets Toolbox provides two categories of tools: Command line functions Graphical interactive tools the first category of tools is made up of functions.

Simulink:

Simulink® is a software package for modeling, simulating, and analyzing dynamic systems.

It supports linear and nonlinear systems, modeled in continuous time, sampled time, or a hybrid of the two. Systems can also be MultiMate, i.e., have different parts that are sampled or updated at different rates. Simulink encourages you to try things out. You can easily build models from scratch, or take an existing model and add to it. Simulations are interactive, so you can change parameters on the fly and immediately see what happens.

A goal of Simulink is to give you a sense of the fun of modeling and simulation, through an environment that encourages you to pose a question, model it, and see what happens. With Simulink, you can move beyond idealized linear models to explore more realistic nonlinear models, factoring in friction, air resistance, gear slippage, hard stops, and the other things that describe real‐world phenomena. Simulink turns your computer into a lab for modeling and analyzing systems that simply wouldn't be possible or practical otherwise, whether the behavior of an automotive clutch system, the flutter of an airplane wing, the dynamics of a predator‐prey model, or the effect of the monetary supply on the economy.



Simulink is also practical. With thousands of engineers around the world using it to model and solve real problems, knowledge of this tool will serve you well throughout your professional career.

Signal Processing Toolbox:

The Signal Processing Toolbox is a collection of tools built on the MATLAB® numeric computing environment.

The toolbox supports a wide range of signal processing operations, from waveform generation to filter design and implementation, parametric modeling, and spectral analysis.

The toolbox provides two categories of tools: Command line functions in the following categories:

Analog and digital filter analysis Digital filter implementation FIR and IIR digital filter design Analog filter design Filter discretization Spectral Windows Transforms Cepstral analysis Statistical signal processing and spectral analysis Parametric modeling Linear Prediction Waveform generation. A suite of interactive graphical user interfaces for Filter design and analysis Window design and analysis Signal plotting and analysis Spectral analysis Filtering signals Signal Processing Toolbox Central Features The Signal Processing Toolbox functions are algorithms, expressed mostly in M‐files, that implement a variety of signal processing tasks.

These toolbox functions are a specialized extension of the MATLAB computational.

ETAP is the most comprehensive analysis platform for the design, simulation, operation, control, optimization, and automation of generation, transmission, distribution, and industrial power systems.



Project Toolbar

The Project Toolbar contains icons that allow you to perform shortcuts of many commonly used functions in PowerStation.

Create Create a new project file

Open Open an existing project file

Save Save the project file

Print Print the one‐line diagram or U/G raceway system

Cut Cut the selected elements from the one‐line diagram or U/G raceway system to the Dumpster

Copy Copy the selected elements from the one‐line diagram or U/G raceway system to the Dumpster

Paste Paste elements from a Dumpster Cell to the one‐line diagram or U/G raceway system

Zoom In Magnify the one‐line diagram or U/G raceway system

Zoom Out Reduce the one‐line diagram or U/G raceway system

Zoom to Fit Page Re‐size the one‐line diagram to fit the window

Check Continuity Check the system continuity for non‐energized elements

Power Calculator Activate PowerStation Calculator that relates MW, MVAR, MVA, kV, Amp, and PF together with either kVA or MVA units

Help Point to a specific area to learn more about PowerStation



Mode Toolbar

ETAP offers a suite of fully integrated software solutions including arc flash, load flow, short circuit, transient stability, relay coordination, cable ampacity, optimal power flow, and more. Its modular functionality can be customized to fit the needs of any company, from small to large power systems.

Edit Mode

Edit mode enables you to build your one‐line diagram, change system connections, edit engineering properties, save your project, and generate schedule reports in Crystal Reports formats. The Edit Toolbars for both AC and DC elements will be displayed to the right of the screen when this mode is active. This mode provides a wide variety of tasks including:

• Drag & Drop Elements • Connect Elements • Change IDs • Cut, Copy, & Paste Elements • Move from Dumpster • Insert OLE Objects • Cut, Copy & OLE Objects • Merge PowerStation Project • Hide/Show Groups of Protective Devices • Rotate Elements • Size Elements • Change Symbols • Edit Properties • Run Schedule Report Manager



Instrumentation Elements:

AC Elements:



DC Elements:

Load Flow Analysis:



Short Circuit Analysis:

Motor Starting Analysis:



Harmonic Analysis:



Transient Stability Analysis:

Optimal Power Flow Analysis:



Reliability Assesment Analysis:

DC Load Flow Analysis:



DC Short Circuit Analysis:

Battery Sizing And Discharge Analysis:



COMMENTS:

MATLAB is very useful and very easy to use software which is basically used for the matrices problems but it is also used for many applications like:

• Matrix computations • Vector Analysis • Differential Equations computations • Integration is possible • Computer language programming • Simulation • Graph Plotation • 2‐D & 3‐D Plotting

ETAP is the most comprehensive analysis platform for the design, simulation, operation, control, optimization, and automation of generation, transmission, distribution, and industrial power systems. This software is used to analyze



very large power systems. ETAP is used for the following types of analysis of any power system:

• Battery Sizing And Discharge Analysis • DC Short Circuit Analysis • DC Load Flow Analysis • Reliability Assesment Analysis • Optimal Power Flow Analysis • Transient Stability Analysis • Harmonic Analysis • Motor Starting Analysis • Short Circuit Analysis • Load Flow Analysis



EXPERIMENT NO: 02

Introduction to Power System Protection

Protection System

A protection scheme in power system is designed to continuously monitor the power system to ensure maximum continuity of electrical supply with minimum damage to life, equipment and property.

Isolation of faulty element

The ill effects of faults are minimized by quickly isolating the faulty element from the rest of the healthy system, thus limiting the disturbance footprint to as small an area in time and space as possible.

FAULTS AND ABNORMAL OPERATING CONDITIONS

Shunt Fault:

“When the path of the load current is cut short because of breakdown of insulation, we say that a ‘short circuit’ has occurred.” These faults due to insulation flashover are many times temporary, i.e. if the arc path is allowed to de‐ionize, by interrupting the electric supply for a sufficient period, then there arc does not restrike after the supply is restored. This process of interruption followed by intentional re‐energization is known as “RECLOSURE”. In low voltage system up to 3 reclosure are attempted, after which the breaker is locked out. The repeated attempts at reclosure, at times, help in burning out the object, which is causing the breakdown of insulation. The reclosure may also be done automatically.

EHV SYSTEM:

In these systems where the damage due to short circuit may be very large and the system stability at stake, only one reclosure is allowed. At times the short circuit may be total sometimes called a dead short circuit or it may be partial short circuit.



METALLIC FAULT:

“A fault which bypasses the entire load current through itself is called a metallic fault”. A metallic fault presents a very low, practically zero, fault resistance. A partial short circuit can be modeled as a non‐zero resistance or impedance parallel with the intended path of current.

ARC RESISTANCE:

Most of the times, the fault resistance is nothing but the resistance of the arc that is formed as a result of flash over. The resistance is highly non‐linear in nature. Early researches have developed models of arc resistance. One such widely used model is due to Warrington, which gives the Arc Resistance as;

Rarc 8750 S 3ut /I1.4

Where

• “S” is the spacing in feet • “t” is the time in seconds • “U” is the velocity of air in mph • “I” is the fault current in ampere

CAUSES OF SHUNT FAULT:

Shunt fault is basically due to failure of insulation. The insulation may fail because of it’s own weakening, or it may fail due to over‐voltage the weakening of insulation may be due to one or more of following factors.

• Ageing • Temperature • Rain, Hail, Snow • Chemical pollution • Foreign objects • Other causes



The over voltage may be either internal due to switching or external due to lightening .

EFFECTS OF SHUNT FAULTS

If the power system just consisted of isolated alternators feeding their own load, then steady state fault currents would not be of much concern.

ISOLATED GENERATOR EXPERINCES A THREE PHASE FAULT

Consider an isolated turbo alternator with a three‐phase short circuit on it’s terminals as shown in fig:

Assuming that;

Internal voltage I p.u

Synchronous impedance Xd 2 p.u



Steady stat short circuit current 0.5 p.u

This current is to small to cause any worry.

However considering;

Sub‐transient impedance Xd ” 0.1 p.u

Sub‐transient current will I ” 10 p.u

FOR INTERCONNECTED POWER SYSTEM

For these systems all the generators and motors will contribute towards the fault current, thus building up the value of the fault current to couple of tens of times to the normal full‐load current. Faults thus cause heavy current to flow. If these current persists for short duration they can cause serious damage to the equipment.

OVERHEATING:

In faulted circuits the over‐current causes the over heating and attendant danger of fire, this over heating also causes the deterioration of the insulation, thus weakening it further. Transformers are known to have suffered mechanical damage to the windings due to fault.

Some important points of inter‐connected power system are:

• The generators in inter connected system must operate in synchronism at all instants.

• The electric power out put from an alternator near the fault drops sharply.

• The mechanical power input remains constant at its pre fault value.

EFFECT OF FAULT:

As mechanical power input remains constant this causes the alternator to accelerate, along with the rotor angle ф starts increasing, thus the alternators start swinging with respect to each other. If the swing goes out of



control alternator will be tripped out. Thus system stability is at sake. Therefore fault need to be isolated and removed as quickly as possible.

CLASSIFICATION OF SHUNT FAULT

PHASE FAULT AND GROUND FAULT

GROUND FAULT:

The fault which involves only one of the phase conductor and ground is called as ground fault.

PHASE FAULT:

The fault which involves two or more phase conductors with or without ground is called as phase fault.

FAULT STATICS WITH REFERENCE TO TYPE OF FAULT

FAULT PROBABILITY OF OCCURANCE SEVERITYL‐G 85% Least L‐L 8% L‐L‐G 5% L‐L‐L 2% Most

FAULT STATICTICS WITH REFERENCE TO POWER SYSTEM ELEMENTS

Further the probability of fault on different elements of power system is different. The transmission lines which are exposed to the vagaries of the atmosphere are most likely to be subjected to these faults. The fault statistics is shown in table:

POWER SYSTEM ELEMENT PROBABILITY OF FAULT %Overhead lines 50

Underground Cables 09 Transformer 10 Generator 07 Switch Gears 12



CT, PT,Relays 12 Phasor Diagram of Voltages and Currents during Various Faults

A fault is accompanied by a build‐up of current, which is obvious. At the same time there is a fall in voltage throughout the power system. If the fault is a metallic fault, the voltage at the fault location is zero. The voltage at the terminals of the generator will also drop, though not drastically. If the source is ideal, there will be no drop in voltage at the generator terminals. Normally the relay is away from the fault location. Thus, as seen from the relay location, a fault is characterized by a build‐up of current, and to a certain extent, collapse of voltage.



Series Fault

These faults occur simply when the path of current is opened. Practically most of the time series fault is converted into shunt fault.

Abnormal Operating Conditions The boundary between the normal and faulty conditions is not crisp. There are certain operating conditions inherent to the operation of the power system which is definitely not normal, but these are not electrical faults either. Some examples are the magnetizing inrush current of a transformer, starting current of an induction motor, and the conditions during power swing.

What are Protective Relays Supposed to Do? Relays are supposed to detect the fault with the help of current and voltage and selectively remove only the faulty part from the rest of the system by operating breakers. This, the relay has to do with utmost selectivity and speed. In a power system, faults are not an everyday occurrence. A typical relay, therefore, spends all of its life monitoring the power system. Thus, relaying is like an insurance against damage due to faults.

Evolution of Power Systems Systems have evolved from isolated generators feeding their own loads to huge power systems spanning an entire country. The evolution has progressed systems to high‐voltage systems and low‐power handling capacities to high power capacities. The requirements imposed on the protective system are linked to the nature of the power system.

Isolated Power System The protection of an isolated power system is simpler because firstly, there is no concentration of generating capacity and secondly, a single synchronous alternator does not suffer from the stability problem as faced by a multi‐machine system. Further, when there is a fault and the protective relays remove the generator from the system, the system may suffer from a blackout unless there is a standby source of power. The steady‐state fault current in a single machine power system may even be less than the full‐load current. Such a fault will, however, cause other effects like speeding up of the generator because of the disturbed balance between the input mechanical power and the output electrical power, and therefore should be quickly attended to. Although, there are no longer any isolated power systems supplying residential or industrial loads, we do encounter such situations in case of emergency diesel generators powering the uninterrupted power supplies as well as critical auxiliaries in a thermal or nuclear power station. Interconnected Power System An interconnected power system has evolved because it is more reliable than an isolated power system. In case of disruption in one part of the system,



power can be fed from alternate paths, thus, maintaining continuity of service. An interconnected power system also makes it possible to implement an economic load dispatch. The generators in an interconnected system could be of varied types such as turbo‐alternators in coal fired, gas fired or nuclear power plants , generators in hydroelectric power plants, wind‐powered generators, fuel cells or even solar‐powered photovoltaic cells. Figure shows a simple interconnected power system. Most of the generators operate at the voltage level of around 20 kV. For bulk transmission of power, voltage levels of the order of 400 kV or higher are used. At the receiving end, the voltage is stepped down to the distribution level, which is further stepped down before it reaches the consumers. It can be seen that the EHV lines are the tie lines which interconnect two or more generators whereas the low voltage lines are radial in nature which terminate in loads at the remote ends. There is interconnection at various EHV voltage levels.



Disadvantages of an Interconnected System • There are other undesirable effects of interconnection. It is • Very difficult to maintain stability • Disturbances quickly propagate throughout the system • Possibility of cascade tripping due to loss of stability is always looming

large • Voltage stability problem • Harmonic distortion propagate throughout the system • Possibility of cyber‐attacks

Various States of Operation of a Power System A power system is a dynamic entity. Its state is likely to drift from one state to the other as shown in the figure. When the power system is operating in steady state, it is said to be operating in normal state. In this state, there is enough generation capacity available to meet the load, therefore, the frequency is stable around the nominal 50Hz or 60 Hz. This state is also characterized by reactive power balance between generation and load.

A Protection System and Its Attributes Following figure shows a protection system for the distance protection of a transmission line, consisting of a CT and a PT, a relay and its associated circuit breaker. Every protection system will have these basic components.



At this stage, we can consider the relay as a black‐box having current and voltage at its input, and an output, in the form of the closure of a normally‐open contact. This output of the relay is wired in the trip circuit of the associated circuit breaker s so as to complete this circuit. The conceptual diagram of a generalized relay is shown in Figure:

Basic Requirements of a Protection System

Sensitivity

The protective system must be alive to the presence of the smallest fault current. The smaller the fault current it can detect, the more sensitive it is.

Selectivity

In detecting the fault and isolating the faulty element, the protective system must be very selective. Ideally, the protective system should zero‐in on the faulty element and isolate it, thus causing minimum disruption to the system.



Speed

The longer the fault persists on the system, the larger is the damage to the system and higher is the possibility that the system will lose stability. Thus, it helps a lot if the entire process of fault detection and removal of the faulty part is accomplished in as short a time as feasible. Therefore, the speed of the protection is very important.

Reliability and Dependability

A protective system is of no use if it is not reliable. There are many ways in which reliability can be built into the system. In general, it is found that simple systems are more reliable. Therefore, we add features like back‐up protection to enhance the reliability and dependability of the protective system.

System Transducers Current transformers and voltage transformers form a very important link between the power system and the protective system. These transducers basically extract the information regarding current and voltage from the power system under protection and pass it on to the protective relays.

Current Transformer The current transformer has two jobs to do.

• Firstly, it steps down the current to such levels that it can be easily handled by the relay current coil. The standard secondary current ratings used in practice are 5 A and 1 A. This frees the relay designer from the actual value of primary current.

• Secondly, it isolates the relay circuitry from the high voltage of the EHV system.

A conventional electromagnetic current transformer is shown in Figure. Ideally, the current transformer should faithfully transform the current without any errors. In practice, there is always some error. The error creeps in, both in magnitude and in phase angle. These errors are known as ratio error and phase angle error.



Voltage Transformer The voltage transformer steps down the high voltage of the line to a level safe enough for the relaying system pressure coil of relay and personnel to handle. The standard secondary voltage on line‐to‐line basis is 110 V. This helps in standardizing the protective relaying equipment irrespective of the value of the primary EHV adopted. A PT primary is connected in parallel at the point where a measurement is desired, unlike a CT whose primary is in series with the line. A conventional electromagnetic VT is shown in Figure:



Circuit Breaker The circuit breaker is an electrically operated switch, which is capable of safely making, as well as breaking short‐circuit currents. The circuit breaker is operated by the output of its associated relay. When the circuit breaker is in the closed condition, its contacts are held closed by the tension of the closing spring. When the trip coil is energized, it releases a latch, causing the stored energy in the closing spring to bring about a quick opening operation.

Organization of Protection The protection is organized in a very logical fashion. The idea is to provide a ring of security around each and every element of the power system. If there is any fault within this ring, the relays associated with it must trip all the allied circuit breakers so as to remove the faulty element from the rest of the power system. This 'ring of security' is called zone of protection. This is depicted in Figure with the help of a simple relay for the protection of a transformer. Without going into the detailed of the differential relaying scheme, we can make the following statements:

Faults within the zone are termed internal faults whereas the faults outside the zone are called external faults. External faults are also known as through faults. The farthest point from the relay location, which is still inside the zone, is called the reach point.

Zones of Protection Various zones for a typical power system are shown in Figure. It can be seen that the adjacent zones overlap; otherwise there could be some portion which is left out and remains unprotected.



Primary and back‐up Protection As already mentioned there are times when the primary protection may fail. This could be due to failure of CT, VT or relay, or failure of circuit breaker. One of the possible causes of the circuit breaker failure is the failure of the trip‐battery due to inadequate maintenance. We must have a second line of defense in such a situation. Therefore, it is a normal practice to provide another zone of protection which should operate and isolate the faulty element in case of primary protection failure. Further, the back‐up protection must wait for the primary protection to operate, before issuing the trip command to its associated circuit breakers. In other words, the operating time of the back‐up protection must be delayed by an appropriate amount over that of the primary protection. Thus, the operating time of the back‐up protection should be equal to the operating time of primary protection plus the operating time of the primary circuit breaker.



Maloperation

There should be proper coordination between the operating time of primary and back‐up protection. It can be seen that the back‐up protection in this case issues trip command to its breaker without waiting for the primary protection to do its job. This results in operation of both the primary and the back‐up, resulting in a longer and unnecessary disruption to the system. It is said that with every additional relay used, there is an increase in the probability of Maloperation.

Various elements of power system that needs protection The power system consists of

• Alternators • Bus bars • Transformers for transmission and distribution • Transmission lines at various voltage levels from EHV to 11kV cables • Induction and synchronous motors • Reactors • Capacitors • Instrument and protective CTs and PTs • Various control and metering equipment etc

Each of these entities needs protection. Each apparatus has a unique set of operating conditions.

Various Principles of Power System Protection The most basic principles that are used in any protection system are following

• Over current protection • Over voltage protection • Distance protection • Differential protection

Normally used protection schemes for different elements

Protection schemes used for different elements of any power system are completely dependant upon the nature of that element. We can not use all protection schemes for every element. Following table shows the protection schemes used for mentioned elements:



ELEMENT Principle Non‐directional

over current

Directionalover current

Differential Distance

Alternator Primary protection

yes yes yes

Bus bar Primary protection

yes

Transformer Primary protection

yes

Transmission line

Primary protection

yes yes yes

Large induction motor

Primary protection yes

yes

COMMENTS

The knowledge about protection system is of great importance. In this experiment, we understand

• What is a protection system? • Different kinds of faults and their effects • Classification of faults • Abnormal operating conditions • Function of a relay • Types of a power system • Properties of a good protection system • Zones of protection

Indeed these necessary to select protection scheme for any power system element to understand the basics of fault effects and regarding protection system.



EXPERIMENT NO: 03

IMPACT OF INDUCTION MOTOR STARTING ON POWER SYSTEM

ELECTRIC MOTOR An electric motor uses electrical energy to produce mechanical energy, through the interaction of magnetic fields and current‐carrying conductors. The reverse process, producing electrical energy from mechanical energy, is accomplished by a generator or dynamo.

Traction motors used on vehicles often perform both tasks. Many types of electric motors can be run as generators, and vice versa.

INDUCTION MOTOR DEFINITION:

An induction motor or asynchronous motor or squirrel‐cage motor is a type of alternating current motor, where power is supplied to the rotor by means of electromagnetic induction.

POWER CONVERSION:

An electric motor converts electrical power to mechanical power in its rotor rotating part . There are several ways to supply power to the rotor. In a DC motor this power is supplied to the armature directly from a DC source, while in an induction motor this power is induced in the rotating device.

ROTATING TRANSFORMER:

An induction motor is sometimes called a rotating transformer because the stator stationary part is essentially the primary side of the transformer and the rotor rotating part is the secondary side. The primary side's current evokes a magnetic field which interacts with the secondary side's emf to produce a resultant torque, henceforth serving the purpose of producing mechanical energy.

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HISTORY:

The induction motor was first realized by Galileo Ferraris in 1885 in Italy. In 1888, Ferraris published his research in a paper to the Royal Academy of Sciences in Turin later, in the same year, Tesla gained U.S. Patent 381,968 where he exposed the theoretical foundations for understanding the way the motor operates.

The induction motor with a cage was invented by Mikhail Dolivo‐Dobrovolsky about a year later. Technological development in the field has improved to where a 100 hp 74.6 kW motor from 1976 takes the same volume as a 7.5 hp 5.5 kW motor did in 1897.

Currently, the most common induction motor is the cage rotor motor.

AC INDUCTION MOTOR Where

n Revolutions per minute rpm

f AC power frequency hertz

p Number of poles per phase an even number

Slip is calculated using:

Where “s” is the slip The rotor speed is:

STARTING OF INDUCTION MOTOR THREE‐PHASE Direct‐on‐line starting : The simplest way to start a three‐phase induction motor is to connect its terminals to the line. This method is often called "direct on line" and abbreviated DOL.



In an induction motor, the magnitude of the induced emf in the rotor circuit is proportional to the stator field and the slip speed the difference between synchronous and rotor speeds of the motor, and the rotor current depends on this emf.

A 3‐phase power supply provides a rotating magnetic field in an induction motor

When the motor is started, the rotor speed is zero. The synchronous speed is constant, based on the frequency of the supplied AC voltage. So the slip speed is equal to the synchronous speed, the slip ratio is 1, and the induced emf in the rotor is large. As a result, a very high current flows through the rotor. This is similar to a transformer with the secondary coil short circuited, which causes the primary coil to draw a high current from the mains.

When an induction motor starts DOL, a very high current is drawn by the stator, in the order of 5 to 9 times the full load current. This high current can, in some motors, damage the windings; in addition, because it causes heavy line voltage drop, other appliances connected to the same line may be affected by the voltage fluctuation. To avoid such effects, several other strategies are employed for starting motors.

STAR‐DELTA STARTERS An induction motor's windings can be connected to a 3‐phase AC line in two different ways:

1 Star Wye



2 Delta

Wye star in Europe , where the windings are connected from phases of the supply to the neutral;

Delta sometimes mesh in Europe , where the windings are connected between phases of the supply.

A delta connection of the machine winding results in a higher voltage at each winding compared to a wye connection the factor is .

A star‐delta starter initially connects the motor in wye, which produces a lower starting current than delta, then switches to delta when the motor has reached a set speed.

DISADVANTAGES:

Disadvantages of this method over DOL starting are:

Lower starting torque, which may be a serious issue with pumps or any devices with significant breakaway torque

Increased complexity, as more contactors and some sort of speed switch or timers are needed

Two shocks to the motor one for the initial start and another when the motor switches from wye to delta

VARIABLE FREQUENCY DRIVES Key information’s are:

Variable‐frequency drives VFD can be of considerable use in starting as well as running motors.

A VFD can easily start a motor at a lower frequency than the AC line, as well as a lower voltage, so that the motor starts with full rated torque and with no inrush of current.

The rotor circuit's impedance increases with slip frequency, which is equal to supply frequency for a stationary rotor,

So running at a lower frequency actually increases torque.

Thus variable frequency drives are used for multiple purposes.



RESISTANCE STARTERS This method is used with slip ring motors where the rotor poles can be accessed by way of the slip rings. Using brushes, variable power resistors are connected in series with the poles. During start‐up the resistance is large and then reduced to zero at full speed.

At start‐up the resistance directly reduces the rotor current and so rotor heating is reduced. Another important advantage is the start‐up torque can be controlled. As well, the resistors generate a phase shift in the field resulting in the magnetic force acting on the rotor having a favorable angle

AUTO‐TRANSFORMER STARTERS Such starters are called as auto starters or compensators, consists of an auto‐transformer.

SERIES REACTOR STARTERS In series reactor starter technology, an impedance in the form of a reactor is introduced in series with the motor terminals, which as a result reduces the motor terminal voltage resulting in a reduction of the starting current; the impedance of the reactor, a function of the current passing through it, gradually reduces as the motor accelerates, and at 95 % speed the reactors are bypassed by a suitable bypass method which enables the motor to run at full voltage and full speed. Air core series reactor starters or a series reactor soft starter is the most common and recommended method for fixed speed motor starting.

SYNCHRONOUS MOTOR A synchronous motor always runs at synchronous speed with 0% slip. The speed of a synchronous motor is determined by the following formula:

For example a 6 pole motor operating on 60Hz power would have speed:



Where

“V” is the speed of the rotor in rpm ,

“f” is the frequency of the AC supply in Hz And

“n” is the number of magnetic poles. Note on the use of p: Some texts refer to number of pole pairs per phase instead of number of poles per phase. For example a 6 pole motor, operating on 60Hz power, would have 3 pole pairs. The equation of synchronous speed then becomes: n 3

PARTS OF SYNCHRONOUS MOTOR A synchronous motor is composed of the following parts:

The stator is the outer shell of the motor, which carries the armature winding. This winding is spatially distributed for poly‐phase AC current. This armature creates a rotating magnetic field inside the motor.

The rotor is the rotating portion of the motor. it carries field winding, which is supplied by a DC source. On excitation, this field winding behaves as a permanent magnet.

The slip rings in the rotor, to supply the DC to the field winding.

STARTING OF SYNCHRONOUS MOTOR Synchronous motors are not self‐starting motors. This property is due to the inertia of the rotor. When the power supply is switched on, the armature



winding and field windings are excited. Instantaneously, the armature winding creates a rotating magnetic field, which revolves at the designated motor speed. The rotor, due to inertia, will not follow the revolving magnetic field. In practice, the rotor should be rotated by some other means near to the motor's synchronous speed to overcome the inertia. Once the rotor nears the synchronous speed, the field winding is excited, and the motor pulls into synchronization.

The following techniques are employed to start a synchronous motor:

A separate motor called pony motor is used to drive the rotor before it locks in into synchronization.

The field winding is shunted or induction motor like arrangements are made so that the synchronous motor starts as an induction motor and locks in to synchronization once it reaches speeds near its synchronous speed.

ADVANTAGES OF SYNCHRONOUS MOTOR Synchronous motors have the following advantages over non‐synchronous motors:

Speed is independent of the load, provided an adequate field current is applied.

Accurate control in speed and position using open loop controls, eg. stepper motors.

They will hold their position when a DC current is applied to both the stator and the rotor windings.

Their power factor can be adjusted to unity by using a proper field current relative to the load. Also, a "capacitive" power factor, current phase leads voltage phase , can be obtained by increasing this current slightly, which can help achieve a better power factor correction for the whole installation.

Their construction allows for increased electrical efficiency when a low speed is required as in ball mills and similar apparatus .



ONE LINE DIAGRAM

POINT UNDER CONSIDERATION

Mtr‐1 Bus‐7



LOAD FLOW ANALYSIS DIAGRAM



STATIC MOTOR STARTING ANALYSIS DIAGRAM



RESPONSE OF DIFFERENT PARAMETERS IN CASE OF STATIC MOTOR STARTING ANALYSIS

MOTOR REACTIVE POWER DEMAND

MOTOR REAL POWER DEMAND



MOTOR TERMINAL VOLTAGE

MOTOR CURRENT



DYNAMIC MOTOR STARTING ANALYSIS DIAGRAM



RESPONSE OF DIFFERENT PARAMETERS IN CASE OF DYNAMIC MOTOR STARTING ANALYSIS

MOTOR REACTIVE POWER DEMAND

MOTOR REAL POWER DEMAND



ACCELERATION TORQUE

MOTOR TERMINAL VOLTAGE



MOTOR CURRENT

MOTOR SLIP



COMMENTS:

In this experiment, we investigate the effect of motor starting current on the power system as motor starting current is many times larger than the normal current. For this purpose, we first take the normal load flow analysis report and then perform motor starting analysis to compare the current value for both cases.

In case of static motor starting analysis:

Motor reactive power demand instantaneously increases from 40KVAR to 80KVAR then attains the previous value which is much lower

Motor real power demand instantaneously increases from 108KW to 160KW then attains the previous value which is much lower

Bus voltage becomes lower at starting instant to a value of 66KV and then achieves the previous high voltage that is 73KV

Motor terminal voltage suddenly becomes lower at starting instant to a value of 48KV and then achieves the previous high voltage that is 64KV

Motor current becomes very high at starting instant to a value of 280KA and then achieves the previous lower current value that is 160KA

In case of dynamic motor starting analysis:

Motor reactive power demand instantaneously increases to 165KVAR then slowly decreases

Motor real power demand slowly exponentially increases Acceleration torque increases exponentially and after some time, it decreases exponentially

Motor terminal voltage is almost at a constant level Motor current becomes very high at starting instant to a value of 360KA and then decrease slowly



EXPERIMENT NO: 04

SELECTION OF CIRCUIT BREAKER FOR DIFFERENT BRANCHES OF A GIVEN POWER SYSTEM USING ETAP

INTRODUCTION POWER SYSTEM PROTECTION Power system protection is a branch of electrical power engineering that deals with the protection of electrical power systems from faults through the isolation of faulted parts from the rest of the electrical network. The objective of a protection scheme is to keep the power system stable by isolating only the components that are under fault, whilst leaving as much of the network as possible still in operation. Thus, protection schemes must apply a very pragmatic and pessimistic approach to clearing system faults. For this reason, the technology and philosophies utilized in protection schemes can often be old and well‐established because they must be very reliable.

COMPONENTS OF PROTECTION SYSTEM

Protection systems usually comprise five components:

• Current and voltage transformers to step down the high voltages and currents of the electrical power system to convenient levels for the relays to deal with;

• Relays to sense the fault and initiate a trip, or disconnection, order;

• Circuit breakers to open/close the system based on relay and auto‐reclosure commands

• Batteries to provide power in case of power disconnection in the system.

• Communication channels to allow analysis of current and voltage at remote terminals of a line and to allow remote tripping of equipment.

For parts of a distribution system, fuses are capable of both sensing and disconnecting faults.



Failures may occur in each part, such as insulation failure, fallen or broken transmission lines, incorrect operation of circuit breakers, short circuits and open circuits. Protection devices are installed with the aims of protection of assets, and ensure continued supply of energy.

CIRCUIT BREAKER A circuit breaker is an automatically‐operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset either manually or automatically to resume normal operation.

Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city.

OPERATION OF BREAKER

All circuit breakers have common features in their operation, although details vary substantially depending on the voltage class, current rating and type of the circuit breaker.

The circuit breaker must detect a fault condition; in low‐voltage circuit breakers this is usually done within the breaker enclosure. Circuit breakers for large currents or high voltages are usually arranged with pilot devices to sense a fault current and to operate the trip opening mechanism. The trip solenoid that releases the latch is usually energized by a separate battery, although some high‐voltage circuit breakers are self‐contained with current transformers, protection relays, and an internal control power source.



Once a fault is detected, contacts within the circuit breaker must open to interrupt the circuit; some mechanically‐stored energy using something such as springs or compressed air contained within the breaker is used to separate the contacts, although some of the energy required may be obtained from the fault current itself. Small circuit breakers may be manually operated; larger units have solenoids to trip the mechanism, and electric motors to restore energy to the springs.

The circuit breaker contacts must carry the load current without excessive heating, and must also withstand the heat of the arc produced when interrupting the circuit. Contacts are made of copper or copper alloys, silver alloys, and other materials. Service life of the contacts is limited by the erosion due to interrupting the arc. Miniature and molded case circuit breakers are usually discarded when the contacts are worn, but power circuit breakers and high‐voltage circuit breakers have replaceable contacts.

When a current is interrupted, an arc is generated. This arc must be contained, cooled, and extinguished in a controlled way, so that the gap between the contacts can again withstand the voltage in the circuit. Different circuit breakers use vacuum, air, insulating gas, or oil as the medium in which the arc forms. Different techniques are used to extinguish the arc including:

• Lengthening of the arc • Intensive cooling in jet chambers • Division into partial arcs • Zero point quenching • Connecting capacitors in parallel with contacts in DC circuits

Finally, once the fault condition has been cleared, the contacts must again be closed to restore power to the interrupted circuit.

ARC INTERUPTION

Miniature low‐voltage circuit breakers use air alone to extinguish the arc. Larger ratings will have metal plates or non‐metallic arc chutes to divide and cool the arc. Magnetic blowout coils deflect the arc into the arc chute.



In larger ratings, oil circuit breakers rely upon vaporization of some of the oil to blast a jet of oil through the arc.

Gas usually sulfur hexafluoride circuit breakers sometimes stretch the arc using a magnetic field, and then rely upon the dielectric strength of the sulfur‐hexafluoride SF6 to quench the stretched arc.

Vacuum circuit breakers have minimal arcing as there is nothing to ionize other than the contact material , so the arc quenches when it is stretched a very small amount 2–3 mm . Vacuum circuit breakers are frequently used in modern medium‐voltage switchgear to 35,000 volts.

Air circuit breakers may use compressed air to blow out the arc, or alternatively, the contacts are rapidly swung into a small sealed chamber, the escaping of the displaced air thus blowing out the arc.

Circuit breakers are usually able to terminate all current very quickly: typically the arc is extinguished between 30 ms and 150 ms after the mechanism has been tripped, depending upon age and construction of the device.

SHORT CIRCUIT CURRENT

Circuit breakers are rated both by the normal current that are expected to carry, and the maximum short‐circuit current that they can safely interrupt.

Under short‐circuit conditions, a current many times greater than normal can exist see maximum prospective short circuit current . When electrical contacts open to interrupt a large current, there is a tendency for an arc to form between the opened contacts, which would allow the current to continue. Therefore, circuit breakers must incorporate various features to divide and extinguish the arc.

In air‐insulated and miniature breakers an arc chutes structure consisting often of metal plates or ceramic ridges cools the arc, and magnetic blowout coils deflect the arc into the arc chute. Larger circuit breakers such as those



used in electrical power distribution may use vacuum, an inert gas such as sulphur hexafluoride or have contacts immersed in oil to suppress the arc.

The maximum short‐circuit current that a breaker can interrupt is determined by testing. Application of a breaker in a circuit with a prospective short‐circuit current higher than the breaker's interrupting capacity rating may result in failure of the breaker to safely interrupt a fault. In a worst‐case scenario the breaker may successfully interrupt the fault, only to explode when reset.

Miniature circuit breakers used to protect control circuits or small appliances may not have sufficient interrupting capacity to use at a panel board; these circuit breakers are called "supplemental circuit protectors" to distinguish them from distribution‐type circuit breakers.

TYPES OF CIRCUIT BREAKER

Many different classifications of circuit breakers can be made, based on their features such as voltage class, construction type, interrupting type, and structural features.

LOW‐VOLTAGE CIRCUIT BREAKER

Low voltage less than 1000 VAC types are common in domestic, commercial and industrial application, include:

• MCB Miniature Circuit Breaker —rated current not be more than 100 A. Trip characteristics normally not adjustable. Thermal or thermal‐magnetic operation. Breakers illustrated above are in this category.

• MCCB Molded Case Circuit Breaker —rated current up to 1000 A. Thermal or thermal‐magnetic operation. Trip current may be adjustable in larger ratings.

• Low voltage power circuit breakers can be mounted in multi‐tiers in LV switchboards or switchgear cabinets.

The characteristics of LV circuit breakers are given by international standards such as IEC 947. These circuit breakers are often installed in draw‐out



enclosures that allow removal and interchange without dismantling the switchgear.

Large low‐voltage molded case and power circuit breakers may have electrical motor operators, allowing them to be tripped opened and closed under remote control. These may form part of an automatic transfer switch system for standby power.

Low‐voltage circuit breakers are also made for direct‐current DC applications, for example DC supplied for subway lines. Special breakers are required for direct current because the arc does not have a natural tendency to go out on each half cycle as for alternating current. A direct current circuit breaker will have blow‐out coils which generate a magnetic field that rapidly stretches the arc when interrupting direct current.

Small circuit breakers are either installed directly in equipment, or are arranged in a breaker panel.

The 10 ampere DIN rail‐mounted thermal‐magnetic miniature circuit breaker is the most common style in modern domestic consumer units and commercial electrical distribution boards throughout Europe. The design includes the following components:

1. Actuator lever ‐ used to manually trip and reset the circuit breaker. Also indicates the status of the circuit breaker On or Off/tripped . Most breakers are designed so they can still trip even if the lever is held or locked in the "on" position. This is sometimes referred to as "free trip" or "positive trip" operation.

2. Actuator mechanism ‐ forces the contacts together or apart.

3. Contacts ‐ Allow current when touching and break the current



when moved apart. 4. Terminals 5. Bimetallic strip 6. Calibration screw ‐ allows the manufacturer to precisely adjust the trip

current of the device after assembly. 7. Solenoid 8. Arc divider/extinguisher

MAGNETIC CIRCUIT BREAKER

Magnetic circuit breakers use a solenoid electromagnet that’s pulling force increases with the current. Certain designs utilize electromagnetic forces in addition to those of the solenoid. The circuit breaker contacts are held closed by a latch. As the current in the solenoid increases beyond the rating of the circuit breaker, the solenoid's pull releases the latch which then allows the contacts to open by spring action. Some types of magnetic breakers incorporate a hydraulic time delay feature using a viscous fluid. The core is restrained by a spring until the current exceeds the breaker rating. During an overload, the speed of the solenoid motion is restricted by the fluid. The delay permits brief current surges beyond normal running current for motor starting, energizing equipment, etc. Short circuit currents provide sufficient solenoid force to release the latch regardless of core position thus bypassing the delay feature. Ambient temperature affects the time delay but does not affect the current rating of a magnetic breaker.

THERMAL MAGNETIC CIRCUIT BREAKER

Thermal magnetic circuit breakers, which are the type found in most distribution boards, incorporate both techniques with the electromagnet responding instantaneously to large surges in current short circuits and the bimetallic strip responding to less extreme but longer‐term over‐current conditions.



COMMON TRIP CIRCUIT BREAKER

Three pole common trip breaker for supplying a three‐phase device. This breaker has a 2A rating

When supplying a branch circuit with more than one live conductor, each live conductor must be protected by a breaker pole. To ensure that all live conductors are interrupted when any pole trips, a "common trip" breaker must be used. These may either contain two or three tripping mechanisms within one case, or for small breakers, may externally tie the poles together via their operating handles. Two pole common trip breakers are common on 120/240 volt systems where 240 volt loads including major appliances or further distribution boards span the two live wires. Three‐pole common trip breakers are typically used to supply three‐phase electric power to large motors or further distribution boards.

Two and four pole breakers are used when there is a need to disconnect the neutral wire, to be sure that no current can flow back through the neutral wire from other loads connected to the same network when people need to touch the wires for maintenance. Separate circuit breakers must never be used for disconnecting live and neutral, because if the neutral gets disconnected while the live conductor stays connected, a dangerous condition arises: the circuit will appear de‐energized appliances will not work , but wires will stay live and RCDs will not trip if someone touches the live wire because RCDs need power to trip . This is why only common trip breakers must be used when switching of the neutral wire is needed.



MEDIUM VOLTAGE CIRCUIT BREAKERS

Medium‐voltage circuit breakers rated between 1 and 72 kV may be assembled into metal‐enclosed switchgear line ups for indoor use, or may be individual components installed outdoors in a substation. Air‐break circuit breakers replaced oil‐filled units for indoor applications, but are now themselves being replaced by vacuum circuit breakers up to about 35 kV . Like the high voltage circuit breakers described below, these are also operated by current sensing protective relays operated through current transformers. The characteristics of MV breakers are given by international standards such as IEC 62271. Medium‐voltage circuit breakers nearly always use separate current sensors and protection relays, instead of relying on built‐in thermal or magnetic over‐current sensors.

Medium‐voltage circuit breakers can be classified by the medium used to extinguish the arc:

• Vacuum circuit breaker

With rated current up to 3000 A, these breakers interrupt the current by creating and extinguishing the arc in a vacuum container. These are generally applied for voltages up to about 35,000 V, which corresponds roughly to the medium‐voltage range of power systems. Vacuum circuit breakers tend to have longer life expectancies between overhaul than do air circuit breakers.

• Air circuit breaker—rated current up to 10,000 A. Trip characteristics are often fully adjustable including configurable trip thresholds and delays. Usually electronically controlled, though some models are microprocessor controlled via an integral electronic trip unit. Often used for main power distribution in large industrial plant, where the breakers are arranged in draw‐out enclosures for ease of maintenance.

• SF6 circuit breakers extinguish the arc in a chamber filled with sulfur hexafluoride gas.

Medium‐voltage circuit breakers may be connected into the circuit by bolted connections to bus bars or wires, especially in outdoor switchyards. Medium‐voltage circuit breakers in switchgear line‐ups are often built with draw‐out construction, allowing the breaker to be removed without disturbing the



power circuit connections, using a motor‐operated or hand‐cranked mechanism to separate the breaker from its enclosure.

HIGH VOLTAGE CIRCUIT BREAKERS

Electrical power transmission networks are protected and controlled by high‐voltage breakers. The definition of high voltage varies but in power transmission work is usually thought to be 72.5 kV or higher, according to a recent definition by the International Electro‐technical Commission IEC . High‐voltage breakers are nearly always solenoid‐operated, with current sensing protective relays operated through current transformers. In substations the protection relay scheme can be complex, protecting equipment and busses from various types of overload or ground/earth fault.

High‐voltage breakers are broadly classified by the medium used to extinguish the arc.

• Bulk oil • Minimum oil • Air blast • Vacuum • SF6

Some of the manufacturers are ABB, GE General Electric , AREVA, Mitsubishi‐Electric, Pennsylvania Breaker, Siemens , Toshiba, Končar HVS, BHEL and others.

Due to environmental and cost concerns over insulating oil spills, most new breakers use SF6 gas to quench the arc.

Circuit breakers can be classified as live tank, where the enclosure that contains the breaking mechanism is at line potential, or dead tank with the enclosure at earth potential. High‐voltage AC circuit breakers are routinely available with ratings up to 765 kV.

High‐voltage circuit breakers used on transmission systems may be arranged to allow a single pole of a three‐phase line to trip, instead of tripping all three poles; for some classes of faults this improves the system stability and availability.



ONE LINE DIAGRAM

FAULTED POINTS

• BUS‐7 • BUS‐13



LOAD FLOW ANALYSIS DIAGRAM



SHORT CIRCUIT ANALYSIS DIAGRAM

BREAKERS OPERATED

• CB‐1 • CB‐3

BREAKERS DATA

Breaker ID Before BUS Normal CurrentAmp

Short Circuit Current

Breaker Interrupting Current

Breaker State

CB‐1 BUS‐7 249 1.2KA 0.5KA OPENCB‐2 BUS‐6 19 0.1KA CLOSEDCB‐3 BUS‐13 9 15KA 0.1KA OPENCB‐5 BUS‐17 243 0.5KA CLOSEDCB‐6 BUS‐6 19 0.1KA CLOSED



ALERT DIAGRAM

COMMENTS:

We find the normal current flowing through BUS‐7 for which we have to design a circuit breaker.

Normal current flowing through BUS‐7 is 246Ampere while through BUS‐13 is 9Amperes.

After that we perform the short circuit analysis to check that how much current can flow in case of fault.

Fault current obtained from short circuit analysis is 1.3KAmpere that is many times larger than the normal operating current

As fault current is greater in magnitude at the fault occurrence event and reduces up to some extent. Keeping in mind this fact, we connect a circuit breaker of suitable operating value of current at which circuit breaker will operate.

In this experiment, we have selected interrupting breaker current as 0.5KAmpere for CB‐1 and 0.1KAmpere for CB‐3 that can be varied to any required value of current.

After connecting the circuit breaker, we again perform the short circuit analysis and observe that the breaker connected to faulty bus is operated and faulty system is isolated.



EXPERIMENT NO: 05 Transient stability analysis of a given power system using ETAP

Transients in Electrical power system

Lightning has long fascinated the technical community. Ben Franklin studied lightning's electrical nature over two centuries ago and Charles R Steinmetz generated artificial lightning in his General Electric laboratory in the 1920's. As someone concerned with premises data communications you need to worry about lightning. Here I will elaborate on why, where and when you should worry about lightning. I'll then discuss how to get protection from it.

It is unfortunate, but a fact of life, that computers, computer related products and process control equipment found in premises data communications environments can be damaged by high‐voltage surges and spikes. Such power surges and spikes are most often caused by lightning strikes. However, there are occasions when the surges and spikes result from any one of a variety of other causes. These causes may include direct contact with power/lightning circuits, static buildup on cables and components, high energy transients coupled into equipment from cables in close proximity, potential differences between grounds to which different equipment’s are connected, miss‐wired systems and even human equipment users who have accumulated large static electricity charge build‐ups on their clothing. In fact, electrostatic discharges from a person can produce peak Voltages up to 15 kV with currents of tens of Amperes in less than 10 microseconds.

A manufacturing environment is particularly susceptible to such surges because of the presence of motors and other high voltage equipment. The essential point to remember is, the effects of surges due to these other sources are no different than those due to lightning. Hence, protection from one will also protect from the other.

When a lightning‐induced power surge is coupled into your computer equipment any one of a number of harmful events may occur.



Semiconductors are prevalent in such equipment. A lightning induced surge will almost always surpass the voltage rating of these devices causing them to fail. Specifically, lightning induced surges usually alter the electrical characteristics of semiconductor devices so that they no longer function effectively. In a few cases, a surge may destroy the semiconductor device. These are called "hard failures." Computer equipment having a hard failure will no longer function at all. It must be repaired with the resulting expense of "downtime" or the expense of a standby unit to take its place.

LIGHTENING SURGES:

In several instances, a lightning‐derived surge may destroy the printed traces in the printed circuit boards of the computer equipment also resulting in hard failures.

Along with the voltage source, lightning can cause a current surge and a resultant induced magnetic field. If the computer contains a magnetic disk then this interfering magnetic field might overwrite and destroy data stored in the disk. Furthermore, the aberrant magnetic field may energize the disk head when it should be quiescent. To you, the user, such behavior will be viewed as the "disk crashing."



Some computer equipment may have magnetic relays. The same aberrant magnetic fields which cause disk crashes may activate relays when they shouldn't be activated, causing unpredictable, unacceptable performance.

Finally, there is the effect of lightning on program logic controllers PLCS which are found in the manufacturing environment. Many of these PLCs use programs stored in ROMS. A lightning‐induced surge can alter the contents of the ROM causing aberrant operation by the PLC.

So these are some of the unhappy things which happen when a computer experiences lightning.

This is a typical reaction and unfortunately it is based on ignorance. True, people may never, or rarely, experience, direct lightning strikes on exposed, in‐building cable feeding into their equipment.

However, it is not uncommon to find computer equipment being fed by buried cable. In this environment, a lightning strike, even several miles away, can induce voltage/current surges which travel through the ground and induce surges along the cable, ultimately causing equipment failure. The equipment user is undoubtedly aware of these failures but usually does not relate them to the occurrence of lightning during thunderstorm activity since the user does not experience a direct strike.

In a way, such induced surges are analogous to chronic high blood pressure in a person; they are "silent killers." In the manufacturing environment, long cable runs are often found connecting sensors, PLCs and computers. These cables are particularly vulnerable to induced surges.

LIGHTENING ARRESTORS:

Metal oxide varistors MOVS provide an improvement over the response time problem of gas tubes. But, operational life is a drawback. MOVs protection characteristic decays and fails completely when subjected to prolong over voltages.



Silicon avalanche diodes have proven to be the most effective means of protecting computer equipment against over voltage transients. Silicon avalanche diodes are able to withstand thousands of high voltage, high current and transient surges without failure. While they can not deal with the surge peaks that gas tubes can, silicon avalanche diodes do provide the fastest response time. Thus, depending upon the principal threat being protected against, devices can be found employing gas tubes, MOVS, or silicon avalanche diodes. This may be awkward, since the threat is never really known in advance. Ideally, the protection device selected should be robust, using all three basic circuit breaker elements. The architecture of such as device is illustrated in Figure 20. This indicates triple stage protection and incorporates gas tubes, MOVs and silicon avalanche diodes as well as various coupling components and a good ground.

With the architecture shown in Figure 20 a lightning strike surge will travel, along the line until it reaches a gas tube. The gas tube dumps extremely high amounts of surge energy directly to earth ground. However, the surge rises very rapidly and the gas tube needs several microseconds to fire.

As a consequence, a delay element is used to slow the propagation of the leading edge wave front, thereby maximizing the effect of the gas tube. For a 90 Volt gas tube, the rapid rise of the surge will result in its firing at about 650 Volts. The delayed surge pulse, now of reduced amplitude, is impressed on the avalanche diode which responds in about one nanosecond or less and can dissipate 1,500 Watts while limiting the voltage to 18 Volts for EIA‐232 circuits. This 18 Volt level is then resistively coupled to the MOV which clamps to 27 Volts. The MOV is additional protection if the avalanche diode capability is exceeded.

As previously mentioned, the connection to earth ground can not be over emphasized. The best earth ground is undoubtedly a cold water pipe.



However, other pipes and building power grounds can also be used. While cold water pipes are good candidates you should even be careful here. A plumber may replace sections of corroded metal pipe with plastic. This would render the pipe useless as a ground.

TRANSIENT STABILITY ANALYSIS IN ETAP

The PowerStation Transient Stability Analysis program is designed to investigate the stability limits of a power system before, during and after system changes or disturbances. The program models dynamic characteristics of a power system, implements the user‐defined events and actions, solves the system network equation and machine differential equations interactively to find out system and machine responses in time domain. From these responses, users can determine the system transient behavior, make stability assessment, find protective device settings, and apply the necessary remedy or enhancement to improve the system stability. The Transient Stability Toolbar section explains how you can launch a transient stability calculation, open and view an output report, select display options, and view plots. The Study Case Editor section explains how to create a new study case, to define parameters for a study case, to create a sequence of switching events and disturbances, to globally define machine dynamical modeling method, to select plot/tabulation devices, etc.

The Display Options section explains what options are available for displaying some key system parameters and the output results on the one‐line diagram, and how to set them.

The Calculation Methods section provides some theoretical backgrounds and quick reference for the fundamentals on transient stability study, which are very helpful for users who do not have extensive experience on running transient stability studies. The Required Data section is a very good reference for you to check if you have prepared all necessary data for transient stability calculations.

The Output Reports section explains and demonstrates the format and organization of the transient stability text reports. The One‐Line Diagram



Displayed Results section explains the available one‐line displaying results and provides one example. The Plots section explains what plots for transient stability are available and how to select and view them.

TRANSIENT STABILITY TOOLBAR

The Transient Stability Toolbar will appear on the screen when you are in the Transient Stability Study mode.

Run Transient Stability

Select a study case from the Study Case Toolbar. Then click on the Run Transient Stability button to perform a transient stability study. A dialog box will appear to ask you to specify the output report name if the output file name is set to Prompt. The transient stability study results will appear on the one‐line diagram and stored in the output report, as well as in the plot file.



Display Options

Click the Display Options button to customize the one‐line diagram annotation options under the transient stability study mode. Also to edit the one‐line diagram display for transient stability calculation results.

Report Manager

Click on Report Manager Button to select a format and view transient stability output report. Transient stability analysis reports are current provided in ASCII formats only, which can be accessed from the Report Manager.



Transient Stability Plots

Click on the Transient Stability Plots button to select and plot the curves of the last plot file. The plot file name is displayed on the Study Case Toolbar. The transient stability plot files have the following extension: .tsp. For more information see plotting section.



Starting Generator Data

To perform a generator start‐up analysis, the following synchronous generator model needs to be selected. This model is adapted from the latest IEEE Standard 1110 “IEEE Guide for Synchronous Generator Modeling Practices in Stability Analyses.” It has one damping winding on each of the direct and quadratic axis.

Turbine ‐ Governor Models

Practically any type of turbine‐governor model in PowerStation can be used in the generator start‐up study, provided there are no other special control functions required.



ONE LINE DIAGRAM:



WAVEFORMS FOR GENERATOR Generator Exciter Current

Generator Exciter Voltage

Explanation As it is clear from graph that as transients occur in system there is a sudden dip in generator excitation voltage at start, this dip in voltage then gets higher value after dipping and as long as transients exists it shows some fluctuations and get stable value when transients get eliminated.



Generator Electrical power

Explanation The effect of transients on generator electrical power is shown in figure. there is slight dip and then alternation in the power values due to alternation in voltage values due to transients, and as transients are being controlled we get stable value of electrical power as obvious from graph. Generator Mechanical Power

Explanation The same is the case with mechanical power as was with electrical power.



Generator Frequency

Generator Rotor Angle:

Explanation As graph shows that there is vibration occurance in rotor of a generator at start due to transient,but as soon as value or effect of transient becomes small the rotor angle degree slows down or it advances towards stable value in synchronous with other generators of the system,



Generator Terminal Current:

Explaination : The effect of transients on generator terminal current is clear from the graph,where it is quite clear that there is sudden almost steep increment in the current magnitude of generator,and then it becomes less than original value and then again comes to the same original current level.

BUS WAVEFORMS Bus Voltages

Explanation The machine current graph shows that the value of current increases sharply at start unlike machine voltage and then it gradually have decline in sinusoidal magnitude variation of current and finally levels off to the original value as clear from graph. As concerned to bus voltage, it is obvious from graph that the bus voltage dips to zero and remain at zero as shown in circuit graph.



Bus Voltage Angle

SYNCHRONOUS MOTOR WAVEFORMS Electrical Power

Explanation The electric power of synchronous motor after a slight increment decreases and then there is a dip in value which then again increases and after that it changed sinusoidaly but gradually decreasing value and at last becomes stable.



Mechanical power

Machine Frequency

Rotor Angle



Machine Connected Voltage

Explanation It is clear from graph that machine connected voltage decreases rapidly as shown by graph and then it got much value to become equal to the original value, but that value slightly increases in magnitude as time proceed as shown in graph. Machine Current



COMMENTS:

Transients are very fast increase in voltage value that exists for a very short interval of time but can damage the system to such an extent, that power failure may occur due to component failure. Transients are of two types, external due to cloud discharging and internal due to switching. However both these cause the system voltage to rise to a dangerous value limits, that must be avoided. The internal occur due to switching out inductive load or switching in capacitive load in the system. Because capacitor provide var’s to our system. Due to transient’s some values relating to voltage and current parameters of different components have different effects. The excitation voltage of the generator decreases while excitation current decreases. The synchronous motor current increases while voltage decreases rapidly for small time and then levels off. As concerned to the frequency it just fluctuates in its original value by just a smaller magnitude which almost negligible. But remain constant for most of the time. The rotor of the generator starts vibrating and is not more synchronized with the system this could lead to more severe vibrations and may lead to more rotors to vibrate. This is very dangerous situation for the health of our system. However after the transients the rotor is brought to the same rotor angle in order to synchronize with the system, to avoid any unwanted situation in the power system. As bus bar is a protecting device so whenever a transient occurs in the system, the bus bar voltage instantly drop to zero as shown in graph. Thus transient either internal or external are very harmful for our system and they must be diminished as soon as possible by proper grounding and other safety measurements.



EXPERIMENT NO: 06 Introduction to Ground Grid Modeling in ETAP

GROUND GRID

An effective substation grounding system typically consists of driven ground rods, buried interconnecting grounding cables or grid, equipment ground mats, connecting cables from the buried grounding grid to metallic parts of structures and equipment, connections to grounded system neutrals, and the ground surface insulating covering material. Currents flowing into the grounding grid from lightning arrester operations, impulse or switching surge flashover of insulators, and line‐to‐ground fault currents from the bus or connected transmission lines all cause potential differences between grounded points in the substation and remote earth. Without a properly designed grounding system, large potential differences can exist between different points within the substation itself. Under normal circumstances, it is current flowing through the grounding grid from line‐to‐ground faults that constitutes the main threat to personnel.

OBJECTIVES OF GROUNDING

An effective grounding system has the following objectives:

Ensure such a degree of human safety that a person working or walking in the vicinity of grounded facilities is not exposed to the danger of a critical electric shock. The touch and step voltages produced in a fault condition have to be at safe values. A safe value is one that will not produce enough current within a body to cause ventricular fibrillation.

Provide means to carry and dissipate electric currents into earth under normal and fault conditions without exceeding any operating and equipment limits or adversely affecting continuity of service.

Provide grounding for lightning impulses and the surges occurring from the switching of substation equipment, which reduces damage to equipment and cable.



Provide a low resistance for the protective relays to see and clear ground faults, which improves protective equipment performance, particularly at minimum fault.

IMPORTANT DEFINITIONS

DC Offset

Difference between the symmetrical current wave and the actual current wave during a power system transient condition is called DC‐offset. Mathematically, the actual fault current can be broken into two parts:

Symmetrical alternating component and Unidirectional dc component

The unidirectional component can be of either polarity, but will not change polarity and will decrease at some predetermined rate.

Earth Current

It is the current that circulates between the grounding system and the ground fault current source that uses the earth as the return path.

Ground Fault Current

It is the current flowing into or out of the earth or an equivalent conductive path during a fault condition involving ground.

Ground Potential Rise GPR

The maximum voltage that a ground grid may attain relative to a distant grounding point assumed to be at the potential of remote earth. The GPR is equal to the product of the earth current and the equivalent impedance of the grounding system.

Mesh Voltage

It is the maximum touch voltage within a mesh of a ground grid.



Soil Resistivity

It is the electrical characteristic of the soil with respect to conductivity. The value is typically given in ohm‐meters.

Step Voltage

The difference in surface potential experienced by a person bridging a distance of 1 meter with his feet without contacting any other grounded object.

Touch Voltage

It is the potential difference between the ground potential rise and the surface potential at the point where a person is standing while at the same time having his hands in contact with a grounded structure.

Transferred Voltage

It is a special case of the touch voltage where a voltage is transferred into or out of the substation from or to a remote point external to the substation site.

AREA OF THE GROUND GRID

The area of the ground grid should be as large as possible, preferably covering the entire substation site.

All of the available area should be used since this variable has the greatest effect in lowering the grid resistance. Measures such as adding additional grid conductor are expensive and do not reduce the grid resistance to the extent that increasing the area does.

In general, the outer grid conductors should be placed on the boundary of the substation site with the substation fence placed a minimum of 3 feet inside the outer conductors. This results in the lowest possible grid resistance and protects persons outside the fence from possibly hazardous touch voltages. It is therefore imperative that the fence and the ground grid layout be coordinated early in the design process.



The simplified design equations require square, rectangular, triangular, T‐shaped, or L‐shaped grids. For preliminary design purposes, on a layout drawing of the substation site, draw in the largest square, rectangular, triangular, T‐shaped, or L‐shaped grids that will fit within the site. These represent the outer grid conductors and will define the area of the grid to be used in the calculations. A square, rectangular, triangular, T‐shaped, or L‐shaped grid site generally requires no additional conductors once the design is complete. For irregular sites, once the design has been completed, additional conductors will be run along the perimeter of the site that were not included in the original grid design and connected to the grid.

This will take advantage of the entire site area available and will result in a more conservative design.

GROUND FAULT CURRENTS

When a substation bus or transmission line is faulted to ground, the flow of ground current in both magnitude and direction depends on the impedances of the various possible paths. The flow may be between portions of a substation ground grid, between the ground grid and surrounding earth, along connected overhead ground wires, or along a combination of all these paths.

The relay engineer is interested in the current magnitudes for all system conditions and fault locations so that protective relays can be applied and coordinating settings made. The designer of the substation grounding system is interested primarily in the maximum amount of fault current expected to flow through the substation grid, especially that portion from or to remote earth, during the service lifetime of the installed design.

Figure illustrates a case governing ground fault current flow. The worst case for fault current flow between the substation grounding grid and surrounding earth in terms of effect on substation safety has to be determined.



The maximum symmetrical rms fault current at the instant of fault initiation is usually obtained from a network analyzer study or by direct computation.

Symmetrical Grid Current

That portion of the symmetrical ground fault current that flows between the grounding grid and surrounding earth may be expressed by:

Ig If . Sf

Where:

Ig rms symmetrical grid current in amperes

If rms symmetrical ground fault current in amperes

Sf Fault current division factor

For the assumption of a sustained flow of the initial ground fault current, the symmetrical grid current can be expressed by:

Ig 3Io . Sf

Where:

Io Symmetrical rms value of Zero Sequence fault current in amperes

For transmission substations, calculate the maximum Io for a single‐phase‐to‐ground fault for both the present station configuration and the ultimate station configuration. Obtain values for all voltage levels in the station. Use the largest of these fault current values.



For distribution stations, since the fault current at distribution stations will not increase significantly over the life of the station as a result of the high impedance of the 34 and 69 kV feeders, the future fault current can be modeled using a suitable growth factor suggest value of 1.1 x For distribution stations, since the fault current at distribution stations will not increase significantly over the life of the station as a result of the high impedance of the 34 and 69 kV feeders, the future fault current can be modeled using a suitable growth factor suggest value of 1.1 x For distribution stations, since the fault current at distribution stations will not increase significantly over the life of the station as a result of the high impedance of the 34 and 69 kV feeders, the future fault current can be modeled using a suitable growth factor suggest value of 1.1 x Io .

For an extremely conservative design, the interrupting rating of the equipment can be used for Io. This value may be as high as ten times the ultimate single‐phase‐to‐ground fault current. Use of such a large safety factor in the initial design may make it difficult to design the grid to meet the tolerable touch and step voltage criteria by any means.

Determine the Split Factor, Sf

The split factor is used to take into account the fact that not all the fault current uses the earth as a return path. Some of the parameters that affect the fault current paths are:

Location of the fault Magnitude of substation ground grid impedance Buried pipes and cables in the vicinity of or directly connected to the substation ground system

Overhead ground wires, neutrals, or other ground return paths

The most accurate method for determining the percentage of the total fault current that flows into the earth is to use a computer program such as EPRI’s SMECC, Substation Maximum Earth Current Computation.



For the purposes of this Bulletin, the graphical method will be used.

Two types of graphs will be presented:

100 percent remote, 0 percent local fault current contribution 25, 50, and 75 percent local, which corresponds to 75, 50, and 25 percent remote fault current contribution

The Decrement Factor, Df

The decrement factor accounts for the asymmetrical fault current wave shape during the early cycles of a fault as a result of the dc current offset. In general, the asymmetrical fault current includes the sub‐transient, transient, and steady‐state ac components, and the dc offset current component. Both the sub‐transient and transient ac components and the dc offset decay exponentially, each having a different attenuation rate. However, in typical applications of this guide, it is assumed that the ac component does not decay with time but remains at its initial value.

The decrement factor can be calculated using:

Where:

tf Time duration of fault in seconds

Ta X/ wR the dc offset time constant in seconds

Maximum Grid Current

During a system fault, the fault current will use the earth as a partial return path to the system neutral.

The current that is injected into the earth during a fault results in a ground potential rise. Typically, only a fraction of the total fault current flows from



the grounding system into the earth. This is due to the transfer of current onto metallic paths such as overhead static shields, water pipelines, etc.

Faults occurring within the substation generally do not produce the worst earth currents since there are direct conductive paths that the fault current can follow to reach the system neutral assuming the substation has a grounded‐wye transformer . The faults that produce the largest ground currents are usually line‐to‐ground faults occurring at some distance away from the substation.

The maximum grid current is the current that flows through the grid to remote earth and is calculated by:

Where:

IG Maximum grid current in amperes

Df Decrement factor for the entire duration of fault t , found for t, given in seconds

Ig rms symmetrical grid current in amperes

Asymmetrical Fault Current

The asymmetrical fault current includes the sub‐transient, transient, and steady‐state ac components, and the dc offset current component and can be defined as shown:

Where:

IF Effective asymmetrical fault current in amperes

If rms symmetrical ground fault current in amperes

Df Decrement factor



The dc offset in the fault current will cause the conductor to reach a higher temperature for the same fault conditions fault current duration and magnitude . In addition, if present, dc offset could result in mechanical forces and absorbed energy being almost four times the value of an equivalent symmetric current case.

GROUND GRID MODELING IN ETAP The Ground Grid Systems program calculates the following:

The Maximum Allowable Current for specified conductors. Warnings are issued if the specified conductor is rated lower than the fault current level

The Step and Touch potentials for any rectangular/triangular/L‐shaped/T‐shaped configuration of a ground grid, with or without ground rods IEEE Std 80 and IEEE Std 665

The tolerable Step and Mesh potentials and compares them with actual, calculated Step and Mesh potentials IEEE Std 80 and IEEE Std 665

Graphic profiles for the absolute Step and Touch voltages, as well as the tables of the voltages at various locations Finite Element Method

The optimum number of parallel ground conductors and rods for a rectangular/triangular/L‐shaped/T‐shaped ground grid. The cost of conductors/rods and the safety of personnel in the vicinity of the substation/generating station during a ground fault are both considered. Design optimizations are performed using a relative cost effectiveness method based on the IEEE Std 80 and IEEE Std 665

The Ground Resistance and Ground Potential rise GPR

Ground Grid Systems Presentation

The GGS presentation is composed of the Top View, Soil View, and 3D View. The Top View is used to edit the ground conductors/rods of a ground grid. The Soil View is used to edit the soil properties of the surface, top, and lower layers of soil. The 3D View is used for the three‐dimensional display of the ground grid. The 3D View also allows the display of the ground grid to rotate,



offering views from various angles. The GGS presentation allows for graphical arrangement of the conductors and rods that represent the ground grid, and to provide a physical environment to conduct ground grid design studies.

Each GGS presentation is a different and independent ground grid system. This concept is different from the multi‐presentation approach of the One‐Line Diagram, where all presentations have the same elements. There is no limit to the number of GGS presentations that can be created.

Create a New Ground Grid Presentation

To create a GGS presentation, a ground grid must first be added to the One‐Line Diagram. Click on the Ground Grid component located on the AC toolbar, and drop the GGS symbol anywhere on the One‐Line Diagram.

Right‐click on any location inside the ground grid box, and select Properties to bring up the Grid Editor. The Grid Editor Dialog box is used to specify grid information, grid styles, equipment information, and to view calculation results. Click on the Grid Presentation button to bring up a GGS presentation.



Double‐clicking on the ground grid box located on the One‐Line Diagram will bring up the Ground‐Grid Project Information dialog box, used to select an IEEE or FEM ‐ Finite Element Method Study Model.

After selecting the IEEE or FEM Study Model, the Ground Grid Systems graphical user interface window will be displayed. Below is a GGS presentation of a ground grid for the FEM Study Model case.



FEM Editor Toolbar

The FEM Editor Toolbar appears when the FEM Study Model is selected, and when in the Ground Grid Systems Edit mode. This toolbar has the following function keys:

Pointer

The cursor takes the shape of the element selected from the Edit Toolbar. Click on the Pointer icon to return the cursor to its original arrow shape, or to move an element placed in the Top View of the GGS presentation.

Conductor

Click on the Conductor icon to create a new conductor and to place it in the Top View of the GGS. For more information on conductors see the Conductor/Rod Editor section for FEM .

Rod

Click on the Rod icon to create a new rod and to place it in the Top View of the GGS. For more information on rods see the Conductor/Rod Editor section for FEM .



FEM Rectangular Shape

Click on the FEM Rectangular Shape icon to create a new FEM grid of rectangular shape and to place it in the Top View of the GGS. For more information on grids see the FEM Group Editor section.

FEM T‐Shape

Click on the FEM T‐Shape icon to create a new FEM T‐shaped grid and to place it in the Top View of the GGS. For more information on grids see the FEM Group Editor section.

FEM L‐Shape

Click on the FEM L‐Shape icon to create a new FEM L‐shaped grid and to place it in the Top View of the GGS. For more information on grids see the FEM Group Editor section.

FEM Triangular Shape

Click on the FEM Triangular Shape icon to create a new FEM grid of triangular shape and to place it in the Top View. For more information on grids see the FEM Group Editor section.

IEEE Edit Toolbar

The IEEE Editor Toolbar appears when the IEEE Study Model is selected, and when in the Ground Grid Systems Edit mode. This toolbar has the following function keys:

Pointer

The cursor takes the shape of the element selected from the Edit Toolbar. Click on the Pointer icon to return the cursor to its original arrow shape, or to move an element placed in the Top View of the GGS presentation.



IEEE Rectangular Shape

Click on the IEEE Rectangular Shape icon to create a new IEEE grid of rectangular shape and to place it in the Top View of the GGS. For more information on grids see the IEEE Group Editor section.

IEEE T‐Shape

The IEEE T‐Shape grid is valid only for the IEEE Std. 80‐2000 method. Click on the IEEE T‐Shape icon to create a new IEEE T‐shaped grid and to place it in the Top View of the GGS. For more information on grids see the IEEE Group Editor section.

IEEE L‐Shape

The IEEE L‐Shape grid is valid only for the IEEE Std 80‐2000 method. Click on the IEEE L‐Shape icon to create a new IEEE L‐shaped grid and to place it in the Top View of the GGS. For more information on grids see the IEEE Group Editor section.

IEEE Triangular Shape

The IEEE Triangular Shape grid is valid only for the IEEE Std 80‐2000 method. Click on the IEEE Triangular Shape icon to create a new IEEE grid of triangular shape and to place it in the Top View. For more information on grids see the IEEE Group Editor section.



Ground Grid Study Method Toolbar

The Ground Grid Study Method Toolbar appears when the GGS Study mode is selected. This toolbar has the following function keys:

Ground‐Grid Calculation

Click on the Ground‐Grid Calculation button to calculate:

Step and Touch mesh Potentials Ground Resistance Ground Potential Rise Tolerable Step and Touch Potential Limits Potential Profiles only for the FEM method



Optimized Conductors

Click on the Optimized Conductors button to calculate the minimum number of conductors that satisfy the tolerable limits for the Step and Touch potentials for a fixed number of ground rods. This optimization function is for IEEE Std methods only.

Optimized Conductors and Rods

Click on the Optimized Conductors button to calculate the optimum numbers of conductors and ground rods needed to limit the Step and Touch potentials. This optimization function is for IEEE Std methods only.

Summary and Warning

Click on this button to open the GRD Analysis Alert View dialog box of Summary and Warning for the Ground Grid Systems Calculation.



Plot Selection

This function is valid only for the FEM method. Click on this button to open the Plot Selection dialog box to select a variety of potential profile plots to review, and click OK to generate the output plots.

Report Manager

Click on this button to open the Ground Grid Design Report Manager dialog box to select a variety of pre‐formatted output plots to review. Select a plot type and click OK to bring up the output plot.

Output Report files can be selected from the Output Report List Box on the Study Case Toolbar shown below:



Stop

The Stop Sign button is normally disabled, and becomes enabled when a Ground Grid Systems Calculation is initiated. Clicking on this button will terminate calculations in progress, and the output reports will be incomplete.

Edit A GGS Conductors, rods, and grids of various shapes are the elements available for adding to the Top View of the Ground Grid Systems presentation. These elements are located on the Edit Toolbar of the GGS module.

Select Elements

Place the cursor on an element located on the Edit toolbar and click the left mouse button. Note that when a grid shape is selected, regardless of the number of conductors or rods it contains, the shape is considered to be one element. If a selected shape is deleted or copied, the shape and its contents will also be deleted or copied. Press the Ctrl key and click on multiple elements to either select or de‐select them.

Add Elements

To add a new element to the GGS presentation, select a new element from the Edit Toolbar by clicking on the appropriate element button. Notice that the shape of the cursor changes to correspond to that of the selected element.

Place the selected element by clicking the mouse anywhere in the Top View section of the GGS presentation, and note that the cursor returns to its original shape. Double‐click on any element in the Edit Toolbar to place multiple copies of the same element in the Top View section of the GGS presentation.



Rules

Elements can be added ONLY in Edit mode Two conductors/rods cannot be added on top of each other Elements cannot be added in the Study mode Only one IEEE shape can be added in the Top View FEM group shapes can overlap each other

Add Conductors

Click on the Conductor button on the FEM Edit Toolbar, move the cursor to the GGS presentation, and click to place the element in the Top View. PowerStation creates the new conductor using default values.

Add Rods

Click on the Rod button on the FEM Edit Toolbar, move the cursor to the GGS presentation, and click to place the element in the Top View. PowerStation creates the new rod using default values.

Add Grid Shapes

Click on the desired Shape button on the FEM Edit Toolbar, move the cursor to the GGS presentation, and click to place the element in the Top View. PowerStation creates the new grid shape using default values.

Add Conductors by Ungrouping FEM Shapes

An FEM shape added in the Top View of a GGS presentation can be ungrouped into individual conductors. To ungroup, move the cursor inside the selected shape, right‐click and select “Ungroup”.

Move / Relocate Elements

When an element is added to a GGS presentation its position coordinates x, y and z are updated automatically in the editor/spreadsheet and in the Help line at the bottom of your screen. The element may be relocated to new coordinates by changing the coordinate values at the editor/spreadsheet x’s, yes and z’s for conductors/rods, and Lx, Ly, Depth, # of Rods and # of



Conductors in X/Y Directions for various typical grid shapes or by dragging the element and watching the Help line change to the desired position.

To drag an element, first select the element to be moved. Place the cursor on top of the selected element, Click and hold the left mouse button, drag the element to the desired position, and release the left button.

Move Conductors/Rods

Select the element, click and hold the left mouse button, drag the element to the new position and release the left button.

Move Shapes

Shapes can be graphically moved within the Top View. Select the shape, click and hold the left mouse button, drag the shape to the new location and release the left button.

Cut Delete Elements

Select the element or group of elements and press the Delete key on the keyboard.

Copy Elements

Select an element or group of elements, click the right mouse button, and select Copy.

Paste

Use the Paste command to copy the selected cells from the Dumpster into the GGS presentation.

Size of Elements

When an element is added to a GGS presentation, its size is set by default. The width and height of grid shapes and the length of conductors can be graphically changed. Select the element and move the cursor to a corner or edge of the element. Once the cursor changes its form, click and hold the left



mouse button to drag the element to its new size. Release the left mouse button once the desired size has been obtained.

Conductor/rod sizes can be change from the spreadsheet or shape editors. When the Length is altered, X1, Y1 and Z1 will remain unchanged, and X2, Y2 and Z2 will change accordingly. The cross‐sectional area of a conductor, the outside diameter and/or length of a rod can only be changed from the conductor or rod Editor.

Rules

Sizing elements can be done in Edit mode ONLY Elements cannot overlap each other

Study Case Editor

The GGS Study Case Editor contains Average Weight, Ambient Temperature, Current Projection Factor, Fault Current Durations, option to input or compute Fault Current Parameters i.e., zero‐sequence fault current, current division factor, and X/R ratio , and Plot Parameters for the Finite Element Method only .

PowerStation allows for the creation and saving of an unlimited number of study cases for each type of study, allowing the user to easily switch between different GGS study cases. This feature is designed to organize the study efforts and to save time. To create a new GGS study case, go to the Study Case Menu on the toolbar and select Create New to bring up the GGS Study Case Editor.



Study Case ID

A study case can be renamed by simply deleting the old Study Case ID and entering a new one. The Study case ID can be up to 25 alphanumeric characters. Use of the Navigator button at the bottom of the Study Case Editor allows the user to go from one study case to another.

Options

In this section, select the average body weight for the person working above the ground grid, and the ambient temperature. The weight is used to calculate the tolerable Step and Touch potentials.



Reports & Plots

Specify the report/plot parameters.

Report Details

Check this box to report intermediate results for an IEEE Std. Method or voltage profiles for the Finite Element Method.

Auto Display of Summary & Alert

Check this box to automatically show the result window for Summary & Warning.

Plot Step

Plot Step is valid only for the FEM Study Model. This value is entered in m/ft, and it is used to find the points or locations where Absolute/Step/Touch potentials need to be computed and plotted. Note that the smaller this number, the more calculations are required, increasing calculation time, but yielding smoother plots. The recommended value is 1 meter. If higher resolution is needed, decrease this number.

Boundary Extension

Enter the boundary extension in m/ft. This value is used to extend the grid boundaries inside which the Absolute/Step/Touch potentials need to be computed.

Fault Durations

Allows the user to specify Fault Current durations

tf

Enter the duration of fault current in seconds to determine decrement factor. The Fault duration tf , tc , and Shock duration ts are normally assumed to be equal, unless the Fault duration is the sum of successive shocks.



tc

Enter in seconds the duration of Fault Current for sizing ground conductors.

ts

Enter in seconds the duration of Shock Current to determine permissible levels for the human body.

Grid Current Factors

In this section, the Corrective Projection Factor and the Current Division Factor can be specified.

Cp

Enter the Corrective Projection Factor in percent, accounting for the relative increase of fault currents during the station lifespan. For a zero future system growth, Cp 100.

Sf

Enter the Current Division Factor in percent, relating the magnitude of Fault current to that of its portion flowing between the grounding grid and the surrounding earth.

Update

Check this box to update/replace the number of conductors/rods in the Conductor/Rod Editor, with the number of conductors/rods calculated by using optimization methods. This box is only valid with the IEEE methods.

Required Data To run a Ground Grid Systems study, the following related data is necessary: Soil Parameters, Grid Data, and System Data. A summary of these data for different types of calculation methods is given in this section.



System Data

System Frequency Average Weight of Worker Ambient Temperature Short Circuit Current Short Circuit Current Division Factor Short Circuit Current Projector Factor Durations of Fault System X/R Ratio Plot Step for FEM model only Boundary Extension for FEM model only

Soil Parameters

Surface Material Resistivity Surface Material Depth Upper Layer Soil Resistivity Upper Layer Soil Depth Lower Layer Soil Resistivity

Ground Conductor Library

Material Conductivity Thermal Coefficient of Resistivity Ko Factor Fusing Temperature Ground Conductor Resistivity Thermal Capacity Factor

Grid Data IEEE Std.’s Only

Shape Material Type Conductor Cross Section Grid Depth Maximum Length of the Grid in the X Direction



Maximum Length of the Grid in the Y Direction Minimum Length of the Grid in the X Direction for IEEE Std 80‐2000 L‐Shaped or T‐Shaped Grids Only

Minimum Length of the Grid in the Y Direction for IEEE Std 80‐2000 L‐Shaped or T‐Shaped Grid Only

Number of Conductors in the X Direction Number of Conductors in the Y Direction Cost

Rod Data IEEE Std.’s Only

Material Type Number of Rods Average Length Diameter Arrangement Cost

Conductor Data FEM model only

Material Type Insulation Cross Section X, Y and Z Coordinates of One End of Conductor X, Y and Z Coordinates of Other End of Conductor Cost

Rod Data FEM model only

Material Type Insulation Diameter X, Y and Z Coordinates of One End of Rod X, Y and Z Coordinates of Other End of Rod Cost



Optional FEM Model Grid Group Data

Shape Material Type Conductor Cross Section Grid Depth Maximum Length of the Grid in the X Direction Maximum Length of the Grid in the Y Direction Minimum Length of the Grid in the X Direction for L‐Shaped or T‐Shaped Grids

Minimum Length of the Grid in the Y Direction for L‐Shaped or T‐Shaped Grids

Number of Conductors in the X Direction Number of Conductors in the Y Direction Cost

Ground Grid Systems Report Manager

Click on the Report Manager Button on the Ground Grid Study Method Toolbar to open the Ground Grid Systems Report Manager dialog box. The Ground Grid Systems Report Manager consists of four pages and provides different formats for the Crystal Reports.



Plot Selection

Plots are used only with the FEM method, and are available for Absolute/Step/Touch Voltages. To select a plot, open up the Plot Selection dialog box by clicking on the Plot Selection button located on the Ground Grid Systems Toolbar.

Plot Selection

The following 3‐D Potential profiles are available for analysis of GGS study case results:

Absolute Voltage

Select to plot an Absolute Potential profile.

Touch Voltage

Select to plot a Touch Potential profile.

Step Voltage

Select to plot a Step Potential profile.

Plot Type

The following plot types are available for analysis of GGS study case results:



3‐D

Plot a 3‐D Potential profile for the Absolute/Touch/Step voltage.

Contour

Plot a Contour Potential profile for the Absolute/Touch/Step voltage.

Display over Limit Voltage

Show areas with potentials exceeding the tolerable limits for 3‐D Touch/Step Potential profiles. This function is disabled when the Contour plot type is selected. A set of sample plots is shown below.



COMMENTS:

Some of the main features of the Ground Grid Systems Analysis Study are summarized below:

Calculate the tolerable Step and Touch potentials Compare potentials against the actual, calculated Step and Touch potentials

Optimize number of conductors with fixed rods based on cost and safety Optimize number of conductors & rods based on cost and safety Calculate the maximum allowable current for specified conductors Compare allowable currents against fault currents Calculate Ground System Resistance Calculate Ground Potential Rise User‐expandable conductor library Allow a two‐layer soil configuration in addition to the surface material Ground grid configurations showing conductor & rod plots Display 3‐D/contour Touch Voltage plots Display 3‐D/contour Step Voltage plots Display 3‐D/contour Absolute Voltage plots Calculate Absolute, Step & Touch potentials at any point in the configuration

Conductor/Rod can be oriented in any possible 3‐Dimensional direction Handle irregular configurations of any shape



EXPERIMENT NO: 07 Ground Grid Modeling of a Given System using ETAP

GROUND GRID

An effective substation grounding system typically consists of driven ground rods, buried interconnecting grounding cables or grid, equipment ground mats, connecting cables from the buried grounding grid to metallic parts of structures and equipment, connections to grounded system neutrals, and the ground surface insulating covering material. Currents flowing into the grounding grid from lightning arrester operations, impulse or switching surge flashover of insulators, and line‐to‐ground fault currents from the bus or connected transmission lines all cause potential differences between grounded points in the substation and remote earth. Without a properly designed grounding system, large potential differences can exist between different points within the substation itself. Under normal circumstances, it is current flowing through the grounding grid from line‐to‐ground faults that constitutes the main threat to personnel.

GROUND GRID MODELING IN ETAP

The Ground Grid Systems program calculates the following:

The Maximum Allowable Current for specified conductors. Warnings are issued if the specified conductor is rated lower than the fault current level

The Step and Touch potentials for any rectangular/triangular/L‐shaped/T‐shaped configuration of a ground grid, with or without ground rods IEEE Std 80 and IEEE Std 665

The tolerable Step and Mesh potentials and compares them with actual, calculated Step and Mesh potentials IEEE Std 80 and IEEE Std 665

Graphic profiles for the absolute Step and Touch voltages, as well as the tables of the voltages at various locations Finite Element Method

The optimum number of parallel ground conductors and rods for a rectangular/triangular/L‐shaped/T‐shaped ground grid. The cost of



conductors/rods and the safety of personnel in the vicinity of the substation/generating station during a ground fault are both considered. Design optimizations are performed using a relative cost effectiveness method based on the IEEE Std 80 and IEEE Std 665


ONE LINE DIAGRAM

Create a New Ground Grid Presentation

To create a GGS presentation, a ground grid must first be added to the One‐Line Diagram. Click on the Ground Grid component located on the AC toolbar, and drop the GGS symbol anywhere on the One‐Line Diagram.



DIAGRAM WITH GROUND GRID

Right‐click on any location inside the ground grid box, and select Properties to bring up the Grid Editor. The Grid Editor Dialog box is used to specify grid information, grid styles, equipment information, and to view calculation results. Click on the Grid Presentation button to bring up a GGS presentation.



Double‐clicking on the ground grid box located on the One‐Line Diagram will bring up the Ground‐Grid Project Information dialog box, used to select an IEEE or FEM ‐ Finite Element Method Study Model.

After selecting the IEEE Study Model, the Ground Grid Systems graphical user‐interface window will be displayed as shown below. Select the T‐shape grid.



Right click on the T‐shape and adjust the dimensions and number of conductors in the following window:



After completing this process, we get the following shape of ground grid:

Ground Grid Study

The Ground Grid Study Method Toolbar appears when the GGS Study mode is selected.

Clicking on the Ground‐Grid Calculation tab and the following shown Alert View window is displayed.



Summary and Warning

Observations:

Calculated Volts Tolerable VoltsTouch 1260.6 427.1Step 2209.3 1216.4 GPR 5677.9 VoltsRg 2.83 Ohm



Summary and Warnings after Complete designing

Using FEM method

The FEM Editor Toolbar appears when the FEM Study Model is selected, and when in the Ground Grid Systems Edit mode. If we use this method, then we get following plots of touch potential and step potential as shown below:



COMMENTS:

Ground‐Grid Calculations are used to calculate:

Step and Touch mesh Potentials Ground Resistance Ground Potential Rise Tolerable Step and Touch Potential Limits Potential Profiles only for the FEM method

In this experiment:

We perform ground grid modeling with low number of rods We observe that the step voltage and the touch voltage are out of tolerable limits as shown in alert view

Then we perform the analysis after adding more number of rods Finally we achieve a position where we do not get any alert and the step voltage and the touch voltage are within tolerable limits

That means that we have modeled the Ground‐Grid according to our requirements



EXPERIMENT NO: 08 Modeling of Single‐Phase Instantaneous Over‐Current Relay using MATLAB

RELAY

A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism, but other operating principles are also used. Relays find applications where it is necessary to control a circuit by a low‐power signal, or where several circuits must be controlled by one signal. The first relays were used in long distance telegraph circuits, repeating the signal coming in from one circuit and re‐transmitting it to another.

TYPES OF RELAYS

Over current Relay

Distance Relay

Differential Relay

And many more…

Over‐Current Relay

The protection in which the relay picks up when the magnitude of current exceeds the pickup level is known as the over‐current protection. Over current includes short‐circuit protection; Short circuits can be Phase faults, Earth faults, Winding faults. Short‐circuit currents are generally several times 5 to 20 full load current. Hence fast fault clearance is always desirable on short circuits. Primary requirements of over‐current protection are: The protection should not operate for starting currents, permissible over current, current surges. To achieve this, the time delay is provided in case of inverse relays. The protection should be coordinated with neighboring over current protection. Over current relay is a basic element of over current protection.



In order for an over current protective device to operate properly, over‐current protective device ratings must be properly selected.

These ratings include voltage, ampere and interrupting rating.

Of the three of the ratings, perhaps the most important and most often overlooked is the interrupting rating.

If the interrupting rating is not properly selected, a serious hazard for equipment and personnel will exist.

Current limiting can be considered as another over current protective device rating, although not all over current protective devices are required to have this characteristic.

Types of Over‐Current Relay

Instantaneous Time over Current Relay:

It operates in a definite time when current exceeds its pick‐up value. It has operating time is constant. In it, there is no intentional time delay. It operates in 0.1s or less

Definite Time over Current Relay:

It operates after a predetermined time, as current exceeds its pick‐up value. Its operating time is constant. Its operation is independent of the magnitude of current above the pick‐up value. It has pick‐up and time dial settings, desired time delay can be set with the help of an intentional time delay mechanism.

Inverse Time over Current Relay:

Over current relay function monitors the general balanced overloading and has current/time settings. This is determined by the overall protective discrimination scheme. There advantage over definite time relays is that they



can have much shorter tripping times can be obtained without any risk to the protection selection process. These are classified in accordance with there characteristic curves, this indicates the speed of the operation. Based on this they are defined as being inverse, very inverse or extremely inverse. The typical settings for these relays are 0.7‐2In normal or rated generator current in 1‐10 second.

Inverse Definite Minimum Time over Current Relay:

It gives inverse time current characteristics at lower values of fault current and definite time characteristics at higher values. An inverse characteristic is obtained if the value of plug setting multiplier is below 10, for values between 10 and 20; characteristics tend towards definite time characteristics. It is widely used for the protection of distribution lines.

Very Inverse Time over Current Relay:

It gives more inverse characteristics than that of IDMT. It is used where there is a reduction in fault current, as the distance from source increases. It is particularly effective with ground faults because of their steep characteristics

Extremely Inverse Time over Current Relay:

It has more inverse characteristics than that of IDMT and very inverse over‐current relay. It is suitable for the protection of machines against overheating. It is for the protection of alternators, transformers, expensive cables, etc



Simulink Diagram in MATLAB for Single Phase Instantaneous Time Over‐Current Relay



Waveform Results in MATLAB for Single Phase Instantaneous Time Over‐Current Relay



COMMENTS:

In this experiment, we designed an instantaneous over‐current relay in MATLAB Simulink and then observed the behavior of this relay.

We observed that the normal current flowing through the system is 100 Amperes, but when the fault occurs in the system, the current flowing is increased from 100 Amperes.

We modeled the circuit such that the breaker must be open just after the current level is increased over 100 Amperes.

In this experiment, we take the results on scope and observed that when current exceeds over 100 Amperes, the breaker is opened instantaneously and our required results are verified.



Experiment#09 Modeling of a Three Phase Instantaneous Over‐Current Relay using MATLAB

Relay:


Type of Relays

Over current Relay

Distance Relay

Differential Relay

And many more…



Functions of Relays:

To detect the presence of fault

Identify the faulted components

Initiate appropriate circuit breaker

Remove the effective component from circuit

Over‐Current Relay The protection in which the relay picks up when the magnitude of current exceeds the pickup level is known as the over‐current protection. Over current includes short‐circuit protection; Short circuits can be Phase faults, Earth faults, Winding faults. Short‐circuit currents are generally several times 5 to 20 full load current. Hence fast fault clearance is always desirable on short circuits. Primary requirements of over‐current protection are: The protection should not operate for starting currents, permissible over current, current surges. To achieve this, the time delay is provided in case of inverse relays. The protection should be coordinated with neighboring over current protection. Over current relay is a basic element of over current protection. In order for an over current protective device to operate properly, over current protective device ratings must be properly selected. These ratings include voltage, ampere and interrupting rating. Of the three of the ratings, perhaps the most important and most often overlooked is the interrupting rating. If the interrupting rating is not properly, selected, a serious hazard for equipment and personnel will exist. Current limiting can be considered as another over current protective device rating, although not all over current protective devices are required to have this characteristic.



Types of Over Current Relay


It operates in a definite time when current exceeds its pick‐up value. It has operating time is constant. In it, there is no intentional time delay. It operates in 0.1s or less.






It gives more inverse characteristics than that of IDMT. It is used where there is a reduction in fault current, as the distance from source increases. It is particularly effective with ground faults because of their steep characteristics


It has more inverse characteristics than that of IDMT and very inverse over‐current relay. It is suitable for the protection of machines against overheating. It is for the protection of alternators, transformers, expensive cables, etc.



Simulink Diagram in MATLAB for Three‐Phase Instantaneous Time Over‐Current Relay

Subsystem:



Inst.Relay:

Waveform Results in MATLAB for Three Phase‐Instantaneous Time Over‐Current Relay



COMMENTS:

In this experiment, we implimented a three phase instantaneous over current relay in MATLAB Simulink.

In this experiment we have used terminators at the outputs that are not needed.

We have implimented a three phase fault at a specified time to ensure the breaker operation at 0.02 on time axis.

When a three phase fault occurs in the system, current exceeds from this value.

Breaker is operated instantaneously at the time when fault occurs and system is protected against the very high current.

This three phase relay can operate also for single phase or two phases fault.



Experiment#10 Modeling of a Differential Relay Using MATLAB

WHAT IS A RELAY?

A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism, but other operating principles are also used. Relays find applications where it is necessary to control a circuit by a low‐power signal, or where several circuits must be controlled by one signal. The first relays were used in long distance telegraph circuits, repeating the signal coming in from one circuit and re‐transmitting it to another. Relays found extensive use in telephone exchanges and early computers to perform logical operations.

A type of relay that can handle the high power required to directly drive an electric motor is called a contractor. Solid‐state relays control power circuits with no moving parts, instead using a semiconductor device to perform switching. Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults; in modern electric power systems these functions are performed by digital instruments still called protection relays. A protective relay is a



automatic sensing device which senses an abnormal condition and causes circuit breaker to isolate faulty element from system. Protective relaying is necessary with almost every electrical power system and no part of it is left unprotected choice of protection depends upon several aspects like

Type and rating of protected equipment and its importance

Location

Probable abnormal condition

Cost

Selectivity ,Sensitivity , Stability ,Reliability ,Fault clearance time

Functions of Relays





Purpose of Relay

Control

Protection

Regulation

Type of Relays

Over current Relay

Distance Relay

Differential Relay etc.

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The matching of CT ratios

Current imbalance produced by tap changing

Dealing with zero sequence currents

Phase shift through the transformer

Magnetizing inrush current

Each of these is considered further below:

The Matching of CT Ratios

The CTs used for the Protection Scheme will normally be selected from a range of current transformers with standard ratios such as 1600/1, 1000/5, 200/1 etc. This could mean that the currents fed into the relay from the two sides of the power transformer may not balance perfectly. Any imbalance must be compensated for and methods used include the application of biased relays and/or the use of the interposing CTs.

Current Imbalance Produced by Tap Changing

A transformer equipped with an on‐load tap changer OLTC will by definition experience a change in voltage ratio as it moves over its tapping range. This in turn changes the ratio of primary to secondary current and produces out‐of‐balance or spill current in the relay. As the transformer taps further from the balance position, so the magnitude of the spill current increases. To make the situation worse, as the load on the transformer increases the magnitude of the spill current increases yet again. And finally through faults could produce spill currents that exceed the setting of the relay. However, none of these conditions is 'in zone' and therefore the protection must remain stable i.e. it must not operate. Biased relays provide the solution.

Magnetizing Inrush Current

When a transformer is first energized, magnetizing inrush has the effect of producing a high magnitude current for a short period of time. This will be seen by the supply side CTs only and could be interpreted as an internal



fault. Precautions must therefore be taken to prevent a protection operation. Solutions include building a time delay feature into the relay and the use of harmonic restraint driven, typically, by the high level of second harmonic associated with inrush current.

Other Issues

Biased Relays

The use of a bias feature within a differential relay permits low settings and fast operating times even when a transformer is fitted with an on‐load tap‐changer. The effect of the bias is to progressively increase the amount of spill current required for operation as the magnitude of through current increases. Biased relays are given a specific characteristic by the manufacturer.

Interposing CTs

The main function of an interposing CT is to balance the currents supplied to the relay where there would otherwise be an imbalance due to the ratios of the main CTs. Interposing CTs are equipped with a wide range of taps that can be selected by the user to achieve the balance required.

As the name suggests, an interposing CT is installed between the secondary winding of the main CT and the relay. They can be used on the primary side or secondary side of the power transformer being protected, or both. Interposing CTs also provide a convenient method of establishing a delta connection for the elimination of zero sequence currents where this is necessary.

Modern Relays

It should be noted that some of the newer digital relays eliminate the need for interposing CTs by enabling essentials such as phase shift, CT ratios and zero sequence current elimination to be programmed directly into the relay.



Simulink Diagram in MATLAB for Differential Relay

SUBSYSTEM



SUBSYSTEM‐1

Waveform Results in MATLAB for Differential Relay



Comments:

It is important to note the direction of the currents as well as the magnitude as they are vectors. It requires a set of current transformers smaller transformers that transform currents down to a level which can be measured at each end of the power line or each side of the transformer.

In this experiment, we modeled a differential relay in MATLAB which provides the essential protection against transformer internal faults and it is useful in power transformers like 500,220 and 132KV.

However it can also be used for the protection of distribution transformer.

First of all we have applied a fault on the secondary side of transformer and ensure the operation of circuit breaker at the instant of fault that was set by us through the timer block.

Then we applied a fault on the primary side and again verify the tripping of circuit breaker.

It was observed that breaker takes a little more time when the fault is on the secondary side as compared to the fault occurrence on primary side of transformer due to larger distance.

It is verified that the differential relay modeled can detect three phase fault as well as fault on any single phase on each side of transformer.



Experiment#11 Comparison between the Step and Touch Potential of a T‐Model and Square Model of Ground Grids under Tolerable and Intolerable in ETAP

THEORY

GROUND GRID

An effective substation grounding system typically consists of driven ground

rods, buried interconnecting grounding cables or grid, equipment ground

mats, connecting cables from the buried grounding grid to metallic parts of

structures and equipment, connections to grounded system neutrals, and the

ground surface insulating covering material. Currents flowing into the

grounding grid from lightning arrester operations, impulse or switching surge

flashover of insulators, and line‐to‐ground fault currents from the bus or

connected transmission lines all cause potential differences between

grounded points in the substation and remote earth. Without a properly

designed grounding system, large potential differences can exist between

different points within the substation itself. Under normal circumstances, it is

current flowing through the grounding grid from line‐to‐ground faults that

constitutes the main threat to personnel. Currents flowing into the grounding

grid from lightning arrester operations, impulse or switching surge flashover

of insulators, and line‐to‐ground fault currents from the bus or connected

transmission lines all cause potential differences between grounded points in

the substation and remote earth.



GROUND GRID MODELING IN ETAP

The Ground Grid Systems program calculates the following:

The Maximum Allowable Current for specified conductors. Warnings

are issued if the specified conductor is rated lower than the fault current

level

The Step and Touch potentials for any rectangular/triangular/L‐

shaped/T‐shaped configuration of a ground grid, with or without

ground rods IEEE Std 80 and IEEE Std 665

The tolerable Step and Mesh potentials and compares them with actual,

calculated Step and Mesh potentials IEEE Std 80 and IEEE Std 665

Graphic profiles for the absolute Step and Touch voltages, as well as the

tables of the voltages at various locations Finite Element Method

The optimum number of parallel ground conductors and rods for a

rectangular/triangular/L‐shaped/T‐shaped ground grid. The cost of

conductors/rods and the safety of personnel in the vicinity of the

substation/generating station during a ground fault are both

considered. Design optimizations are performed using a relative cost

effectiveness method based on the IEEE Std 80 and IEEE Std 665


OBJECTIVES OF GROUNDING

An effective grounding system has the following objectives:



Ensure such a degree of human safety that a person working or walking

in the vicinity of grounded facilities is not exposed to the danger of a

critical electric shock. The touch and step voltages produced in a fault

condition have to be at safe values. A safe value is one that will not

produce enough current within a body to cause ventricular fibrillation.

Provide means to carry and dissipate electric currents into earth under

normal and fault conditions without exceeding any operating and

equipment limits or adversely affecting continuity of service.

Provide grounding for lightning impulses and the surges occurring from

the switching of substation equipment, which reduces damage to

equipment and cable.

Provide a low resistance for the protective relays to see and clear

ground faults, which improves protective equipment performance,

particularly at minimum fault.

Step Voltage

The difference in surface potential experienced by a person bridging a

distance of 1 meter with his feet without contacting any other grounded

object.

Touch Voltage

It is the potential difference between the ground potential rise and the surface

potential at the point where a person is standing while at the same time

having his hands in contact with a grounded structure.



SINGLE LINE DIAGRAM



INTOLERABLE RECTANGULAR SHAPE GROUND GRID

ALERTS



TOLERABLE RECTANGULAR SHAPE GROUND GRID

ALERTS



INTOLERABLE T‐SHAPE GROUND GRID

ALERTS



TOLERABLE T‐SHAPE GROUND GRID

ALERTS



Comments

There are following major types of ground grids according to their shape:

Rectangular Triangular L‐shaped T‐shaped

In this experiment, we have used two types of ground grid for comparison that are Rectangular‐shaped and T‐shaped.

First we perform the analysis for intolerable limits for both types of ground grids. Then perform the analysis for tolerable limits.

We observed that the number of rods used in case of T‐shaped ground grid are required in greater quantity for tolerable limits as compared to Rectangular‐shaped ground grid.

Due to greater number of rods requirement, T‐shaped ground grid is much expensive than the Rectangular shaped ground grid. That is why we can say that the Rectangular shaped ground grid is better than T‐shaped.



Experiment#12 Modeling of an Over‐Current Relay using ETAP

WHAT IS A RELAY?


Type of Relays

Over current Relay

Distance Relay

Differential Relay

And many more…








Over‐Current Relay The protection in which the relay picks up when the magnitude of current exceeds the pickup level is known as the over‐current protection. Over current includes short‐circuit protection; Short circuits can be Phase faults, Earth faults, Winding faults. Short‐circuit currents are generally several times 5 to 20 full load current. Hence fast fault clearance is always desirable on short circuits. Primary requirements of over‐current protection are: The protection should not operate for starting currents, permissible over current, current surges. To achieve this, the time delay is provided in case of inverse relays. The protection should be coordinated with neighboring over current protection. Over current relay is a basic element of over current protection. In order for an over current protective device to operate properly, over current protective device ratings must be properly selected. These ratings include voltage, ampere and interrupting rating. Of the three of the ratings, perhaps the most important and most often overlooked is the interrupting rating. If the interrupting rating is not properly; Selected, a serious hazard for equipment and personnel will exist. Current limiting can be considered as another over current protective device rating, although not all over current protective devices are required to have this characteristic.











It gives more inverse characteristics than that of IDMT. It is used where there is a reduction in fault current, as the distance from source increases. It is particularly effective with ground faults because of their steep characteristics.



CURRENT TRANSFORMER CT

In electrical engineering, a current transformer CT is used for measurement of electric currents. Current transformers, together with voltage transformers VT potential transformers PT , are known as instrument transformers. When current in a circuit is too high to directly apply to measuring instruments, a current transformer produces a reduced current accurately proportional to the current in



the circuit, which can be conveniently connected to measuring and recording instruments. A current transformer also isolates the measuring instruments from what may be very high voltage in the monitored circuit. Current transformers are commonly used in metering and protective relays in the electrical power industry.

Accuracy of CT

The accuracy of a CT is directly related to a number of factors including:

Burden

Burden class/saturation class

Rating factor

Load

External electromagnetic fields

Temperature and

Physical configuration.

The selected tap, for multi‐ratio CT's

CIRCUIT BREAKER

A circuit breaker is an automatically‐operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset either manually or automatically to resume normal operation. Circuit breakers are made in varying sizes, from small devices that



protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city.

SINGLE LINE DIAGRAM



SINGLE LINE DIAGRAM WITH FAULT‐1

ALERTS DIAGRAM




ALERTS DIAGRAM



COMMENTS:

An "overcurrent relay" is a type of protective relay which operates when the load current exceeds a preset value. The ANSI device number is 50 for an instantaneous over current IOC , 51 for a time over current TOC . In a typical application the overcurrent relay is connected to a current transformer and calibrated to operate at or above a specific current level. When the relay operates, one or more contacts will operate and energize to trip open a circuit breaker.

In this experiment we have used one current transformer that is connected to the over current relay.

When a fault occur in the system, the amount of current flowing through that section increases and current transformer provides the relay a sense of fault by changing its current.

After sensing the fault, the relay operates the circuit breaker and isoltes the faulty system from the normal system.

We have verified that the relay is operating for both faults added in the system by selecting two different faulty points.



Experiment#13 Modeling of a Differential Relay Using ETAP

WHAT IS A RELAY?

A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism, but other operating principles are also used. Relays find applications where it is necessary to control a circuit by a low‐power signal, or where several circuits must be controlled by one signal. The first relays were used in long distance telegraph circuits, repeating the signal coming in from one circuit and re‐transmitting it to another. Relays found extensive use in telephone exchanges and early computers to perform logical operations.

A type of relay that can handle the high power required to directly drive an electric motor is called a contractor. Solid‐state relays control power circuits with no moving parts, instead using a semiconductor device to perform switching. Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults; in modern electric power systems these functions are performed by digital instruments still called protection relays. A protective relay is a



automatic sensing device which senses an abnormal condition and causes circuit breaker to isolate faulty element from system. Protective relaying is necessary with almost every electrical power system and no part of it is left unprotected choice of protection depends upon several aspects like

Type and rating of protected equipment and its importance

Location

Probable abnormal condition

Cost

Selectivity ,Sensitivity , Stability ,Reliability ,Fault clearance time

Functions of Relays





Purpose of Relay

Control

Protection

Regulation

Type of Relays

Over current Relay

Distance Relay

Differential Relay etc.

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The matching of CT ratios

Current imbalance produced by tap changing

Dealing with zero sequence currents

Phase shift through the transformer

Magnetizing inrush current

Each of these is considered further below:

The Matching of CT Ratios

The CTs used for the Protection Scheme will normally be selected from a range of current transformers with standard ratios such as 1600/1, 1000/5, 200/1 etc. This could mean that the currents fed into the relay from the two sides of the power transformer may not balance perfectly. Any imbalance must be compensated for and methods used include the application of biased relays and/or the use of the interposing CTs.

Current Imbalance Produced by Tap Changing

A transformer equipped with an on‐load tap changer OLTC will by definition experience a change in voltage ratio as it moves over its tapping range. This in turn changes the ratio of primary to secondary current and produces out‐of‐balance or spill current in the relay. As the transformer taps further from the balance position, so the magnitude of the spill current increases. To make the situation worse, as the load on the transformer increases the magnitude of the spill current increases yet again. And finally through faults could produce spill currents that exceed the setting of the relay. However, none of these conditions is 'in zone' and therefore the protection must remain stable i.e. it must not operate. Biased relays provide the solution.

Magnetizing Inrush Current

When a transformer is first energized, magnetizing inrush has the effect of producing a high magnitude current for a short period of time.



This will be seen by the supply side CTs only and could be interpreted as an internal fault. Precautions must therefore be taken to prevent a protection operation. Solutions include building a time delay feature into the relay and the use of harmonic restraint driven, typically, by the high level of second harmonic associated with inrush current.

Other Issues

Biased Relays

The use of a bias feature within a differential relay permits low settings and fast operating times even when a transformer is fitted with an on‐load tap‐changer. The effect of the bias is to progressively increase the amount of spill current required for operation as the magnitude of through current increases. Biased relays are given a specific characteristic by the manufacturer.

Interposing CTs

The main function of an interposing CT is to balance the currents supplied to the relay where there would otherwise be an imbalance due to the ratios of the main CTs. Interposing CTs are equipped with a wide range of taps that can be selected by the user to achieve the balance required.

As the name suggests, an interposing CT is installed between the secondary winding of the main CT and the relay. They can be used on the primary side or secondary side of the power transformer being protected, or both. Interposing CTs also provide a convenient method of establishing a delta connection for the elimination of zero sequence currents where this is necessary.

Modern Relays

It should be noted that some of the newer digital relays eliminate the need for interposing CTs by enabling essentials such as phase shift, CT ratios and zero sequence current elimination to be programmed directly into the relay.



CURRENT TRANSFORMER CT

In electrical engineering, a current transformer CT is used for measurement of electric currents. Current transformers, together with voltage transformers VT potential transformers PT , are known as instrument transformers. When current in a circuit is too high to directly apply to measuring instruments, a current transformer produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments. A current transformer also isolates the measuring instruments from what may be very high voltage in the monitored circuit. Current transformers are commonly used in metering and protective relays in the electrical power industry.

Accuracy of CT

The accuracy of a CT is directly related to a number of factors including:

Burden

Burden class/saturation class

Rating factor

Load

External electromagnetic fields

Temperature and

The selected tap, for multi‐ratio CT's

CIRCUIT BREAKER

A circuit breaker is an automatically‐operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow. Unlike a fuse, which operates once



and then has to be replaced, a circuit breaker can be reset either manually or automatically to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city.

SINGLE LINE DIAGRAM




ALERTS DIAGRAM




ALERTS DIAGRAM



Comments:

It is important to note the direction of the currents as well as the magnitude as they are vectors. It requires a set of current transformers smaller transformers that transform currents down to a level which can be measured at each end of the power line or each side of the transformer.

In this experiment, we modeled a differential relay in ETAP which provides the essential protection against transformer internal faults and it is useful in power transformers like 500,220 and 132KV.

However it can also be used for the protection of distribution transformer.

Here we have used two CT’s, one on primary side of transformer and the other on secondary side. These CT’s are directly connected to the differential relay that is sensing the difference between the secondary side currents of both CT’s.

First of all we have applied a fault on the secondary side of transformer and ensure the operation of circuit breaker at the instant of fault through the signal provided by the relay.

Then we applied a fault on the primary side and again verify the tripping of circuit breaker through the relay signal.

It is verified that the differential relay modeled can detect three phase fault as well as fault on any single phase on each side of transformer.

Moreover the differential relay can only sense the faults that are present in the internal zone of both CT’s.



Experiment#14 Modeling of a Definite Time Over‐Current Relay using MATLAB

WHAT IS A RELAY?


Type of Relays

Over current Relay

Distance Relay

Differential Relay

And many more…








Over‐Current Relay The protection in which the relay picks up when the magnitude of current exceeds the pickup level is known as the over‐current protection. Over current includes short‐circuit protection; Short circuits can be Phase faults, Earth faults, Winding faults. Short‐circuit currents are generally several times 5 to 20 full load current. Hence fast fault clearance is always desirable on short circuits. Primary requirements of over‐current protection are: The protection should not operate for starting currents, permissible over current, current surges. To achieve this, the time delay is provided in case of inverse relays. The protection should be coordinated with neighboring over current protection. Over current relay is a basic element of over current protection. In order for an over current protective device to operate properly, over current protective device ratings must be properly selected. These ratings include voltage, ampere and interrupting rating. Of the three of the ratings, perhaps the most important and most often overlooked is the interrupting rating. If the interrupting rating is not properly; Selected, a serious hazard for equipment and personnel will exist. Current limiting can be considered as another over current protective device rating, although not all over current protective devices are required to have this characteristic.











It gives more inverse characteristics than that of IDMT. It is used where there is a reduction in fault current, as the distance from source increases. It is particularly effective with ground faults because of their steep characteristics.





Simulink Diagram in MATLAB for Definite Time Over‐Current Relay



Waveform Results in MATLAB for Definite Time Over‐Current Relay



COMMENTS:

As clear from the name, the definite time over‐current relay operates after a predetermined time, as current exceeds its pick‐up value. Its operating time is constant. Its operation is independent of the magnitude of current above the pick‐up value. It has pick‐up and time dial settings, desired time delay can be set with the help of an intentional time delay mechanism.

The relay modeled in this experiment has a constant time delay of 1 second.

When any fault occurs in the power system, the relay senses the occurrence of fault and check it upto the time delay provided in the setting. If the fault is removed in between that time, then relay will not operate the circuit breaker.

Relay will operate the circuit breaker if fault occurrence time is greater than the time delay given in the setting.

For example in this experiment, there is a fault in the system from 0 to 0.5 second but this fault time is smaller than the time delay 1 second that is why the relay does not operate during this fault. After that another fault occur from 1 to 2.1 seconds, now the fault time 1.1 second is greater than the delay time 1 second . It is observed that the relay is operated during this fault time which verifies the definite time relay operation.

Documents

59926214 Power System Protection Lab Manual