Unit Protection

UNIT PROTECTION

Boiler Protections• Loss of FSSS DC for > 2 Sec• Both ID Fan Trip• Both FD fan Trip• Both Air Heater Trip• Loss of all fuel• Flame Failure• Drum Level High / Low• Furnace vaccum High / Low• Boiler manual Trip• ALL BFP’s Trip• Turbine to Boiler Trip

Boiler Trip

Boiler Lockout

Turbine Protections

• Main Steam Temperature Low• Axial Shift High• Lub Oil Pressure Low• ESV/IV Closed• Turbine Manual Trip• HPH’s Level High High• Loss of 6.6KV Voltage

Turbine Lockout

Integrated Protections

Boiler Lockout Master Trip Relay

Generator Lockout

Turbine Lock out

86Y86T

86GT 86B

86G

FCB Contact 2/37 2/32

POWER SYSTEM PROTECTION KEY ASPECTS

• Reliability• Security• Sensitivity• Selectivity• Zone over lapping• co-ordination• Primary & Back up Relays• Speed

KEY ASPECTS• Reliability

• Security/Stability

• Sensitivity

• Protection zones

• Coordination

• Primary Relays

• Back up Relays

• Probability that the system will function correctly when required to act(for a fault in it’s zone)

• Refrain from unwanted operation in the absence of fault or fault out side it’s zone

• Ability of the system to detect the threshold value of an abnormal condition to initiate protective action.

• Regions of primary sensitivity• Determination of graded settings

to achieve selectivity

• Relays with in a particular zone that should operate for prescribed abnormalities with in that zone

• Relays outside a given primary protection zone, independently of the primary Relays.

Generator Protection Requirement

Generator faults are considered to be serious since they may cause severe and costly damage to insulation, windings, and the core may also produce severe mechanical torsional shock to shafts and couplings.

Fault current may continue to flow for many seconds even after the generator is tripped, because of trapped flux within the machine, thereby increasing the amount of fault damage.

As a consequence, for faults in or near the generator that produce high magnitudes of short-circuit currents, some form of high-speed protection is normally used to trip and shut down the machine as quickly as possible in order to minimize damage.

TRIP LOGIC OF GENERATOR PROTECTION

• TWO INDEPENDENT CHANNELS WITH INDEPENDENT CT/VT INPUTS/DC SUPPLY/TRIP RELAY

CLASS A TRIPS• ALL ELECTRICAL TRIP• TRIP TURBINE , FIELD, GENERATOR,GT,UT

CLASS-B TRIP• MECHANICAL TRIPS• AVOID OVER SPEEDING OF TURBINE DUE TO STEAM ENTRAPPED IN TURBINE.

TURBINE TRIP SIGNAL IS GIVEN FIRST AND THE ACTIVE POWER, SENSED BY THE LOW FORWARD RELAY (32G) GIVES THE TRIP SIGNAL TO THE UNIT BREAKER & FIELD BREAKER AFTER A TIME DELAY.

• IN GCB SCHEME, ONLY GCB AND FIELD IS TRIPPED,KEEPING UAT CHARGED THROUGH GT

• IN NON GCB SCHEME, HV CB,FIELD,UT LCV CB ARE TRIPPED.

• Class C• Trips only HV CB

Gen Stator Thermal Protection Field Thermal Protection Gen stator fault Protection Gen rotor field Protection Gen abnormal operating conditions System backup Protection Power transformer Protection

Generator Protection

Generator Protection

Stator Thermal protection

Thermal protection for the generator stator core and windings Generator overload

Winding Temperature Over current

Failure of cooling systems RTDs Thermocouple Flow and pressure sensor

Localized hot spots caused by core lamination insulation failures or by localized or rapidly developing winding failures Generator Core monitor

Generator Protection

Turbine-generator short time thermal capability for balanced three-phase loading

Generator Protection

Generator Field Thermal protection

Thermal Protection Direct rotor Body temperature measurement not possible Core Monitor may detect overheating

Protection for field over excitation IDMT/ Definite Time Excitation limiters

Generator Protection

Generator field short time thermal capability

Generator Grounding Practices

It is common practice to ground all types of generators through some form of external impedance

limit the mechanical stresses and fault damage in the machine, to limit transient voltages during faults, and to provide a means for detecting ground faults within the

machine.

Typical Grounding practicesUngroundedSolid GroundingHigh-impedance groundingLow-resistance groundingReactance groundingGrounding-transformer grounding

Generator Grounding Practices

High Impedance Grounding– High resistance grounding

• The high-resistance grounding method utilizes a resistor connected across the secondary of the distribution transformer to limit the maximum ground fault current.

• For a single-phase-to-ground fault at the machine terminals, the primary fault current will be limited to a value in the range of about 3 A to 25 A.

– Ground fault neutralizer grounding The ground fault neutralizer grounding method utilizes a secondary tunable

reactor to limit the maximum ground fault current. Low –resistance grounding

In this method, a resistor is connected directly between the generator neutral and ground.

For a single-phase-to-ground fault at its terminals the primary fault current will be limited to a value in the range of about 200 A up to 150% of rated full-load current.

Resistor cost and size usually preclude the use of resistors.

Current based system– For generators with split neutrals with all six terminals brought out on

neutral side.– Delayed low-set o/c relay which senses the current in the connection

between the neutrals of the stator windings

• Voltage based system – Relay compares the neutral NGT sec voltage and Genertaor terminal

open delta voltage. – Balance during external E/F or normal condition– During inter turn fault open delta voltage will be developed and NGT

sec voltage will be zero, resulting in a differential voltage which makes the relay operate.

Typical setting

Definite time type relays: minimum setting with 1 sec delay.

INTERTURN PROTECTION

Inter turn protection

Split Phase Protection

Voltga

Voltage Based

NEGATIVE SEQUENCE PROTECTION

Negative sequence protection• NEGATIVE SEQUENCE PROTECTION FOR GENERATOR PROTECTS THE GENERATOR

FROM EXCESSIVE HEATING IN THE ROTOR RESULTING FROM UNBALANCED STATOR CURRENTS

• CAUSED DUE TO– ONE POLE OPEN IN LINE– ONE POLE OPEN OF A CIRCUIT BREAKER– CLOSE IN UNCLEARED UNBALANCED FAULTS

• THE NEGETIVE SEQUENCE PROTECTION RELAYS SHALL BE SET TO THE NPS CAPABILITY OF THE MACHINE WHICH IS

• K = I22X T

• TYP FOR 500 MW PERMISSIVE NEG SEQ CURRENT = 5 – 8 % OF STATOR CURRENT

PERMISSIVE I22X T = 5 – 10

SETTINGS ADOPTED FOR NTPCI2 = = 7.5 % I2

2XT = 8.3

NEGATIVE SEQUENCE PROTECTION

TYPICAL NPS CAPABILITY

Negative sequence protection

Loss of field protection

Loss of field protection

• ACTS AS AN INDUCTION GENERATOR• INDUCED EDDY CURRENTS IN THE FIELD WINDING, ROTOR BODY,

WEDGES AND RETAINING RINGS • MW FLOW IN TO THE SYSTEM/ MVAR FLOWS IN TO THE MACHINE.• THE APPARENT IMP TRAVELS TO THE FORTH QUADRANT OF X-Y

PLANE

• METHOD OF DETECTION:

MINIMUM IMPEDANCE WITH U/V

SOME RELAYS ARE SET IN THE ADMITTANCE PLANE MATCHING WITH THE CAPABILITY CURVE OF THE MACHINE

TRIP CHARACTERISTICS OF LOSS OF FIELD PROTECTION

REVERSE /LOW FORWARD POWER INTERLOCK

LOW FORWARD AND REVERSE POWER INTER LOCK

• To allow entrapped steam in the turbine to be utilized to avoid damage of the turbine blade.

• To protect the machine from motoring action• Trip under class B after a short time delay in

case the turbine is already tripped ( typ set at 2 sec)

• Trip under class A, after a long time delay if turbine is not tripped (typically set at 10 sec)

• Power setting typ 0.5 % of rated power

Rev power/ LFPR INTERLOCK

• STATOR EARTH FAULT PROTECTION

• E/F CURRENT IS LIMITTED TO 10A• THIS MINIMIZES THE DAMAGE• FIRST FAULT LESS CRITICAL• NEEDS CLEARANCE AS

IT MAY DEVELOP INTO A PH TO PH FAULT SECOND FAULT WILL RESULT IN VERY HIGH

CURRENT

• TWO TYPES:• 100 % E/F• 95 % E/F

EFFECT OF STATOR E/F

Results In Voltage Shift Of Gen Neutral W.R.T Ground.

Detected By Voltage Relay Connected Across Grounding Resistor Or From The Generator Terminal Through Open Delta VT

Protect Approx 95% Of Stator Wdg

Typical Setting:– For Definite Time Delay Type: 5%of 110 V Ie, 5.5 V

At 1 Sec

95 % Stator Earth Fault

April 9, 2023 32PMI Revision 00

Three ways of providing E/F protection:• Voltage relay connected across grounding

resistor• A current relay connected to CT provided in

grounding transformer• Voltage relay connected to open delta in

generator VT

EARTH FAULT PROTECTION

April 9, 2023 33PMI Revision 00

100 % Stator E/F Protection

• Third Harmonic Principle• Relay responds to the reduction of the 3rd Harmonic

Component • For a Stator Phase-to-ground fault at or near the

Generator Neutral, there will be an increase in third Harmonic Voltage at The Generator Terminals, which Will Cause Relay Operation.

100% SEF based on third harmonics measurements

Disadvantages

Due to design variations, certain generating units may not produce sufficient third harmonic voltages.

This method does not protect the machine during stand still conditions.

100% stator earth fault protection (Low freq. injection principle)

20 Hz

RE

max.200 V

I

20 Hz

RE

max.200 V

I

Detects the ground faults by injecting a low frequency signal (say 20 hz) at the neutral earthing transformer and monitor the earth current in the winding.

COMPARISION BETWEEN E/F PROTECTIONS

ROTOR EARTH FAULT PROTECTION

• FIRST ROTOR E/F DOES NOT CAUSE IMMEDIATE DAMAGE• SECOND E/F RESULTS IN A WDG SC OF ROTOR • CAUSE MAGNETIC UNBALANCE/MECH FORCES /DAMAGE• METHODS OF DETECTION

– POTENTIOMETER METHOD• A CENTRE TAPED RESISTOR IS CONNECTED ACROSS THE MAIN

FIELD WINDING • THE CENTRE TAP IS CONNECTED TO EARTH THROUGH A VOLTAGE

RELAY• AN EARTH FAULT ON THE FIELD WINDING WILL PRODUCE

VOLTAGE IN THE RELAY, MAXIMUM VOLTAGE OCCURRING FOR END FAULTS

• A BLIND SPOT EXISTS AT THE TAPPING POINT, TO AVOID THIS , THE TAPPING POINT IS VARIED WITH A PUSH BUTTON OR SWITCH , AND IS TESTED PERIODICALLY TO DETECT BLIND ZONE

• SETTING IS 5% OF FIELD VOLTAGE

LOW FREQUENCY INJECTION METHODMODERN ROTOR EARTH FAULT PROT ECTION RELAY OPERATES ON THE PRINCIPLE OF LOW FREQUENCY INJECTION INTO THE FIELD WINDING VIA CAPACITORS.

CORRESPONDING CURRENT OR RESISTANCE DURING E/F IS SENSED

TYP SETTING (500 MW)

ALARM 40 K OHM TIME = 10 SEC

TRIP 5 K OHM TIME = 1 SEC

ACTUAL VALUES OF SETTING SHALL BE DECIDED AT SITE DURING COMMISSIONING TO ACCERTAIN THE HEALTHY VALUE OF THE PARTICULAR M/C.

ROTOR E/F PROT (contd)

Under and Over Voltage Protection:27/59

l Backup to generator’s AVRl V<: Protects against stalling

of auxiliary cooling fans or pumps at low voltage

l V>: Protects insulation from damage for sustained overvoltages

l V<1 & V>1: IDMT or DT operation

l V<2 & V>2: Definite time delayed

Time

VoltageV>1

Idmt curve (V>1)

t = k / (M-1)

M = Multiple of settingk = Time multiplier

MiCOM-P340-40

V/Hz Overfluxing Protection (24)

V f K

Primary function to detect overfluxing during machine run-up

Alarm : Definite time characteristic to initiate corrective action

Trip : IDMT or DT characteristic to clear overfluxing condition

Settings

Pick-up 1.5 to 3.0 i.e. 110V x 1.05 = 2.3150Hz

DT setting range 0 to 100 secondsMiCOM-P340-41

Typical Over Fluxing Withstand Capability

O/V PROTECTION

. TYP SETTINGS OF A 3 STAGE O/V RELAY IS AS FOLLOWS ALARM 110 % 2 SEC TRIP 120 % 1 SEC

140 % INSTANTANEOUS

U/F O/F PROTECTION

TYPICAL SETTING: U/F O/FALARM - 47.8HZ 1 SEC 51 Hz 1 SECTRIP - 47.4 HZ 2 SEC 51.5Hz 2.5 SECSETTING NEED TO BE CO-ORDINATED WITH THE RESPECTIVE GRID

AGENCY AND THE ISLANDING SCHEME SETTINGS AND THE M/C CAPABILITY.

NTPC Vindhyachal

Local Breaker Back up Protection (LBBGT-50Z):

All the protections of the Generator transformer and unit auxiliary transformers finally operate the Generator master trip relay. This master trip relay issues tripping command to Generator bay breaker. In the event, the Generator breaker does not open within preset time say 200 ms, the LBB scheme is energized.

For uncleared system fault

The backup protection is time delayed to coordinate with the zone 3 setting of lines

Detected by– over current – impedance– Impedance type preferred as the line is provided with

distance relays Setting should be made to cover the GT imp and the longest

line impedance. Setting should take care of the infeed from other generators

connected to the same bus also. Time setting 1.5 –2 sec

Backup impedance protection

Stator fault Protection

High Speed Differential protection– Will detect Phase to Phase Faults, Double phase faults involving

earth– Single phase to Earth will not be detected due to limited earth

fault current available.

Two types of high-speed differential relays are commonly used for stator phase fault detection:– High-impedance differential– Biased differential

High Impedance Differential Relay

Use two sets of identical dedicated CTs. PS class CT with stringent parameters to be used This scheme has higher sensitivity than the percentage

differential relay. Through fault stability achieved by using stabilising resistors

in the relay circuit.

High Impedance Differential

I1 I1

i1 i1’

i1 – i1’

Equipment to be protectedI2

i2i1 + i2Stabilising

Resistance

Operating relay

Biased Type Diff Relay

Less stringent CT parameters. CTs can be shared with other protections. Through fault stability achieved through biasing. CT mismatch (typ of the order of 1:5 ) can be accommodated. More suitable for numerical integrated protection systems as the CTs can be shared

for many functions. Modern numerical relays have flexible settings for

Low Impedance Differential

I1 I1

i1 i1’

i1 – i1’

Equipment to be protectedI2

i2

i1 + i2

Biasing/Restraining element

Differentialelement

Overall Differential Protection:

.This protection is provided to detect the faults of Generator, Generator transformer, unit auxiliary transformer.

• This is three winding type biased percentage differentialrelay.

• In case of any fault within the Generator neutral toswitchyard the unbalance will be created in the relay and relay

will operate.•This relay is fast operating (20ms) and is considered as primary

protection of Generator unit protection system. •Operation of this relay isolates the set from the system.

•The relay should have immunity to 2nd and 5th harmonic to avoid operation due to magnetizing in rush current and over

excitation of transformer.

April 9, 2023 52PMI Revision 00

87 GT

50Z

GT

GENERATOR

GT OVERALL DIFFERENTIAL PROTECTIION

Igt

Iuat1

Ig

Iuat2

NUMERICAL RELAYS

• These relays combine several type of Protections into one relay.

• These are micro processor based and can perform various type of calculation by programming.

S.K. Singhal 53

Numerical Relays

• They also display values of Current, Voltage, Power factor at the time of fault as well as during normal running.

• They can record and store number of faults

and events.

S.K. Singhal 54

DISADVANTAGE OF NUMERICAL RELAYS

• Failure of one relay can make a number of protections inoperative.

• Except changing of cards, no maintenance is possible

• Become obsolete very fast and manufacturers are not able to provide services for a long time.

• Requires cleaner and humid free / dry environment as compared to conventional relay.

S.K. Singhal 55

59

78

81

64G

Core-1

Core-3Core-2

Core-4

P1

P2

P1

P2

Resi

stor

Gen. Diff. Prot. (87G)AVR (Auto)

Gen.

LK

To Excit. Trf.To PT Ckt.

MeteringField Failure Prot. (40)

Neg. Phase Seq. (46)

Back Up Imp. Prot. (21G)

Reverse Power Prot.(37G)

Gen. Diff. Protection (87G)

Gen. O/A Diff. Protection (87G)

Over Voltage Relay

Out OF Step Relay

Under/ Over Freq. Relay

100% Stator E/F Protection

Gen. Trf. O/A (87GT) Diff. Protection

P2P1P1P2

Gen. Trf. O/A (87GT) Diff. Protection

UAT O/C (50/51) Protection

UAT Ref. Protection (64 Ref.)

UAT Ref. Protection (64 Ref.)

UAT

52U

GENERATOR PROTECTION SYSTEM

S.K. Singhal 56

• Differential ProtectionCurrent differential relaying can be used to protect network transformers. The relays are connected to current transformers on the high side and low side of the network transformer. The net operating current to the relays is the difference between input and output currents to the network transformer zone of protection. Differential relaying provides a clearly defined zone of protection.

• Biased type with adjustable bias setting of 10-50%• Triple pole• Harmonic restraint feature• No of bias wdgs as applicable• Operating current setting of 15% or less

• Over current protection– Given on LV side as primary protection– On HV side as back up protection– IDMT or def time as applicable– Need to be co-coordinated ( for time and current)with down stream

protection Directional if the power flow is bi-directional

• Back up earth fault protection– Single pole– Def time /IDMT As applicable– Time delay 0.3 – 3 sec for def time relay– Standard curves for IDMT– Setting ranges to suit the application

• Over fluxing– Operate on V/F principle– Inverse characteristics to suit the transformer o/f withstand capability– Shall have alarm and trip stages

Restricted earth fault Protection Single pole Generally high impedance type preferred Setting rage of 5-20% 0r 10 – 40 % Stable for through faults Include stabilizing resistor for through fault stability

NTPC Vindhyachal

Generator Out of Step (Pole slip) Protection:

This relay is connected between Generator neutral C.Ts. and phase side P.T.

When there is sudden increase or decrease of load in power system may be due to fault, switching ON/OFF of large loaded lines which disturbs the balance of power of the system

causing oscillations is called power swing. If the system recovers immediately, stable operation is resumed and generating set can sustain such small swings. However, if the

system is not recovered within say few seconds power swing become so large that synchronism between gen. and system is lost (which is called pole slip).

The pole slip is detected by the impedance relay. It detects the locus of the machine and system impedance and its residence time in the reactance diagram. Generally if the

impedance remains more than 25 ms in the impedance diagram it is considered as pole slip. The relay should not operate on normal recoverable power swings. This protection is very important for the mechanical protection of the machine as the power swings causes

severe pulsating stress on the T.G. set shaft. It is necessary to isolate the machine on unrecoverable power swings.

LA

Core-3

Core-2

Core-1

G.T. O/A Diff. Protection (87 GT)

G.T. Ref. Protection (64 R)

Link Line Diff. Protection (87 LL)

G.T. Ref. Protection (64 Ref.)

P2 P1

GENERATOR TRANSFORMER PROTECTION SYSTEM

S.K. Singhal 62

STANDARD PROTECTIONS USED FOR TRANSFORMERS

• Differential• Over current• Earth fault• Restricted earth fault• Over fluxing• Mechanical protection

buchholtzpressure relief device

• Temperature protectionWTI/OTI

GENERATOR Xer PROTECTION

• REF Protection• Back-up Earth Fault Protection• Overfluxing Protection• Differential Protection• Oil & Wdg Temperature Protection• Buckholtz Protection• PRV Protection