ETAP - Harmonics.pdf · Slide 12. Phase Angle Relationship • Fundamental Frequency. ETAP.IT. ETAP µ ¯íã 6³ Ó Ê ã ... ETAP.IT. ETAP µ ¯íã 6³ Ó Ê ã

© 1996-2009 Operation Technology, Inc. – Workshop Notes: Harmonics

Harmonic Analysis

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ETAP مرجع فارسی آموزشETAP.IR

Slide 2© 1996-2009 Operation Technology, Inc. - Workshop Notes: Harmonics

Types of Power Quality

Problems

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Waveform Distortion

• Primary Types of Waveform Distortion

– DC Offset

– Harmonics

– Interharmonics

– Notching

– Noise ETAP.IT



Harmonics

• One special category of power quality

problems

• “Harmonics are voltages and/or currents

present in an electrical system at some

multiple of the fundamental frequency.”

(IEEE Std 399, Brown Book)ETAP.IT



Nonlinear Loads

• Sinusoidal voltage

applied to a simple

nonlinear resistor

• Increasing the

voltage by a few

percent may cause

current to doubleETAP.IT



Fourier Representation

• Any periodic waveform can be expressed as a sum of sinusoids

• The sum of the sinusoids is referred to as Fourier Series (6-pulse)

)cos(

13cos13

111cos

11

17cos

7

13cos

5

1(cos

32

1h

hh

dac

thI

tttttII

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Harmonic Sources

• Utilities (Power Grid)

– Known as “Background Harmonic”

– Pollution from other irresponsible customers

– SVC, HVDC, FACTS, …

– Usually a voltage source

• Synchronous Generators

– Due to Pitch (can be eliminated by fractional-pitch winding) and Saturation

– Usually a voltage source

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Harmonic Sources (cont’d)

• Transformers

– Due to magnetizing branch saturation

– Only at lightly loaded condition

– Usually a current source

• Power Electronic Devices

– Charger, Converter, Inverter, UPS, VFD, SVC, HVDC,

FACTS, …

– Due to switching actions

– Either a voltage source or a current source

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Harmonic Sources (cont’d)

• Other Non-Linear Loads

– Arc furnaces, discharge lighting, …

– Due to unstable and non-linear process

– Either a voltage source or a current source

• In general, any load that is applied to a

power system that requires other than a

sinusoidal currentETAP.IT



Harmonic I and V

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Classification of Harmonics

• Harmonics may be classified as:

– Characteristic Harmonics

Generally produced by power converters

– Non-Characteristic Harmonics

Typically produced by arc furnaces and discharge

lighting (from non-periodical waveforms)ETAP.IT



Phase Angle Relationship

• Fundamental Frequency

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• Third Order

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• Fifth Order

• Seventh Order

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Order vs. Sequence

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Characteristic Harmonics

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Characteristic Harmonics

(cont’d)

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Harmonic Spectrum

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Harmonic-Related Problems

• Motors and Generators

– Increased heating due to iron and copper losses

– Reduced efficiency and torque

– Higher audible noise

– Cogging or crawling

– Mechanical oscillations

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(cont’d)

• Transformers

– Parasitic heating

– Increased copper, stray flux and iron losses

• Capacitors (var compensators)

– Possibility of system resonance

– Increased heating and voltage stress

– Shortened capacitor life

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(cont’d)• Power Cables

– Involved in system resonance

– Voltage stress and corona leading to dielectric failure

– Heating and derating

• Neutrals of four-wire systems (480/277V; 120/208V)

– Overheating

• Fuses

– Blowing

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(cont’d)• Switchgears

– Increased heating and losses

– Reduced steady-state current carrying capability

– Shortened insulation components life

• Relays

– Possibility of misoperation

• Metering

– Affected readings

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(cont’d)

• Communication Systems

– Interference by higher frequency electromagnetic field

• Electronic Equipment (computers, PLC)

– Misoperation

• System

– Resonance (serial and parallel)

– Poor power factor

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Parallel Resonance

• Total impedance at resonance frequency increases

• High circulating current will flow in the capacitance-inductance loop

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Parallel Resonance

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Capacitor Banks

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Capacitor Banks

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Capacitor Banks

Say, Seventh Harmonic Current = 5% of 1100A = 55 A

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Capacitor Banks

Resistance = 1% including cable and transformer

CAF = X/R = 7*0.0069/0.0012 =40.25

Resonant Current = 55*40.25 = 2214 A

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Parallel Resonance (cont’d)

Cause:

Impacts: 1. Excessive capacitor fuse

operation

2. Capacitor failures

3. Incorrect relay tripping

4. Telephone interference

5. Overheating of equipment

Source inductance resonates with

capacitor bank at a frequency

excited by the facilities harmonic

sources

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Harmonic Distortion Measurements

• Total Harmonic Distortion (THD)

– Also known as Harmonic Distortion Factor (HDF), is the most popular index to measure the level of harmonic distortion to voltage and current

– Ratio of the RMS of all harmonics to the fundamental component

– For an ideal system THD = 0%

– Potential heating value of the harmonics relative to the fundamental

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Harmonic Distortion Measurements (cont’d)

1

2

2

F

F

THD

i

Where Fi is the amplitude of the ith harmonic,

and F1 is that for the fundamental component.

– Good indicator of additional losses due to current flowing through a conductor

– Not a good indicator of voltage stress in a capacitor (related to peak value of voltage waveform, not its heating value)

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Harmonic Distortion Example

Find THD for this waveform

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Harmonic Example

• Find THD for this Harmonic Spectrum

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Adjustable Speed Drive –

Current Distortion

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Adjustable Speed Drive –

Voltage Distortion

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Harmonic Distortion Measurements (cont’d)• Individual Harmonic Distortion (IHD)

- Ratio of a given harmonic to fundamental

- To track magnitude of individual harmonic

1F

FIHD i

• Root Mean Square (RMS) - Total

- Root Mean Square of fundamental plus all

harmonics

- Equal to fundamental RMS if Harmonics are

zero

1

2

iFRMS

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• Arithmetic Summation (ASUM)

– Arithmetic summation of magnitudes of all

components (fundamental and all harmonics)

– Directly adds magnitudes of all components to

estimate crest value of voltage and current

– Evaluation of the maximum withstanding ratings

of a device

1

iFASUM

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• Telephone Influence Factor (TIF)

– Weighted THD

– Weights based on interference to an audio

signal in the same frequency range

– Current TIF shows impact on adjacent

communication systems

2

1

2

1

i

ii

F

FW

TIF

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• I*T Product (I*T)

– A product current components (fundamental

and harmonics) and weighting factors

H

h

hh TITI1

2)(

where Ih = current component

Th= weighting factor

h = harmonic order (h=1 for fundamental)

H = maximum harmonic order to account

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Triplen Harmonics

• Odd multiples of thethird harmonic(h = 3, 9, 15, 21, …)

• Important issue forgrounded-wye systemswith neutral current

• Overloading and TIF problems

• Misoperation of devices due to presence of harmonics on the neutral

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Triplen Harmonics

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Winding Connections

• Delta winding provides ampere turn balance

• Triplen Harmonics cannot flow

• When currents are balanced Triplens

behave as Zero Sequence currents

• Used in Utility Distribution Substations

• Delta winding connected to Transmission

• Balanced Triplens can flow

• Present in equal proportions on both sides

• Many loads are served in this fashion

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Implications

• Neutral connections are susceptible to overheating when serving single-phase loads on the Y side that have high 3rd Harmonic

• Measuring current on delta side will not show the triplens and therefore do not give a true idea of the heating the transformer is subjected to

• The flow of triplens can be interrupted by appropriate isolation transformer connection

• Removing the neutral connection in one or both Y windings blocks the flow of Triplen harmonic current

• Three legged core transformers behave as if they have a “phantom” delta tertiary winding

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Modeling in Harmonic

Analysis

• Motors and Machines

– Represented by their equivalent negative

sequence reactance

• Lines and Cables

– Series impedance for low frequencies

– Long line correction including transposition and

distributed capacitance

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Modeling in Harmonic

Analysis (cont’d)

• Transformers

– Leakage impedance

– Magnetizing impedance

• Loads

– Static loads reduce peak resonant impedance

– Motor loads shift resonant frequency due to

motor inductance

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Reducing System

Harmonics• Add Passive Filters

– Shunt or Single Tuned Filters

– Broadband Filters or Band Pass Filters

– Provide low impedance path for harmonic

current

– Least expensive

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Reducing System

Harmonics (cont’d)

• Increase Pulse Numbers

– Increasing pulse number of convert circuits

– Limited by practical control problems

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Reducing System

Harmonics (cont’d)• Apply Transformer Phase Shifting

– Using Phase Shifting Transformers

– Achieve higher pulse operation of the total

converter installation

• In ETAP

– Phase shift is specified in the tab page of the

transformer editorETAP.IT



Reducing System

Harmonics (cont’d)• Either standard phase shift or special phase

shift can be used

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Reducing System

Harmonics (cont’d)• Add Active Filters

– Instantly adapts to changing source and load

conditions

– Costly

– MVA LimitationETAP.IT



Voltage Distortion Limits

Recommended Practices for Utilities (IEEE

519): Bus Voltage

At

PCC

Individual

Distortion

(%)

Total Voltage

Distortion

THD (%)

69 kV and below 3.0 5.0

69.001 kV through 161kV 1.5 2.5

161.001 and above 1.0 1.5

In ETAP:

Specify Harmonic Distortion Limits in Harmonic

Page of Bus Editor:ETAP.IT



Current Distortion Limits

Recommended Practices for General

Distribution Systems (IEEE 519):

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Documents

ETAP - Harmonics.pdf · Slide 12. Phase Angle Relationship • Fundamental Frequency. ETAP.IT. ETAP µ ¯íã 6³ Ó Ê ã ... ETAP.IT. ETAP µ ¯íã 6³ Ó Ê ã