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

Harmonic Analysis

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

Types of Power Quality

Problems

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Harmonics

One special category of power qualityproblems

Harmonics are voltages and/or currentspresent in an electrical system at some

multiple of the fundamental frequency.

(IEEE Std 399, Brown Book)

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Nonlinear Loads

Sinusoidal voltageapplied to a simple

nonlinear resistor

Increasing the

voltage by a few

percent may cause

current to double

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Fourier Representation

Any periodicwaveform can beexpressed as a sumof sinusoids

The sum of thesinusoids is referredto as Fourier Series

(6-pulse)

)cos(

13cos13

111cos

11

17cos

7

13cos

5

1(cos

32

1

h

h

h

dac

thI

tttttII

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7/53Slide 7 1996-2009 Operation Technology, Inc. - Workshop Notes: Harmonics

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 (contd)

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 (contd)

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 current

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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 dischargelighting (from non-periodical waveforms)

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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

(contd)

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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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(contd)

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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(contd)

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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(contd)

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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(contd)

Communication Systems

Interference by higher frequencyelectromagnetic field

Electronic Equipment (computers, PLC)

Misoperation

System Resonance (serial and parallel)

Poor power factor

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Parallel Resonance

Total impedance at resonance frequencyincreases

High circulating current will flow in thecapacitance-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 (contd)

Cause:

Impacts: 1. Excessive capacitor fuse

operation

2. Capacitor failures3. Incorrect relay tripping

4. Telephone interference

5. Overheating of equipment

Source inductance resonates withcapacitor bank at a frequency

excited by the facilities harmonic

sources

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H i Di t ti

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Harmonic DistortionMeasurements (contd)

1

2

2

F

F

THD

i

Where Fiis the amplitude of the ith harmonic,

and F1

is that for the fundamental component.

Good indicator of additional losses due tocurrent flowing through a conductor

Not a good indicator of voltage stress in acapacitor (related to peak value of voltage

waveform, not its heating value)

H i Di t ti

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Harmonic DistortionExample

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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H i Di t ti

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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

H i Di t ti

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Arithmetic Summation (ASUM)Arithmetic summation of magnitudes of all

components (fundamental and all harmonics)

Directly adds magnitudes of all components toestimate crest value of voltage and current

Evaluation of the maximum withstanding ratings

of a device

1

iFASUM

H i Di t ti

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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

H i Di t ti

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I*T Product (I*T)A product current components (fundamental

and harmonics) and weighting factors

H

h

hh TITI

1

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 for

grounded-wye systemswith neutral current

Overloading and TIF problems

Misoperation of devices due to presence ofharmonics on the neutral

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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 thathave high 3rd Harmonic

Measuring current on delta side will not show thetriplens and therefore do not give a true idea of the

heating the transformer is subjected to

The flow of triplens can be interrupted by appropriateisolation transformer connection

Removing the neutral connection in one or both Ywindings blocks the flow of Triplen harmonic current

Three legged core transformers behave as if they havea phantom delta tertiary winding

M d li i H i

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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

M d li i H i

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Modeling in Harmonic

Analysis (contd)

Transformers

Leakage impedance

Magnetizing impedance

Loads

Static loads reduce peak resonant impedance

Motor loads shift resonant frequency due to

motor inductance

R d i S t

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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 harmoniccurrent

Least expensive

R d i S t

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Reducing System

Harmonics (contd)

Increase Pulse Numbers

Increasing pulse number of convert circuits

Limited by practical control problems

R d i S t

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Reducing System

Harmonics (contd)

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 editor

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R d i S t

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Reducing System

Harmonics (contd)

Add Active Filters

Instantly adapts to changing source and load

conditions

Costly

MVA Limitation

V lt Di t ti Li it

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Voltage Distortion Limits

Recommended Practices for Utilities (IEEE

519): Bus VoltageAt

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 HarmonicPage of Bus Editor:

C t Di t ti Li it

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Current Distortion Limits

Recommended Practices for General

Distribution Systems (IEEE 519):

13 - Harmonics

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