understanding of harmonics in power distribution system

8/12/2019 Understanding of Harmonics in Power Distribution System

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Understanding of Harmonics

in Power Distribution System


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What are Power System Harmonics?

Harmonic: a mathematical definition, generally used whentalking about Integral orders of Fundamental frequencies

Power system harmonics: currents or voltages withfrequencies that are integer multiples (h=0,1,2,N) of the

fundamental power frequency [1]

1stharmonic: 60Hz

2ndharmonic: 120Hz

3rdharmonic: 180Hz

Figure: 1 [2]


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How are Harmonics Produced ?

Power system harmonics: presenting deviations from a perfectsinusoidal-waveform(voltage or current waveform).

The distortion comes from a Nonlinearity caused by saturation,electronic-switching and nonl inear electric loads,Inrush/Temporal/Arc/Converter/Limiter/Threshold Type Loads.


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Current vs. Voltage Harmonics

Harmonic current flowing through the AC PowerSystem impedance result in harmonic voltage-drop at the load bus and along the Feeder!!

Figure: 3 [3]


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How to Quantify Harmonic Distortion?

Total Harmonic Distortion-THD: the

contribution of all harmonic frequency

Currents/Voltages to the fundamental current. [3]

The level of THD-for Current or Voltage is

directly related to the frequencies and amplitudes

of the Offending Quasi-Steady State persistent

Harmonics. Individual Distortion Factor-(DF)-h quantify

Distortion at hharmonic-order


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Calculation of THD

THD: Ratio of the RMS of the harmoniccontent to the RMS of the Fundamental [3]

(Eq-1)

Current THD-I

(Eq-2)

Voltage THD-V

(Eq-3)


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Negative Effects of Harmonics

Overheating and premature failure of distribution transformers [1]

Increasing iron and copper losses or eddy currents due to stray flux losses

Overheating and mechanical oscillations in the motor-load system [1] Producing rotating magnitude field, which is opposite to the fundamental

magnitude field. Overheating and damage of neutral ground conductors [2]

Trouble sustained type Harmonics: 3rd, 9th, 15th

A 3-phase 4-wire system: single phase harmonic will add rather thancancel on the neutral conductor

Malfunction/Mal-Operation of Sensitive Tele-control and

Protection Relaying


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Harmonics and Series Resonance Circuit

The voltage of upstream AC Network can be alsodistorted due to series/parallel resonance formed by

capacitance of the capacitor bank and System/load

inductance : Ca cause high harmonic current circulation

through the capacitors[5]

Parallel Resonance can also lead to high voltage distortion.

Figure 5: Series resonance circuit and its equivalent circuit [5]


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Measure Equipments of Harmonics

Digital Oscilloscope:Wave shape, THD and Amplitude of each harmonic

True RMS Multi-Meter:

Giving correct readings for distortion-free sine waves and typicallyreading low when the current waveform is distorted

Use of Harmonic Meters-Single Phase or three Phase

Figure 6: True RMS Multi-Meter[3]


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Standards for Harmonics Limitation

IEEE/IEC

IEEE 519-1992 Standard: Recommended Practices andRequirements for Harmonic Control in Electrical PowerSystems(Current Distortion Limits for 120v-69kv DS)

Table 1: Current Harmonic Limits [4]

Ratio

Iscc / Iload

Harmonic odd

numbers (35)

THD-i

< 20 4.0 % 0.3 % 5.0 %

20 - 50 7.0 % 0.5 % 8.0 %

50 - 100 10.0 % 0.7 % 12.0 %

>1000 15.0 % 1.4 % 20.0 %


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Standard of Harmonics Limitation

(contd)

IEEE 519-1992 Standard: Recommended Practices andRequirements for Harmonic Control in Electrical PowerSystems(Voltage Distortion Limits)

Table 2: Voltage Harmonic Limits[4]

Bus Voltage Voltage Harmonic limitas (%) of Fundamental

THD-v (%)

= 161 Kv 1.0 1.5


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Harmonics Filter Types [6]

Isolating harmonic current to protect electricalequipment from damage due to harmonic voltage

distortion

Passive Filter-Low cost:

Built-up by combinations of capacitors, inductors(reactors) and resistors

most common and available for all voltage levels

Active Power Filter APF:

Inserting negative phase compensating harmonics intothe AC-Network, thus eliminating the undesirableharmonics on the AC Power Network.

APF-Used only for for low voltage networks


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Conclusions

The harmonic distortion principally comes fromNonl inear-Type Loads.

The application of power electronics is causingincreased level of harmonics due to Switching!!

Harmonic distortion can cause seriousFailure/Damage problems.

Harmonics are important aspect of poweroperation that requires Mitigation!!

Over-Sizing and Power Filtering methods arecommonly used to limit Overheating Effects ofSustained Harmonics.


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understanding of harmonics in power distribution system

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