Understanding of Harmonics in Power Distribution System
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Transcript of 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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