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Power ystem armonics Pacific Gas and Electric (电力系统armonics太平洋天然气和电力)
Pacific Gas and
Electric Company
Power System Harmonics
What are power system harmonics?
Ideally, voltage and current waveforms are perfect sinusoids. However,
because of the increased popularity of electronic and other non-linear loads, these
waveforms quite often become distorted. This deviation from a perfect sine wave
can be represented by harmonics—sinusoidal components having a frequency that
is an integral multiple of the fundamental frequency (see Figure 1). Thus, a pure
voltage or current sine wave has no distortion and no harmonics, and a non-
sinusoidal wave has distortion and harmonics. To quantify the distortion, the term
total harmonic distortion (THD) is used. The term expresses the distortion as a
percentage of the fundamental (pure sine) of voltage and current waveforms.
D istorted w aveform ( 60 Hz = Fundam ental + 3rd H arm onic)
Fundamental (60 Hz)
3rd Harm onic ( 180 Hz)
rd
Figure 1: Distorted Waveform Composed of Fundamental and 3 Harmonic. THD approximately 30%
Why are voltage and current harmonics a problem?
Current harmonics are a problem because they cause increased losses in the
customer and utility power system components. Transformers are especially
sensitive to this problem and may need to be derated to as much as 50% capacity
when feeding loads with extremely distorted current waveforms (current total
harmonic distortion above 100%).
1
ANSI/IEEE C57.110-1986 (IEEE Recommended Practice for Establishing
Transformer Capacity When Supplying Nonsinusoidal Load Currents) states that a
transformer subject to nonsinusoidal load current having more than 5% total
harmonics distortion needs to
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