Power ystem armonics Pacific Gas and Electric (电力系统armonics太平洋天然气和电力).pdf

Power ystem armonics Pacific Gas and Electric (电力系统armonics太平洋天然气和电力).pdf

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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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