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  In an ideal system, a clean power supply, current and voltages are sinusoidal. When only with linear components (resistors, inductors and capacitors), a simple circuit, the current flowing to the applied voltage proportional to the current flowing through the sine wave.
   In the actual power supply system, due to the presence of non-linear loads, when current flows through the non-linear relationship between the applied voltage when the load on the formation of non-sinusoidal current. Any periodic waveform can be decomposed into a fundamental frequency sine wave plus many harmonic frequency sine wave. Harmonic frequencies are integer multiples of the fundamental frequency, for example, the base frequency of 50Hz, the second harmonic is 100Hz, the third harmonic was 150Hz. Therefore, the current waveform distortion may have second harmonic, third harmonic ...... until the thirtieth may harmonic components.