By using a method of Fourier decomposition，any periodical AC signal can be deco- mposed to many sine waves and DC component. Some sine waves are harmonic whose frequency does not equal to the frequency of fundamental component. The more non-linear loads are used in power system, the more asymmetrical three-phase voltages, and the more server voltage fluctuation are. Though the efficiency can be enhanced when sine wave passes through non-linear loads, the sine wave distortion appears, and then harmonic current passes through loads. When the distorted currents exist in power system, it will result in distorted voltages and bring on the problem of harmonic pollution.
The harm of the harmonic wave in power system is serious, such as depressing the efficiency of production, transfers and utilization of electric power, heating up electric equipment, bringing vibration and noises, aging the insulation, cutting the life-span, even bringing on fire. In power system, it can excite partial parallel resonance or serial resonance, enlarge the harmonic wave, trigger relay protection system in error and disturb electric power energy metering equipment. Meanwhile, the harmonic wave can disturb many other electric and communication equipment out of the power system.
There are some conventional detection methods for power system harmonics, such as analog band-pass/band-stop filter, detection method based on FT or detection method based on instantaneous reactive power theory. Some new detection methods -- *improved FT method, detection method based on neural network and detection method based on wavelet analysis--come forth and are used more and more widely.
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