The electronic and electrical equipment is used in the low voltage utility network, the power supply voltage is the sine wave, but the current waveform is not the sine wave, it may have more or less distortion. Much of the electrical equipment may cause the network voltage waveform distortion and cause the decrease of the power quality.
Figure : High voltage rectifying circuit and the corresponding distortion current waveform
A periodic function can be decomposed as the Fourier series to signify the sum of multistage sine function, namely the periodic function can be regarded as the composition of the fundamental wave of the sine function and the ultra harmonics.
Therefore, we can decompose the current waveform distortion of the equipment as base wave and high harmonic; we can also analyze the current waveform distortion degree of the equipment by using the specific instrument to measure the higher harmonic contents. These higher harmonic current components can be called as the harmonic current for short.
Figure : Fourier spread schemes of the current waveform distortion
When the power grid has excessive harmonic current, it will not only reduce the generator efficiency, but also cause the damage of the generator and the grid equipment and affects the normal work of the grid user’s equipment, such as the wrong computer operation, rolling TV picture.
The harmonic current test is not suitable for the electrical and electronic products powered by the utility of low voltage, ac and dc batteries.
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