Harmonic: refers to the current contained in the frequency of integer multiple of the fundamental wave electric quantity, generally refers to the periodic non-sinusoidal electric quantity Fourier series decomposition, the rest is greater than the fundamental frequency of the electric quantity generated.Broadly speaking, since the effective component of AC network is a single frequency of power frequency, any component different from power frequency can be called harmonic.

The cause of harmonic generation: because the sinusoidal voltage is applied to the nonlinear load, when the current flows through the load, it does not show a linear relationship with the added voltage, and the fundamental current is distorted to form a non-sinusoidal current, that is, there are harmonics in the circuit.The main nonlinear load is UPS, switching power supply, rectifier, frequency converter, inverter and so on.

There are three reasons for the source of switching power harmonics:

(1) Incoming harmonics from the power grid.

Because the power network is struck by lightning or lightning induction will produce a very high range of surge voltage, connected to other electrical equipment on the power network when working harmonics and the surge voltage generated by switching will also cause harmonics.

(2) The rectifier and filter circuit at the input end of the switching power supply is a combination of nonlinear elements and energy storage elements, which leads to the current waveform distortion and is pulse-like.

(3) The DC/DC transformation of the switching power supply works in the high frequency switching state.In high frequency DC/DC converter rectifier diode add reverse voltage, due to the accumulation of charge by the forward current don't disappear immediately, the diode can't immediately stop and reverse current generation, reverse recovery, if the voltage spike and surge current is bigger, also under the action of inductance spike pulse contains abundant harmonic.

Hazards caused by harmonics:

For transformers, harmonic current can increase copper loss.Compared with the sinusoidal current and voltage of pure fundamental wave, the overall effect of harmonics on the transformer is higher temperature rise.It is worth noting that these additional losses due to harmonics will increase in proportion to the square of the current and frequency, resulting in a reduction in the fundamental load capacity of the transformer.At the same time, harmonics will also lead to the increase of transformer noise.

What are the methods of harmonic suppression?

(1) the use of power factor correction circuit, the use of active power factor correction and passive power factor correction technology

(2) access to the AC grid filter, between the grid and switching power supply access to the AC grid filter can effectively suppress the harmonic network incoming.