Harm of Harmonics to Electric Power and Electrical Equipment (I)

1. Harm to power capacitors

When there is a harmonic in the power grid, the voltage at the terminal increases after the capacitor is input, and the current through the capacitor increases more, so that the power loss of the capacitor increases. For the film-paper composite dielectric capacitor, although the power loss is allowed to be 1.38 times the power loss when there is no harmonic; the power loss when the harmonic is allowed for the full-film capacitor is 1.43 times that of the harmonic-free, but if The high harmonic content exceeds the allowable condition of the capacitor, which will cause the capacitor to over-current and overload. The power loss exceeds the above value, causing the capacitor to generate abnormal heat. Under the action of electric field and temperature, the insulating medium will accelerate aging. Especially when the capacitor is put into the power grid whose voltage has been distorted, the harmonics of the power grid can be aggravated, that is, the harmonic expansion phenomenon occurs. In addition, the presence of harmonics tends to make the voltage appear as a cusp waveform, and the peak voltage wave is easy to induce partial discharge in the medium, and because of the large voltage change rate, the partial discharge intensity is large, and the insulating medium can accelerate the aging, thereby shortening The life of the capacitor. In general, for every 10% increase in voltage, the life of a capacitor is reduced by about 1/2. Moreover, in the case of severe harmonics, the capacitor may also bulge, puncture or explode.

2. Harm to power cables

Due to the high frequency rise of the harmonic order, the larger the cross-sectional area of the cable conductor, the more obvious the skin effect is, which leads to an increase in the AC resistance of the conductor, so that the allowable passing current of the cable is reduced. In addition, the resistance of the cable, the system bus side and the line inductance are connected in series with the system, and the capacitance and capacitance of the capacitor and the line for improving the power factor are connected in parallel with the system, and resonance may occur under a certain value of inductance and capacitance.