How To Solve The Problem Of Harmonic Amplification in Pure Capacitive Reactive Power Compensation?

The selection of compensation type for reactive power compensation cabinets is a very important step. Whether the selection is correct or not directly affects the efficiency and reliability of the compensation cabinets. The key basis for selecting the compensation type is the degree of harmonic pollution. Due to the increasing use of nonlinear loads in power distribution systems, the problem of harmonic pollution caused by them has become increasingly serious. The reactive power compensation capacitor is one of the equipment most affected by harmonics among power distribution devices. Harmonics not only cause overload of the capacitor, reduce the voltage value, and shorten the service life, but also may cause grid resonance and serious electrical accidents. Moreover, incorrect selection of compensation type can also cause harmonic amplification, further aggravating the degree of harmonic pollution in the distribution network. Therefore, the correct compensation type must be selected according to the degree of harmonic pollution.

When high-order harmonics are present in the system, the conventional practice of using pure capacitors as reactive power compensation will encounter the following problems:
(1) Since the capacitive reactance of the capacitor is inversely proportional to the frequency, when the frequency is higher, the capacitive reactance of the capacitor becomes lower. As a result, more harmonics will pass through the capacitor, causing the capacitor to overload and be damaged.

(2) Since pure capacitor compensation will amplify the harmonic current on the system side, the amplified current flowing through the capacitor will cause overload of the capacitor. Moreover, when the amplified harmonic current flows into the system side, it will generate greater harmonic voltage distortion under the influence of the amplified harmonic current, thereby causing harm to the power supply equipment and electrical equipment throughout the system.

(3) Due to the existence of harmonic currents, corresponding harmonic voltages of the same frequency will be generated in the system. When the harmonic voltages are superimposed on the fundamental voltage, they may exceed the withstand voltage level of the capacitor. Moreover, high-order harmonics in the system may cause resonance, resulting in higher resonant overvoltages and damaging the capacitor. For capacitors, when the voltage increases by 10%, their service life is halved.

(4) When the terminal voltage of the capacitor is a non-sinusoidal wave, additional active power loss occurs in the capacitor dielectric, resulting in extra heat generation, which causes the temperature of the capacitor to rise and accelerates the aging of the capacitor dielectric. For every 8%-10% increase in temperature, the service life of the capacitor is halved.

Therefore, the conventional pure capacitor compensation method is not suitable for use in distribution systems with a large number of nonlinear components. In the reactive power compensation circuit, adding an inductor in series with a capacitor to change its impedance characteristics under harmonic action, so that in the case of harmonic action, the reactive power compensation circuit connected is not capacitive in nature, thereby avoiding the amplification of harmonics and resonance.