1. The influence of harmonics on shunt capacitors

       When the system voltage waveform is distorted, these non-sinusoidal waveforms can be decomposed into a 50H2 fundamental wave and high-order harmonics of various multiples of frequency by digital analysis. The superposition of the fundamental wave and various harmonics is the distorted waveform. . For capacitors, there are generally no even multiples of harmonics. In addition, the phase current of the capacitor bank connected in the neutral point of the star connection and the phase voltage of the capacitor bank connected in the delta connection do not include the harmonics of the third order and its integer multiples. Therefore, the 5th, 7th, 11th, and 13th harmonics are mainly considered. Among these high-order harmonics, the 5th harmonic is the most significant.

       Due to the effect of high-order harmonics in the system voltage, the capacitor will generate overcurrent and overload, and the multiples of the two are not the same. At the same time, the distortion of the electrical waveform caused by a certain high-order harmonic current is far more serious than the distortion of the voltage waveform. For example, the voltage waveform of a system includes the fundamental wave and the 5th harmonic (other high-order harmonics account for a small proportion). The fundamental voltage is equal to the rated voltage, and the 5th harmonic voltage is 26.45% of the rated voltage. After calculation, The results show that: in this case, the capacitor bank overvoltage is 3.4%, the overcurrent is 65.6%, the capacitor's reactive output is overloaded by 35%, and the reactive power meter reads 171.2% of the capacitor bank's rated reactive power Qe. The distorted power generated by the 5th harmonic is as high as 105% of the rated reactive power of the capacitor. Furthermore, capacitors are more sensitive to the response of harmonic voltages. In the case of harmonic resonance, the capacitor may be overloaded twice, causing serious damage to the capacitor or inoperability.

2. Limits of harmonics

       When there is a harmonic source in the system and the safe operation of the capacitor is affected, the author believes that the user (harmonic source) should first take corresponding measures to reduce the higher harmonic components. There are many ways to suppress high-order harmonics, such as increasing the number of phases of the rectifier, limiting the connection capacity of large-scale rectifiers in the power grid, and using tuning reactors, micro-inductance resistors and filter capacitors to make special high-order harmonic filtering devices. Secondly, it should be considered because of the existence of harmonics in the system voltage, which measures can be adopted to limit the amplification of harmonics. At present, in order to protect shunt capacitors used for reactive power compensation, the most effective method is to connect reactors in series in the capacitor loop. The inductive reactance value of the reactor should be determined according to the following method.

Obviously, the larger the series reactor XL, the smaller the harmonic current In. In order to avoid nth and above harmonic resonance, the natural frequency should be smaller than n, let X = XS+XL, when the nth harmonic resonance , That is

      As mentioned above, for the capacitor circuit, the 5th, 7th, 11th etc. harmonics play the main role. In the actual power grid, in order to avoid harmonic resonance of the 5th and above, it can be seen from equation (3) that when n = 5, X > 0 should be satisfied. Because XS is very small, X = XL should be used for 04XC, so XL should be made XL > ; 0.04XC.

      When n = 5, that is, when 5XL=, that is, XL=0.04Xc, the 5th harmonic voltage on the bus is zero, so that the 5th harmonic voltage is not amplified, which is exactly what is desired.

      In summary, in order to limit the 5th harmonic current, XL> 0.04Xc should be used, and to reduce the 5th harmonic voltage on the bus, XL = 0.04Xc should be used. Both of the above should be taken into account, usually XL = 0.06Xc .

      Generally, XL = 0.07Xc can target more non-linear loads.