When the capacitor is re-inputed after re-injection, if the thyristor is turned on (the capacitor is connected to the grid), the grid voltage and the residual voltage of the capacitor are greatly different, and the voltage on the capacitor cannot be abruptly changed, resulting in a large current surge (closing) Inrush current), this impact is likely to damage the thyristor or bring high frequency impact to the grid. In order to prevent the inrush current from being impacted when the capacitor is input, it is necessary to select the ideal timing for the thyristor to be triggered, that is, to ensure that the grid voltage and the capacitor residual voltage are equal and the polarity is the same when the thyristor is turned on, and it is necessary to measure the residual voltage of the capacitor in advance. To solve this problem, consider the following options:
1. Add discharge resistor. After each removal of the capacitor, the capacitor is discharged through a special discharge resistor, so that the residual voltage of the capacitor is close to zero, and the thyristor is put into operation when the grid voltage is zero. This solution requires an increase in the cost of the reactive power compensation device, and the circuit for automatically connecting the discharge resistor after the capacitor is removed is also complicated.

2. The capacitor is pre-charged. It is pre-charged before being put into the capacitor, charged to the peak of the grid voltage, and the thyristor is triggered at the peak of the grid voltage. This method will make the main circuit complicated and prolong the investment time of the capacitor.

3. The main circuit uses thyristor and diode anti-parallel. The voltage of the capacitor is always maintained at the peak of the grid voltage before the capacitor is put in. Once the capacitor voltage is lower than the peak value of the grid voltage, the diode will charge its voltage to the peak voltage of the grid. As long as the thyristor is triggered at the peak of the grid voltage, current surges can be avoided.

4. Zero voltage triggering method to detect the voltage across the thyristor. Due to the uncertainty of the residual voltage of the capacitor, the voltage on the thyristor is a value that cannot be calculated from the grid voltage, but the voltage between the grid voltage and the residual voltage of the capacitor can be determined by detecting the voltage across the thyristor (anode and cathode). The thyristor is triggered when it is detected that the voltage across the thyristor is equal (the voltage difference is zero).