Why overvoltage occurs when capacitor current is cut off at zero crossing

After the capacitor is cut off, it will produce over-voltage. The common explanation for this problem is that the over-voltage of the capacitor is caused by the re ignition of the switch contact during the cut-off process. The over-voltage caused by arc reburning is affected by many factors, including contact breaking time, system inductance, reburning time and so on. The mechanism analysis is very complex, so we will not discuss it here.

The capacitor controlled by thyristor will also produce overvoltage after being cut off. This phenomenon can not be explained by arc reburning, because the thyristor will not produce arc reburning. Thyristor can only be turned off when the current crosses zero, so the capacitor controlled by thyristor must be cut off when the current crosses zero, so we will discuss the overvoltage phenomenon with the capacitor controlled by thyristor.

For the single-phase capacitor controlled by thyristor, there is no over-voltage after switching off. Due to the 90 degree phase difference between the voltage and current of the capacitor, the moment when the current of the capacitor is zero is the moment when the voltage is the highest. Therefore, we can determine that the voltage of the single-phase capacitor controlled by thyristor after being cut off is equal to the peak value of the power supply voltage, that is, 1.414 times of the effective value of the power supply voltage, which is within the withstand voltage range of the capacitor.

For the three-phase capacitor controlled by thyristor, the situation is completely different. The removal process of three-phase capacitor is much more complex than that of single-phase capacitor. The abnormal high voltage will remain after the removal of three-phase power capacitor controlled by thyristor, which may pose a threat to the safety of the capacitor.