The power capacitor is very sensitive to voltage. Because the loss of capacitor is proportional to the square of voltage, overvoltage will make the capacitor heat seriously, the insulation of capacitor will accelerate aging, shorten its life, and even break through electric shock.

Generally, the grid voltage should be lower than the rated voltage of the capacitor itself, and higher than 10% of its rated voltage, so as to prolong the service life of the capacitor. It should be noted that higher operating voltage and higher operating temperature cannot occur at the same time. Therefore, when the working voltage is 1.1 times the rated voltage, cooling measures need to be taken.

The rated voltage of the capacitor is at least equal to the operating voltage of the power grid, and the influence of the capacitor itself should also be considered. It should be noted that the operating voltage of the power grid sometimes differs greatly from the nominal voltage of the power grid. In addition, when the capacitor is connected, the voltage from the power supply to the capacitor installation will rise, and the harmonic voltage will also rise.

Considering the above factors, the rated voltage level of capacitor shall be determined at least 5% higher than the nominal voltage of circuit.

For example, 380V circuits use at least 400V capacitors. Secondly, the user should choose according to the longer duration and higher continuous voltage of the actual use situation. Especially when the capacitor circuit is in series with reactor, the voltage on the capacitor terminal will rise due to the series reactor, which will exceed the operation voltage of the power grid. Rated voltage of capacitor behind series reactor = system voltage / (1-k (reactance rate)).

For example, for a series capacitor with 12% reactance rate, the rated voltage of the capacitor should be 400V / (1-0.12) = 455v.