Unbalance protection is often used in high‐voltage power capacitor banks as their main relay protection. Based on the analysis of the force of the capacitor, we show that the capacitance of the oil‐filled capacitor will change with the change of the field strength, and we deduce the theoretical equation of the capacitance change with the voltage. On the basis of this, we study the relationship between the capacitance and the voltage. Taking a real capacitor device as an example, the effect of voltage on unbalance leveling is studied by Monte Carlo simulations. The results show that the capacitance of the oil‐impregnated high‐voltage capacitor is affected by the voltage and may lead to the failure of the unbalance protection leveling. © 2018 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.