According to the abnormal voltage deviation of 110 kV section I and II bus in a power plant,the DCS system data,on-line examination,theoretical calculation and electrical test are comprehensive analyzed in this paper.Thinking that the 110 kV bus voltage anomaly is caused by some internal capacitor unit short circuit in the CVT.Then return the equipment to the factory,the result of the preliminary analysis is verified by the disassembly inspection,at the same time some capacitor unit quality defects are founded out,such as insulation material weakness and bad dipping property.Accordingly,require the manufacturer to replace all the products of this batch,then propose some treatment measures and suggestions for the same type equipment.