To ensure the accuracy and reliability of a voltage transformer

it is necessary to calibrate its accuracy.However

the traditional calibration method needs to be carried out periodically in the condition of outage

and some-times it cannot find transformer’s fault due to longer calibration time. In view of the fact

this paper proposes a volt-age transformer error online calibration system based on the open-closed capacitor sensing method. In this method

two open-closed capacitors and other auxiliary devices are used to compose a voltage sensor

and it is taken as the standard sensor. The outputs of the two open-closed capacitors are compared with each other to achieve self-test of sensor accuracy

and the average value of the outputs is used as the final output. Hence

the accuracy and reliability of the calibration are greatly improved. Test results show that the present system works within 20%～120% of the rated voltage

the variation of the ratio error is less than 0.037%

and the variation of the phase error is less than 0.45′.