Active on-site calibration technology of current transformer based on non-rated frequency current injection

This paper proposes an active on-site calibration method through background current cancellation and non-rated current injection. It can measure the error of the current transformer in service from 1% to 120% rated current percentage without power supply interruption. In order to establish the error relationship between rated frequency and arbitrary frequency, a theoretical analysis of current transformer calibration at the arbitrary frequency has been developed by means of the equivalent circuit. It describes a method to determine the phase angle and ratio errors of the measuring transformers at arbitrary frequencies on the basis of the calibrated error values at rated frequency. To prove the theoretical analysis, an experimental validation was carried out. The experimental results demonstrate that this active onsite calibration is a valid tool for the evaluation of current transformer performances. The calibration results showed that, for both cases (non-rated frequency calibration and mixing frequency calibration), the difference between mean ratio error and rated frequency ratio error was lower than 0.01%, and the difference between mean phase error and rated frequency phase error was lower than 1', which meets the requirement of the 0.2 accuracy class calibration.