Calibration system for high-voltage transformers with multi-harmonic distorted waveforms

Non-ideal frequency response and voltage non-linearity of high-voltage instrument transformers directly impact the accuracy of high-voltage measurements in the presence of harmonics. Characterization of voltage instrument transformer frequency response is generally performed at low voltage with sinusoidal test signals. Those results may not accurately represent the response of the instrument transformer when subjected to distorted waveforms in modern power systems. This paper presents a calibration system for the evaluation of harmonic measurement accuracy of single-phase instrument transformers with rated voltages up to 132/√3 kV using multi-tone test signals containing a high-voltage fundamental component and superimposed harmonics up to 10 kHz. Such signals would represent the realistic operating conditions of high-voltage instrument transformers. The performance of the developed system was analyzed by obtaining the voltage ratio and phase displacement of the reference capacitive voltage divider from 50 Hz to 10 kHz at 1 kV with an expanded uncertainty of 0.07% for the voltage ratio error and 0.09 crad for the phase error. The effect of voltage linearity at harmonic frequencies and the effect of the fundamental voltage on the errors of the measurements of harmonics were also evaluated. The results prove that the system is capable of calibrating high-voltage instrument transformers under distorted waveform conditions with an uncertainty of 0.13 % for ratio and 0.15 crad for phase error at harmonic frequencies from 100 Hz to 10 kHz, and 0.02 % and 0.04 crad at the fundamental power frequency of 50 Hz.