Measurement of Voltage Transformer Errors using a Self-calibrating Multi-ratio Capacitive Divider System

Abstract

A new portable voltage transformer (VT) calibration system has been developed, based on an existing fixed laboratory system. The existing system is based on a high voltage compressed gas capacitor in the upper arm of a voltage divider, and a range of precision air capacitors in the lower arm, with the errors being balanced via the use of inductive voltage dividers. The new system utilises the same high voltage compressed gas capacitor in the upper arm but in the lower arm uses small, class 1, multi-layer ceramic capacitors. Instead of balancing the system with inductive voltage dividers, a direct measurement of the VT errors is made with the use of an integrating amplifier and two digital multimeters (DMMs). One DMM measures the secondary voltage and the other measures the relative phase and amplitude of the error voltage from an integrating amplifier. Using a VT with the nominal ratio of 10:1, and the ability of switching several of the lower arm ceramic capacitors into the upper arm, and then following a sequence of measurements, all the relative capacitance values can be calculated using a mathematical build-up process. The new portable VT calibration system has achieved a typical measurement uncertainty for voltage error and phase displacement of better than 0.003% and 0.003 crad respectively. It can test VTs with applied primary voltages from 30 V to 220 kV, and secondary voltages from 10 V to 300 V, with the ratio settings of the capacitive divider in the range of 0.1 to 2200. The system has been optimised for operating at 50 Hz and 60 Hz, but theoretically it could be used for higher frequencies.