The absolute calibration of voltage transformers

An account is given of equipment and measuring techniques in use at the National Standards Laboratory, Australia, for calibrating voltage transformers. The equipment employs 3-terminal air-dielectric capacitors to form a voltage divider, the ratio and phase defect angle of which are determined by a self-contained build-up technique. Owing to the use of virtually loss-free capacitors, the phase angle of the divider is negligibly small. Two methods are described for calibrating transformers. In one, ratio balance is obtained by means of a small auxiliary variable-ratio voltage transformer, and phase balance by current injection into one of the detector bridge junctions. In the other method, ratio balance is provided by a variable 3-terminal air-capacitor in the low-voltage arm of the divider, and phase balance by voltage injection in series with that arm, a feedback amplifier providing a low-impedance injection circuit. In both methods the accuracy of measurement is approximately 2 parts in 105 in ratio and 0.05¿ in phase angle. The air-dielectric capacitors used in the voltage divider are tested to ensure that they have no significant voltage coefficient of capacitance or loss angle.