A current-comparator-based load loss standard for in-situ calibration of transformer loss measuring systems

A current-comparator technique for obtaining a load-loss standard with adjustable loss tangent (or power factor) for in-situ calibration of transformer loss-measuring systems is described. The standard, driven by the test voltage through a low-loss, high voltage compressed-gas-dielectric reference capacitor, simulates an inductive load that can be operated with load currents of up to 1000 A. The equivalent rating of the load is determined by the voltage of the high-voltage reference capacitor. Adjustment of the loss tangent (or power factor) is achieved by comparing the load current, using the current comparator, with in-phase and quadrature reference currents derived from the test voltage. The standard features three current ranges of 10 A, 100 A, and 1000 A. The maximum current of each range is available at five different voltage levels which are determined by the voltage rating of the high-voltage reference capacitor. The standard also features a direct-reading loss tangent (tan δ) adjustment in the range from 0 to 0.1 with five-digit resolution and an accuracy of better than 20 parts per million