A Phantom Burden for Current Transformer Calibration

A technique is presented for applying a burden to a current transf ormer being calibrated in which the power and reactive volt-amperes are returned to the supply rather than being absorbed in a passive network. The transfer is accomplished either by the magnetic shield if a compensated current comparator is used as the ratio standard, or by an auxiliary current transformer in circuits which employ other standards. Electronic circuitry provides the necessary volt-amperes for exciting the magnetic shield of the current comparator or the auxiliary current transformer, and the current transformer being calibrated, to produce the desired power transfer. The circuit requirements are analyzed and equipment which can develop burdens with both in-phase and quadrature components up to 10 ohms, at secondary currents up to 10 amperes, and for a 0.6 percent accuracy class current transformer or better, is described. The burden setting, which is independent of current, may be made by in-phase and quadrature dials to an accuracy of about one percent of full scale.