An Electronic Self-Balancing Instrument Transformer Testing Device

A self-balancing instrument transformer testing device is described by which the voltage and current errors and phase displacements of instrument transformers can be read off or recorded directly with a printer or digital instrument. The circuitry correr sponds to the usual difference circuit in which the test transforme- is compared with a standard transformer. The voltage difference between the secondary voltages of the test and the standard transformer is balanced by a voltage that is separated into two components, one being in phase with the voltage of the standard transformer (reference voltage) and the other in quadrature. The direct voltages with which the reference voltages are multiplied to generate the balancing voltage are a measure of the errors and the phase displacements of the instrument transformers. The circuits generating the reference voltages, the zero voltage amplifier, the phase-selective rectifier, and the multiplier circuit are described. The device is designed for measuring the errors of current transformers between 1 and 200 percent of rated current and the errors of voltage transformers between 40 and 120 percent of the rated voltage, to an accuracy of about 1 percent of full scale. It may also be used to measure the impedance of resistors, inductors, and capacitors.