A Transistorized-Bushing Potential Device

A transistorized-bushing potential device has been developed which provides an economical means of obtaining instrument and protective-relay voltages from power transmission systems. The device described in this paper includes a power supply, input circuits, and four transistor amplifiers. In the application described, the input signals which are to be amplified are obtained from taps on the condenser bushings of a 46-kv power circuit breaker. The device output voltages are proportional to the system line and zero-sequence voltages and are used to energize the directional element potential coils of induction-type overcurrent relays. Power for the amplifiers is obtained from a remote operating transformer and from a current transformer in the power circuit breaker. The paper describes the circuitry used and shows test results.