Means for detecting the inception of corona discharges within induction apparatus

Claim 1

Corona-detecting means for detecting the inception of corona discharges within single phase induction apparatus that comprises a tank containing a liquid dielectric, a high voltage winding within the tank immersed in said liquid, and a high voltage bushing comprising a conductor extending into the tank for carrying current to and from said winding, said corona-detecting means comprising: a. corona-level detector means sensitive to the voltage on the power circuit phase in which said conductor and winding are connected for developing a first electrical signal pulse in response to the appearance of corona-produced, radio-frequency voltage exceeding a predetermined amplitude on said power circuit phase during any one of predetermined corona-level sampling periods, b. a pressure-wave detector comprising a transducer arranged to receive and sensitive to ultrasonic pressure waves in said liquid dielectric for developing a second electrical signal pulse in response to the receipt of ultrasonic pressure waves exceeding a predetermined amplitude by said transducer during any one of predetermined pressure-wave sampling periods, c. a coincidence circuit to which said first and second signal pulses are supplied for developing a first-stage output signal in response to receipt of both said first and second signal pulses during any one of predetermined monitoring periods, d. and counting means for developing a second-stage output in response to the occurrence of a predetermined minimum number of said first-stage output signals over a predetermined period of time comprising a plurality of said monitoring periods.