Ultrasonic Corona Detection in Transformers

Corona detection in liquid-filled transformers is becoming recognized as a significant means of ascertaining the integrity of the insulation system. One shortcoming of detectors previously used was their limitation in indicating only the presence or absence of corona without regard to physical location. Techniques have been developed now to utilize the grossly different travel times of the electrical and pressure signals produced by a corona discharge to determine the location of the source. The high-frequency electrical signal arrives at the transformer terminals instantaneously and triggers the oscilloscope sweep. Pressure waves, traveling at much slower speeds through the insulation, arrive at the transducer and are converted to electric signals, which are displayed as the vertical deflection on the oscilloscope screen. The time interval between the start of the oscilloscope sweep and the first vertical deflection is a measure of the distance between the corona source and pressure transducer. Triangulation procedures may be required to discern the exact location of the corona source. These procedures may be used during factory insulation tests or on transformers operating in the field.