Development of Partial Discharge Diagnosis Method by Focusing on Ultrasonic Frequency

An early detection of abnormality through partial discharge measurement is important for the maintenance of the power quality of equipment. Thus, our group has developed a diagnostic method measuring the ultrasonic waves associated with the partial discharge occurring in a metal casing. As a characteristic test of ultrasonic waves, the discharge current and the ultrasonic wave signal associated with the partial discharge were simultaneously measured. An analysis of each detected signal revealed that there is a possibility that the frequency components of the ultrasonic wave signal have a correlation with the partial discharge charge amount. Ultrasonic waves are elastic, thus the thermal expansion of the medium has an influence on the waveform. Therefore, it is considered a reasonable result that the frequency characteristics of the ultrasonic waves are associated with a partial discharge. Incidentally, it is necessary to estimate the partial discharge charge amount in the case of diagnosing insulation degradation. The partial discharge charge amount is usually determined by the signal strength. Thus, it is necessary to specify the location of the partial discharge occurrence, because the signal intensity of ultrasonic waves decreases with distance. However, by using the knowledge about the frequency characteristics obtained in this work, it becomes possible to estimate the partial discharge charge amount through the ultrasonic wave frequency, without specifying the partial discharge location. In order to evaluate the possibility of the application to actual equipment, our diagnosis method was tested on an oil filled equipment with insulation failure. The test was performed monthly over a three-month period. Each resulting estimated charge amount was less than 1000 pC. Further, a dissolved gas analysis was also carried out each time. The result indicated there was no noticeable change in the acetylene content. Acetylene is generated when a discharge of over 1000 pC occurs. From the above results, the transition value is considered as a reasonable result. In the future, we will try to determine the characteristics of an ultrasonic wave signal accompanying a partial discharge. We also plan to improve the accuracy of a partial discharge diagnostic method.