岡田 翔

It is well-known that acoustic waves are generated by partial discharge (PD), however, the wave generation mechanism remains unclear. To this end, in this study, a PD test is performed to measure the acoustic waves caused by PD in insulating oil. A needle-plate electrode is applied to generate PD, and the acoustic waves are detected by an acoustic emission (AE) sensor. The PD current and AE signal associated with PD are simultaneously measured, and their relationship is analyzed. The results indicate that the frequency components of an AE signal vary with the PD current duration time. Therefore, the amount of PD charge used as a parameter in dielectric degradation can be derived from the time integral of the PD current. Thus, the acoustic frequency can help estimate the amount of PD charge and the degradation of oil-immersed electric equipment. Furthermore, a possibility of increasing the accuracy of the PD diagnosis method by focusing on AE signal frequency is established.

Ozone has much attention in many industrial fields. To materialize higher ozone generation efficiency, it is necessary to elucidate the relationship between discharge characteristics for discharge electrode system and ozone generation concentration. To clarify the improvement of mechanism on the multiple needle-to-plane configurations, the discharge characteristics and ozone generation characteristics for a multiple needle electrode system with various needle tip distances have been investigated. In this experiment, the maximum ozone yield at needle tip interval of d = 0.5 mm was improved by the minimum discharge power, the local maximal at d = 0.5 mm of discharge volume and the same ozone generation compared with those for d = 0 mm and d = 1 mm. © 2020 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

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.