上野 秀樹

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.

© 2020 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. 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.

© Springer Nature Switzerland AG 2020. The abstract should summarize the contents of the paper in short terms, i.e. 150—250 words. Partial discharge characteristics of new developed foamed wire with various insulated layers with low dielectric permittivity by including small air cavities in insulating layer have been studied. Partial discharge inception voltages (PDIV) have been investigated. PDIV depends on insulation layer of enamelled wire clearly, that is, PDIV for enamelled wire using an insulator with lower dielectric permittivity foamed polyimide becomes higher by increasing air cavities in the insulated layer. Furthermore, possibility of partial discharge generation in air cavities in the insulating layer has been also studied. By theoretical electric field analysis, electric field of each cavity is relaxed lower than that at the gap between wires due to presence of a great number of cavities in foamed insulating layer, resulting in suppression of partial discharge in the cavities. Partial discharge initiates in gap between wires before discharge generation in cavities of the new developed wire. From V - t characteristics measurements, the evaluated lifetime for enamelled wire using foamed wire is improved comparing conventional unfoamed one.

© Springer Nature Switzerland AG 2020. 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.

A discharge phenomenon was investigated to clarify basic property on cavitation bubble plasma in sodium chloride solution. The discharge plasma emission was observed by using charge coupled device camera with a high speed gated image intensifier unit. Generation and annihilation of plasma in the same cavitation bubbles were continuously repeated. The arch-like discharge path was formed on upper parts of the opposite electrodes because of cavitation bubbles produced on the upper parts of the electrodes besides the rotor part., which gradually extended to the direction that the solution flowed through. The maximum height of the arch-like discharge path was 4 mm, meanwhile the voltage of 160 pulses was applied between the electrodes. It was suggested that these phenomena were repeated in the different cavitation bubbles.

We have investigatud the characteristics of radiatud electromagnetic (EM) waves from positive and negative partial0discharges (PD) in epoxy resin qnd cross-linked polyethylene. Wu found that there is a correlatyon among the EM level from PD, the positive PD current, and elestrical trees. Therefore, the growth of an electrical tree produses a lot of positive PD. We havu also investigated the characteristics of the frequency region of EM waves from PD in air, insulating oil, and liquid epoxy in addition to the above insulators. EM waves were detected in the frequency region of 40 MHz to 300 MHz from positive and negative PD in epoxy resin and cross-linked polyethylene. EM waves were also detected in the frequency region of 40 MHz to 150 MHz from positive and negative PD in air. In the case of insulating oil and liquid epoxy, EM waves were detected in the frequency regions of 40 MHz to 150 MHz from positive PD, and 40 MHz to 250 MHz from negative PD. The frequency region differed depending on the material and the discharge polarity. Our investigation indicates that the cause is differences in electric field strength at the time of PD occurrence.

A multiple needles electrode system is one of the good configurations because of easy generation of corona and dielectric barrier discharge suitable to ozone production. On this configuration, discharge and ozone generation characteristics are affected by needle configuration such as needle density, adjacent needles, but their influence has not been clarified in detail until now. In this study, the relationship between barrier discharge characteristics and ozone generation under AC voltage application has been investigated for triple needles-plane configuration with two different needle-tip curvatures (r = 15, 80 m) in dry air, which is a simpler multiple needles configuration. Characteristics of barrier discharge and ozone generation depend on the needle-tip distance and need-tip radius. In the case of the multiple needles configuration with a smaller needle-tip's distance, the barrier discharge characteristics have been strongly affected by the adjacent needles each other. Especially, the ozone production characteristics depend on the needles configurations. The configuration with a smaller needles distance has good ozone productivity compared with other configurations. In addition, ozone concentration and ozone generation efficiency for the triple needles with r = 80 m were higher than those for the needles with r = 15 m. These results can be interpreted by the effect of the space charges and accumulated charges induced by corona discharge and barrier discharge on the space for ozone generation.

We describe the characteristics of a creeping discharge developed in a narrow gap with a backside electrode, under microsecond pulse voltage application. In a gas mixture of 3% SF(6) in N(2), the flashover voltage at pressures of 0.2 and 0.3 MPa decreased by approximately 5% compared to that of 0.1 MPa when using both a filamentous straight backside electrode and a filamentous curved backside electrode. V-characteristics with decreased flashover voltage were obtained with SF(6) mixtures in N(2). We obtained V-characteristics of the flashover voltage for SF(6) concentrations in the range of D = 3 to 100% for a filamentous backside electrode. The flashover voltages obtained with a filamentous straight backside electrode were higher than those obtained with a filamentous curved backside electrode. From corona observation using an ultra-high-speed camera, it was found that the flashover is generated along the backside electrode. (C) 2010 Wiley Periodicals, Inc. Electr Eng Jpn, 171(1): 1-9, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20955

Barrier discharge characteristics have been investigated for a twin needles-to-plane electrode configuration in dry air. The characteristics of barrier discharge under ac voltage application have been investigated for various distances between two needle tips (d = 1.0-4.0 mm). We have found that corona discharge behavior strongly depends on needle-tip distance. In the case of a twin-needles configuration with a long needle-tip distance (d = 4.0 mm), discharges from the two needle tips develop into a dielectric barrier with almost a straight path. On the contrary, the development of repulsive discharges from two needle tips in the gap between needles and a barrier was obtained for the shortest needle-tip distance investigated here (d = 1.0 mm) and it was enhanced by increasing the peak voltage. From detailed time-resolved observations, development of repulsive discharge was observed only during positive polarity upon ac voltage application. Moreover, the degree of repulsion increased with increasing applied voltage of positive polarity. The observed unique discharge behavior can be interpreted as the effect of field relaxation induced not only by charge accumulation on the barrier surface, which is markedly enhanced at a short needle-tip distance, but also by space charge by coronas between two needles.

Ultrasound is a well-known diagnosis tool that can be used for the condition assessment of insulating systems. For this special purpose, various investigations on the detectability of the included inhomogeneities are performed: The resolution of ultrasonic measurements on high voltage insulations has been evaluated by using standardized Landolt rings in silicone rubber and epoxy resin. Although a dependency exists on the ring position, the diagnostic system that is used is able to image the gap with a sub-millimeter size. In other words, the system has recognition ability for two different objects close to such a distance. Regarding detectability, it has been found that in addition to ball-shaped inhomogeneities, the system can also detect thin fibrous objects with a diameter of a few micrometers. Finally, the results are used to detect and characterize electrical trees in silicone rubber.

The flashover voltage with a backside electrode was found to be lower than that without the backside electrode. Under microsecond pulse voltage application, we describe the characteristics of a creeping discharge developed in the narrow gap with the backside electrode. Using a CCD camera and ultrahigh-speed camera, we observed the corona extension processes. The lowest flashover voltage was found to be obtained at positive polarity with a SF6 content D = 3%. In the corona extension obtained using an ultrahigh-speed camera, peculiar differences were observed in the corona extension process. The corona extension increased, and rapid flashover was observed at D = 3%. Using a CCD camera, small coronas were detected from the backside electrode. (c) 2006 Wiley Periodicals, Inc.