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.

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.

Streamer corona discharge was generated by applying nanosecond-pulsed voltage with many different pulse rise times. Pulsed voltage having pulse rise time of 7-40 ns was applied to a coaxial chamber consisting of a center wire, 1 mm in diameter, and an outer electrode, 20 mm in diameter, in atmospheric pressure air. Streamer propagation in the discharge was observed using a streak camera. The propagation velocity increased with a shortening rise time of the pulsed voltage. © 2006 IEEE.

We have investigated the gas temperature during pulsed discharge. The gas temperature is one of the most important parameters for application of pulsed discharge in atmospheric gases. At atmospheric pressure gases, the gas temperature can be assumed equal to its rotational temperature. In the present work, the rotational temperature of the nitrogen molecules in a coaxial discharge reactor was obtained by synthesizing measured and calculated spectra. The emission spectrum was obtained from the second positive system of the nitrogen molecule due to the transition from the C Π state to the B Π state. As the result, rapidly increasing of the temperature (approximately 60 K) was observed at the boundary between the streamer and the glow-like discharge phases. In this paper, the spatial and the temporal dependences of the rotational temperature during pulsed discharge are reported. © 2011 The Japan Society of Applied Physics. 3 3 u g

Non-thermal plasma has been widely used in many applications, such as ozone generation, control of NOx and SOx from exhaust gases, dioxin decomposition, volatile organic compounds (VOCs) removal and laser excitation. Recently, it is demonstrated by our research group that the non-thermal plasma produced using the nano-seconds pulsed discharge has higher energy efficiencies of ozone generation and removal of pollutants in gases. However, the physical characteristics of nano-seconds pulsed streamer discharge is still not clear enough. In this paper, the axial propagations of nano-seconds pulsed discharge in a coaxial reactor were investigated by observing its streak images. In the results, influence of applied voltages and feeding gas composition against nano-seconds pulsed discharge propagation velocity are reported. © 2011 IEEE.

Nonthermal plasmas have been widely used for various applications. Observation of a discharge plasma is an essential aspect for understanding the plasma physics of this growing field. In this paper, the propagation of a general pulsed discharge having a 100-ns pulse duration is observed by taking framing and streak images and spectroscopic measurement. The results showed that two discharge phases exist in the general pulsed discharge, namely, a streamer discharge and the following glowlike discharge. Between these two phases, the electrode gap impedance changed dramatically which could cause impedance mismatching between the power generator and the electrode. In addition, the gas temperature increased about 150 K during the glowlike discharge, which causes further energy loss in plasma-enhanced chemical reactions. Consequently, it was decided to remove the glowlike discharge phase and to only have the streamer discharge. A nanosecond pulsed power generator having a pulse duration of 5 ns was developed, and the observed discharge propagation ended before it shifts to the glowlike discharge. The streamer propagation velocity with the nanosecond pulsed discharge was 6.0-8.0 mm/ns, which is much faster than that of a general pulsed discharge, and showed little difference between positive and negative voltage polarities. © 2010 IEEE.

Pulsed streamer discharges have been extensively used in many applications such as control of NOx and SO from exhaust gases, treatment of dioxins, removal of volatile organic compounds, generation of ozone and laser excitation. An operation with a high-energy efficiency is necessary for practical applications. It is very important to know the propagation mechanism of streamer discharges in order to improve the energy efficiency of pulsed discharge systems. In this paper, the emission from pulsed streamer discharges in a, coaxial electrode system in air at 0.1 MPa was observed using a high-speed gated intensified charge-coupled display camera. A concentric wire-cylinder electrodes configuration was used. Positive and negative pulsed voltages having a width of about 200 ns were applied to the central electrode. From the results, the streamer discharges were initiated at the inner electrode and terminated at the outer electrode. It is suggested that the propagation velocity of the streamer discharges is being influenced strongly by the rise time and the polarity of the pulsed voltage. 2