Masakazu Higashiyama

Partial discharge phenomena in artificial cavity in epoxy cast resin insulation system

It is crucial for proper insulation design of cast resin transformer to consider voids and delamination which might exist in cast molding because of several surface boundaries between resin and conductor. Such defects in the insulator lead to reduction of the life of the apparatus. In this report, we investigate the relation between the void size and apparent charge of partial discharge (PD) occurring in a model simulating the insulation system of cast resin transformer. It is also important to determine necessary PD detection sensitivity of PD test in a factory as well as in a field. In this paper, we also discuss the detection sensitivity by considering the relation between charge and void radius using formula derived by L.Niemyer, M.Pedersen and their colleagues in the late 1980?s to apply to the case of the cast resin transformer. Experiments were also performed to obtain PD inception voltage (PDIV) of epoxy resin slab including an artificial void with a given size. Investigation was also made on the effect of X-ray irradiation to the epoxy resin on PDIV characteristics.

Effect of X-ray irradiation on partial discharge inception and extinction voltage characteristics of cast resin transformer insulation system

In general, it is hard for discharge to be initiated in a small void existing in solid insulating material because of insufficient initial electrons. For this reason, the discharge occurs after a long delay even at a critical electric field to initiate discharge. In this study, the initial electrons are supplied by X-ray irradiation to a cavity formed in epoxy resin (ER) sample. We have investigated PD characteristics of ER with spherical void. Additionally, we investigated the effect of X-ray irradiation on PD inception voltage (PDIV) and PD extinction voltage (PDEV) of the sample with delamination. It was found that the lowest and stable PDIV was brought by the X-ray irradiation for the samples with the cavity and delamination. Furthermore, we estimated quantitatively attenuation of X-ray dose by the structure such as winding, an iron core of a cast resin transformer. Discharge time lag was estimated from X-ray dose, and compared with that measured for the ER sample with the cavity. This report also deals with the effect of X-ray irradiation on PD generating characteristics in various artificial detects of epoxy insulation system simulating insulation of winding, delamination, and void.

Study on time lag of void discharge in epoxy resin by considering attenuation of X-ray irradiation dose

It is crucial for proper insulation design of cast resin transformer to consider voids and delamination which might exist in cast molding process and/or under long-term operation because of several surface boundaries between resin and conductor. Should such defects in the insulation system exist, it would lead to reduction of the life of the apparatus. In this report, we investigate X-ray irradiation induced discharge of spherical void in epoxy resin. Physical consideration of the effect of X-ray irradiation on void discharges in epoxy resin was also made. Time lag of void discharges in epoxy resin was also made with attenuation of X-ray irradiation dose considered.

Partial discharges of small-air-gap in cast-resin insulation system

Partial discharges of small-air-gap for the molded insulation system has been investigated in view of the Paschen curve. It was found that Vs (partial discharge inception voltage: PDIV) in a smaller Pd (pressure times gap distance) region than so-called Paschens minimum was determined in terms of glow-like discharge with micro-second order of discharge-current probably due to the ionic current for both MGM (Metal-Gap-Metal) and MGI (Metal-Gap-Insulation) electrode systems. In the Paschens minimum and the larger Pd regions for MGM, the partial discharge behavior showed a streamer-like discharge current waveform with relatively short rise-time of an order of 10 ns or less and short discharge-duration. For the MGI electrode system, on the other hand, the discharge type was found to show glow-like discharge with small discharge current of less than few A. The glow-like discharge, Townsend and/or streamer discharges were also recognized by the high speed CCD camera photographs.

Study of direction identification of partial discharge using multi small loop sensors

This paper describes the study on the identification location of partial discharge (PD) which occur at a faulty point in electric equipment. We identified the location of PD utilizing arrival time differences of electromagnetic wave emitted from PD between each loop sensor. An attempt is made to perform direction identification of PD source using the four small loop sensors placed in a fixed interval each other. It is found that PD direction can be identified with angle error of about plus or minus 25 degrees.

X-ray irradiation induced discharge of spherical void in epoxy resin

It is crucial for proper insulation design of cast resin transformer to consider voids and delamination which might exist in cast molding process and/or under long-term operation because of several surface boundaries between resin and conductor. Should such defects in the insulation system exist, it would lead to reduction of the life of the apparatus. In this report, we investigate the relation between the void size and apparent charge of partial discharge (PD) occurring in a model simulating the insulation system of cast resin transformer. It is also important to determine necessary PD detection sensitivity of PD test in a factory as well as in a field. In addition, we investigate X-ray irradiation induced discharge of spherical void in epoxy resin. Physical consideration of the effect of X-ray irradiation on void discharges in epoxy resin was also made. Time lag of void discharges in epoxy resin was also made with attenuation of X-ray irradiation dose considered.

Breakdown voltage of interelectrode gap between sphere-plane electrodes under ac voltage

The breakdown voltage in a small spacing under dc voltage application has been well described with the Paschen law. The present paper differs from the previous works in that we employ ac voltage. The electrode system employed is sphere-plane electrodes and the sphere and/or sphere electrodes are coated with an epoxy resin. The breakdown voltage of air gap of the Metal (M) - Air gap (G) - M, MG-Epoxy layer (I), IGI electrode systems is measured using the advantage of the excited particles from the previous breakdown under a fast rate-of rise of ac voltage. It is found that the breakdown voltage decreases by employing the epoxy coating on electrode in the whole range of the production Pd from 0.3 to 90 Pa·m. In the range of the small product Pd from 0.3 to 5 Pa·m, the possibility of breakdown becomes high when the polarity of brass electrode is negative for MGI electrode system. It is concluded that the epoxy coated electrode decreases the breakdown of air and enhances electric field for the small spacing.

Impulse and subsequent AC partial discharge properties in cast resin transformer

Withstand lightning impulse voltage test is generally carried out for guaranteeing equipments insulation performance. On the other hand, it is concerned that other insulation performances are influenced by applying impulse voltage. For example, an AC PD (partial discharge) test changes after a number of impulse voltage prestress is applied to a cast resin transformer (CRTr). This paper deals with detection of impulse PD and subsequent ac partial discharge properties in an actual CRTr having artificial defects in manufacturing process. The lightning impulse voltage prestress effect is evaluated by measuring PD inception voltage (PDIV) and PD extinction voltage (PDEV) before and after the prestress.