Teruhiko Maeda

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.

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.

A sensing system for degradation diagnosis of oil-filled transformers

Under normal operating conditions the insulation of power transformers undergoes gradual deterioration that produces various degradation by-products. Properties of the insulation, such as degree of paper polymerization, also change. To evaluate the degradation state of the insulation the detection and analysis of the degradation by-products have been widely performed. However, the diagnostic techniques presently used are rather complicated and time consuming. This paper describes a new diagnostic technique for oil-filled transformers based on a semiconductor-type sensor. The technique can be used for in-situ measurements in the field, requires very small amounts of oil for analysis and allows for rapid test data analysis. The detection of ketonic by-products with the newly developed sensor was found to be very useful as a diagnostic tool.<<ETX>>

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.

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.

Influence of defect pressure on insulation characteristics of molded transformer model

This paper discusses the influence of pressure in a defect on time evolution of Partial discharge (PD) characteristics and lifetime characteristics of an insulation model of a molded transformer. Experimental results show that the time evolution of PD characteristics for a defect at atmospheric pressure is different from that for a defect at a reduced pressure. Secondly, the lifetime of a sample with an atmospheric pressure defect is short, since samples containing a defect at atmospheric pressure can lead to breakdown with a small amount of PD charge.

A swelling characteristics of PET in oil

The swelling behavior of PET (polyethylene terepthalate) film immersed in insulating liquid was investigated, and the swelling kinetics of the PET was evaluated. It was found that the swelling behavior can be measured more effectively by a weight measurement than by a thickness measurement. The swelling process was found to be proportional to the square root of time. The swelling of the PET caused a change in the glass transition temperature, which can be explained by Foxs equation.<<ETX>>