Noboru Hosokawa

Dielectric strength of transformer insulation at DC polarity reversal

Among special insulation tests for DC power converter transformers, polarity reversal tests can be substituted by applied AC voltage tests from a stress similarity in oil ducts. In order to investigate equivalent levels of other insulation tests to polarity reversal tests, the dielectric strength of a transformer at polarity reversals was examined by use of a model of typical oil/paper composite insulation. From the comparison of polarity reversal and other insulation tests, it was found that dielectric strength of transformer insulation at polarity reversals is phenomenally similar to that under switching impulse stresses. Equivalent AC test voltage to polarity reversal test was also estimated from the experimental results. The breakdown characteristics of polarity reversal tests showed that partial discharges at polarity reversals impose less harmful stress on insulation barriers than those at usual AC or impulse tests.

Major factors influencing static electrification in an aged transformer

Increase in static electrification was experienced during the service operation of aged power transformers. The increase was noted a few years after on-site replace of the bushings and conservator. Analysis of the dissolved gas in the transformer oil suggested that slow intrusion of air, which could have led to the increase in static electrification, took place after the on-site work. However, the electrical characteristics of the oil did not demonstrate increase in static electrification. The investigation on the aged oil by ECT (electrostatic charging tendency) measurement suggested the possibility that some deteriorated compound in oil was adsorbed to the pressboard and increased the static electrification. However, the ECT measurement with the aged pressboard did not demonstrate enough increase, which suggested the necessity of the evaluation of the accumulated charge on the pressboard for assessing static electrification in transformers. At the static electrification tests with the model transformer using the aged pressboard and oil, strong acceleration effect of oxygen on the increase of static electrification was demonstrated. The rapid increase of the static electrification was succeeded by gradual decrease later. This suggested the possibility that yet unknown transient products by oxygen injection played strong role on the static electrification.

Principal decomposition by-products generated at various abnormalities in gas-insulated transformers

It is pointed out that the application of decomposition gas analysis based on GC and IC (gas and ion chromatography) methods provides a promising practical means of predictive diagnosis of gas-insulated transformers. As the first step of developing a diagnostic method, model tests simulating important modes of abnormalities in a gas-insulated transformer were carried out. Major components characteristic of each abnormality such as SO/sub 2/ in the case of metal overheat and SO/sub 2/F/sub 2/ in the case of partial discharges were identified by the experiments. Attention is also paid to the effect of material differences, which is a major factor differentiating the gas-insulated transformer from other gas-insulated apparatus. >

Distinction of the partial discharge source in oil by two frequency correlation method

A method based on frequency characteristics to distinguish partial discharge (PD) source in oil filled transformer is proposed. Frequency spectrum of electromagnetic wave from two typical PD sources, i.e., protrusion of metal and air void in oil was investigated. It was found that metal protrusion gave PD signal with stronger component in the frequency range above several hundreds MHz compared with air void. Since the acquisition of frequency spectrum is difficult in practical transformer test condition, we tried to apply the two frequency correlation method to distinguish the two PD sources. In the method, intensity of two frequency components of each PD signal is plotted on the x-y plane, where the horizontal axis represents the intensity of low frequency band and vertical axis represents that of high frequency band, which is a method to characterize a PD source on the x-y plane. The two PD sources were plotted clearly on the different area of the two frequency correlation graph, that is, metal protrusion yielded PD signal with higher frequency component than air void. We concluded that the method is effective to identify the PD source in oil filled transformer.

Increase of Static Electrification in Aged Transformers

Increase in static electrification and discharge generation was experienced during the service operation of aged power transformers of both shell and core-form. However, the electrical characteristics of the oil did not demonstrate increase in static electrification. The investigation on the aged oil by measurement of electrostatic charging tendency suggested the possibility that some deteriorated compound in oil was adsorbed to the pressboard and increased the static electrification. However, the measurement of electrostatic charging tendency with the aged pressboard did not demonstrate enough increase, whereas accumulated charge measurement of the aged pressboard confirmed strong increase in static electrification. On the other hand, the measurement of accumulated charge by potential measurement confirmed strong increase of charge accumulation by the pressboard aging. It was suggested that measurement of accumulated charge was suitable for the assessment of static electrification in aged transformers.