Tsutomu Tsukui

An Analysis of Combined Stress Degradation of Rotating Machine Insulation

This paper presents a new method for estimating the life of rotating machine insulation under combined stresses. Since there already exist experimental formulae for each stress degradation (thermal, voltage, and mechanical), the method aims to derive a new formula for multiple stress degradation. The residual breakdown voltage should be a unified measure of degradation. In the method, the amounts of each single stress degradation are converted into that of voltage degradation, with the aid of the unified measure. The total degradation is estimated by summing up these amounts. When it attains the critical value of an insulation, the insulation loses its properties. Voltage endurance curves have been expressed as straight lines on either log-log or semi-log coordinate. Our new method of estimating the life of insulation under combined stresses is applicable to both cases. The interactive effects between stresses can be represented as the change of voltage degradation rate, and the residual breakdown voltage characteristics can be explained with no contradictions to the life characteristics.

Correlations between Nondestructive and Destructive Tests on High-Voltage Coil Insulations for Rotating Miachines

The correlations between results of nondestructive and destructive tests on high-voltage coil insulations for rotating machines were investigated and the following findings were obtained. Breakdown voltage on application of a commercial line voltage (50 Hz) is correlated to the voltage at which a partial discharge bridges the insulation layer. Breakdown voltage on application of a direct voltage is correlated with the discharge magnitude, dielectric constant and dielectric loss tangent corresponding to void discharge in the insulation layer. Breakdown under an impulse waveform voltage is in some respects similar to that under commercial frequency but, because of the short duration of voltage application, it is correlated with the magnitude of electrical resistance, and this correlation is related to the ease with which the bridge phenomenon of discharge in the insulation layer propagates. Thus, the correlation between breakdown voltage and electrical characteristics in nondestructive tests varies with the voltage waveform involved. In tapewound laminated structures, as in micaceous insulatopms, breakdown occurs along the lap of tape, so that characteristic phenomena of the breakdown process for various voltage waveforms combining mutually correlated characteristics, are observed.

Aging Properties of Mica Insulation Systems Impregnated with Solventless Resin for Generator Stator Coils

In these days, the solventless resin-impregnated mica insulation systems are extensively adopted to the large powergenerator stator-coil insulation systems as a result of rapid development of synthetic resin in recent years. Various destructive and nondestructive tests with these samples were made. The electrical properties were compared between the initial coils (which were measured by the electrical properties at the acceptance test) and the used coils (which were taken out from the generator winding that was used for about ten years). It is said that the winding insulation systems will change during long running times because of a number of factors such as thermal, mechanical, and electrical. Studies by the present authors show that used coils were not deteriorated by mechanical stress because electrical breakdown voltages did not decline in comparison with those of the initial coils, and also they were not deteriorated by electrical stress because the breakdown voltages of coils at the line side were lower than at the earth side. Therefore, some changes of the electrical properties in the nondestructive test seemed to result from thermal effects. As mentioned above, it is proved that solventless resin-impregnated mica insulation systems have kept good electrical properties in service for over ten years.

Aging properties of mica insulation systems impregnated with solventless resin for generator stator coils

As stated in the above, the non-destructive test with solventless coil having been kept in operation for approximately 10 years showed some changes in the electrical properties, but the breakdown voltage remained unchanged. When the electrical properties of solventless coil was compared with those of asphalt coil obtained form the literature, the solventless coil was proved to be thermally stable, less hydroscopic, its electrical properties being less affected by aging.