Yasuhiko Kako

Flashover Characteristics of Extremely Long Gaps in Transformer Oil under Non-Uniform Field Conditions

Flashover characteristics of rod-rod and rod-plane electrode configurations in transformer oil are studied for extremely long gap lengths in the range of 150 to 1000 mm under conditions of lightning impulse, switching impulse, and alternating current voltages. The relation between flahsover voltage and gap length and the polarity reversal effect for switching impulse voltages are described in detail.

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.

Capability of Insulation Diagnosis for Mica-Resin Insulated Coils

A group of bar coils were subjected to accelerated insulation degradation, and the correlations between various nondestructive test results and insulation breakdown voltage were investigated. Nondestructive test values measured for only the flat part of the insulation layer of the coils showed good correlation with breakdown voltage. For the nondestructive test results measured on the entire insulation, including both flat parts and edge parts, better correlation with breakdown voltage was observed with those results which depend most strongly on the properties of the flat (or the slot) part. The edge part of the insulation does not undergo degradation even under static flexural strain; voltage degradation is saturated at an early date. In the flat part of the insulation, degradation proceeds at the same pace under either flexural voltage aging. Thus, better correlation with breakdown voltage is shown by those nondestructive measurements which more clearly express the change in the flat part of the insulation and occur without saturation of degradation. Among these correlations, it was found that as far as static flexural strain and voltage degradation are concerned, they fit on the same correlation curve.

New Voltage-Endurance Curves for Combined Thermal and Electrical Aging of Coil Insulation

The paper describes a method for estimating the life of high-voltage coil insulation under multiple stress conditions. Thermal and electrical stresses are considered. The final aim is to predict the life of insulation under normal voltage at normal temperature. The lifetime of the insulation is defined as the time until the residual ac short-time breakdown voltage decreases to the withstand test voltage, 2E+3 kV, where E is the rated voltage. We propose a new V-t curve, in which the lives under various multiple stress conditions can be interpolated by a graphical method. For example, from a series of accelerated aging tests on class B, 11 kV mica-epoxy coil insulation, new V-t curves for various temperatures have been derived. The life of the insulation under normal voltage (6.4 kV) at normal temperature (130 ° C) was estimated to be about 49 years. However, for real machine windings we must consider the effect of additional stresses, mechanical and environmental, and therefore, the revised lifetime of the insulation was assumed to be about 19 years. It was concluded that detailed examinations or insulation diagnosis should be made on machine insulation in service for over 15 years.

Study on Corona Suppression for Fusion Reactor Coils

Fusion reactor coils are subjected to 50 Hz ac voltages (for overvoltage tests) and impulsive voltages whose rise time ranges from 1 ¿s to 1 ms (for fusion experiments). This paper shows that SiC paint is ideal as a corona suppression coating for such coils. Also, in a fusion reactor, two coils may be arranged in parallel facing each other across a narrow air gap. In such parallel coils, the junction of the conducting paint and SiC paint may shift. This paper shows that, in such cases, the corona starting voltage at the air gap is lowered and a method to raise it is proposed.

A Simple Method to Derive Correlation Curves for Insulation Diagnosis

In insulation diagnosis it is essential to know the correlation between some nondestructive measurements and the residual breakdown voltage. All the nondestructive measurements should be translated to the assumed residual breakdown voltages with the aid of correlation curves. This paper discusses a simple method to derive the correlation curves. Model coils of epoxy-mica splittings insulation were subjected to voltage-endurance tests. Nondestructive tests were also conducted at alloted intervals during the voltage application. The change of the residual breakdown voltage with time was assumed on the basis of certain calculations. The measured nondestructive values were related to the assumed residual breakdown voltages at each time of measurement, and the correlation curves were drawn. In our previous paper [l], the correlation curves were derived from nondestructive and destructive test data of a number of aged coils. In our new method, we made use of only a few coils and were able to derive the same correlation curves.

New Evaluation Method of Dielectric Materials Using a Microwave Technique

A new nondestructive method of evaluating changes in characteristics or detecting internal defects in insulation materials with microwave techniques is discussed. The main feature of the method is that the amplitude and phase angle of a detected signal are displayed in a polar co-ordinate system, which allows changes in both the dielectric constant and loss tangent of a dielectric material to be obtained at the same time. The results of experiment on deterioration, the detection of foreign materials, and differences in the inner structure of complex insulation materials are shown.