Kenzo Kadotani

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.

A Proposal for Insulation Diagnosis of 3 KV Motor Stator Windings

This paper presents a proposal for insulation diagnosis of 3 kV induction motor stator windings of mica splittings and solvent-type thermosetting resin insulation. Nondestructive and destructive tests were conducted on 29 windings it operation for 15 years or more. The average breakdown voltage (BDV), Vave of the coils was estimated using 10 to 12 coils from each winding. The relationship between Vave and Vmin, the BDV of the winding and the BDV of the worst coil in the winding, was determined. The criterion for insulation renewal, generally Vmin¿7 kV (the withstand test voltage), could also be expressed in terms of Vave, with Vave¿l5 kV. Several nondestructive measurements were also examined for their ability to forecast Vave or Vmin. Although the dissipation factor tip-up (¿tan¿) should be related to Vave, no useful correlation was found. Partial discharge starting voltage (Vi) and the first turning point voltage (Pil) on the I-V curve in an ac current test proved to have comparatively good correlation with Vmin. A criterion for insulation diagnosis of that insulation is proposed, i.e. when Vi or Pil has decreased to 3 kV, the rated voltage, the insulation of the winding should be replaced.

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.

An Approach for Insulation Diagnosis of Mica-Resin Coils

Used coils from a hydro-generator and an induction motor both in operation for many years, and also model coils subjected to accelerated agings were examined concerning the correlationship between the residual breakdown voltage VR and two nondestructive measurements ¿and Qm. It was found that VR could be expressed by the following equation, Uniformly degraded coils and partially and abnormally degraded coils could be classified by plotting the nondestructive measurements on X ¿Y coordinates where the X axis represents log Qm and Y axis, ¿.

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.

An Analysis of Multi-Factor Aging of Mica-Epoxy Insulation Systems by the Infinite Sequential Stress Method

In most cases the infinite sequential stressing method can be applied in the analysis of multi-factor aging of insulation systems. Although the simultaneous multi-factor aging characteristics cannot be reproduced strictly by sequential stress tests, the characteristic equation for multi-factor aging can be derived by combining all single-factor aging characteristics in sequential stress aging. Generally, such characteristic equations can be represented only in numerical form, but in the case of mica-epoxy insulation systems in which the value of n in the inverse n-th power law remains unchanged during thermal aging, and also the value of thermal aging rate remains unchanged during voltage aging, the characteristic equations for multi-factor aging can be derived in an analytical form. The mechanical aging characteristics determined by repeated bending tests seem to be of two types, uniform degradation, similar to thermal aging, and local degradation, similar to voltage aging. The ratio of these two types depends on the mica (mica-paper or mica-splitting). Characteristic equations for simultaneous thermal, voltage, and mechanical aging also can be derived by the infinite sequential stressing method. This method assumes an infinite repetition of the four kinds of aging: thermal, voltage, uniform mechanical, and local mechanical.

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.

Mechanical Properties of Low Thermal Contraction GFRP

Liquid helium duct spacers in large superconducting magnets must have high compressive strength to withstand large electromagnetic forces. They must also show a low thermal contraction, in the direction perpendicular to the laminations, to maintain windings stability at liquid helium temperature. To attain higher glass content or higher density, we adopted glass roving cloth with mineral fillers and unidirectional woven glass cloths. E-,S-, and R-glass cloths were examined as reinforcements and epoxy and bismaleimide triazine, as matrix resins.

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.

An analysis of insulation ageing of generator stator windings

The ‘D Map’ expression for insulation diagnosis was revised to fit for actual large generator windings with transposition of conductors.