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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.