Evaluation of electrical insulation quality for high voltage stator windings machines through visual inspection of dissected coils

The long term reliability of armature windings for high voltage machines can be predicted with a high level of confidence based on properly performed dissection of sample coils. Coil dissection and analysis is a very efficient and quick way of evaluating insulation quality. This article will describe the basic methodology for coil dissection and will give examples of different types of insulation deficiencies for insulation made both with the Vacuum Pressure Impregnation (VPI) process and the Resin Rich (RR) process (also known as B-stage tape process). Examples of good quality of insulation will be also presented. This article in no way is intended to cover the whole spectrum of all possible problems that can be found in dissected coils. The purpose of the article is to provide a guideline for coil evaluation based on the authors' experience. Special discussion is also given for artifacts that could be created during the section preparations. Coil dissection for failure evaluation and Root Cause Analysis is not discussed in this article. In the case of electrical failure, a special methodology of dissection is required. That approach will be presented in another follow up article by the same authors.