Identification of discharges in electrical machines-a manufacturing perspective

For many years, indications of the overall manufacturing quality of insulation systems for new stator windings have been provided by the results of loss tangent and breakdown tests carried out on randomly selected coils. Routine quality control tests have been used to confirm that a minimum level performance is maintained throughout the production sequence. The only standard measure of the condition of the main insulation in the completed winding has been loss tangent values. No acceptance limits are applied due to variable influences of stress grading and end winding geometry on the results obtained. After machines have been installed, an increased desire to ensure high service reliability has led to many users undertaking regular monitoring activities employing a wide variety of techniques. Procedures have been developed in response to situations which have been encountered. Significant experience has therefore been gained with particular approaches and this, in turn, has led to them being incorporated into specifications for future supplies. As a manufacturer, it has become necessary to evaluate a number of these methods to ensure conformance with purchaser´s requirements. They can be categorised according to whether they are intended to identify discharge sites within the body of the insulation or on its surfaces