Diagnostic testing of high-voltage machine insulation. A review of ten years' experience in the field

Following a review of the basic philosophy of preventive maintenance testing and the theory of degradation mechanisms leading to failure, available test methods used for assessing the state of high-voltage machine insulation are discussed. Preferred test methods introduced ten years ago for field measurements between the winding and the grounded core are detailed, together with the reasons for their selection and their significance. Test data obtained from measurements on nearly five hundred machines are presented in terms of eight criteria and four groupings based on rated voltage, namely 3-4.9 kV, 5-7.9 kV, 8-12.9 kV, and 13 kV and above. To illustrate the practical application of the diagnostic data, a number of case histories are briefly outlined. It is concluded that a standardised programme of nondestructive measurements carried out periodically on high-voltage stator windings can identify trends in generalised degradation, reduce unplanned outages and allow refurbishment to be carried out at an early stage. Of the several criteria used, the measurements of integrated discharge energy and associated charge/voltage loop trace displays have been particularly helpful in indicating changes in structural integrity associated with cumulative degradation. They have also been of value in detecting anomalous discharging. The need is recognised for additional diagnostic tests to be developed to detect localised defects.