Partial discharge patterns and the identification of defects in high voltage stator windings insulation

Reliable operation of high voltage (HV) equipment depends largely on the integrity of its insulation. Therefore, the condition of insulating systems, and the early prediction of faults and problems are of major concern to high voltage equipment's manufacturers and users. They allow corrective maintenance and planning and reduce the overall maintenance costs. Partial Electrical discharges (PD) are both a factor that can cause the degradation, ageing and breakdown of HV insulation systems, and an indicator of the actual state and condition of the insulation. Thus, reliable periodic measurement and interpretation of PD activities are of great importance to manufacturer and user of HV equipment. They can enable the detection of a range of insulation problems, which can then be addressed in order to prevent the occurrence of failure and controlled outage of equipment. The introduction of computers and digital equipment has greatly benefited the detection, measurement and identification of PDs. It has allowed better analysis and interpretation of complicated discharge distributions, and providing fast data acquisition, processing and vast amounts of memory space for data storage. An on-line partial discharge detection system has been developed by ELECO (Dowding & Mills), based on more than 12 years experience of PD field tests. The system has been used over the years to monitor, and evaluate the condition of insulating used in the stator windings of HV machines. This paper discuses the relation between the discharge patterns and identification of insulation problems in high voltage stator windings, and how this can help the manufacturer and user to assess ageing and controlled outage of equipment. Practical typical examples from site tests, demonstrating this relationship, are also presented.