Discrepancies of PD patterns from a fixed metal object on oil-paper insulation

Partial discharge (PD) has long been thought to be one of the most important indications of the condition of high power plant, not only as quality assurance by manufacturers but also by plant operators during the service. Both types of measurement take place as a short period of activity during which, higher than working stress is applied. In the test regimes, the widely used phi-q-n pattern of discharge activity, when compared with existing PD characteristic databases from different defect forms, is crucial to determining the fault type and severity and in diagnosing the condition of the insulation system. The availability and validity such a database is essential and there have been unremitting efforts in both research and industrial applications. In this paper the characteristics of phi-q-n patterns for metal particles entrapped in oil-paper are extensively investigated using an IEC60270 PD measurement system. The test pieces are formed by wrapping a brass conductor with insulation paper, at the final layer of paper a spherical steel ball is included to simulate the trapped particle. The complete construct is immersed in oil so that a natural defect in a transformer is vividly simulated. Different values for the key factors are applied to reveal the resulted changes of phi-q-n pattern of discharge activity. Conventional criteria are calculated to characterize the different patterns. Oil samples for DGA are also collected, before and after each stressing period