Partial discharge activity in distribution MOSAs due to internal moisture

In order to evaluate if metal oxide surge arrester present any partial discharge (PD) in case of internal moisture and how applicable it is as a condition monitoring diagnostic method, 8 distribution class (Ucov = 20 kV) metal oxide surge arresters (6 silicone housed and 2 Ethylen-Vinylacetate (EVA) housed) from four different manufacturers (A, B, C and D) were submitted to an immersion test for 19 days. The partial discharge and leakage current levels were measured daily. Parameters like largest repeatedly occurring PD magnitude, discharge current and PD repetition rate were recorded for later comparison with power loss calculated from the leakage current. In some of the arresters internal moisture was noticed, however, partial discharge measurement was not as sensitive as power loss. For those samples identified with internal moisture, the partial discharge activity showed a special behavior keeping a limited amplitude itself but presenting a high repetition rate.