Influence of bow-tie trees on residual breakdown voltage of dry-cured XLPE power cables after accelerated water immersion aging test

This paper describes the change of the residual AC breakdown voltage of 6.6 kV dry-cured XLPE cables after an accelerated later immersion aging test. The accelerated test was carried out at room temperature. The vented trees were not observed. Therefore we investigated the bow-tie trees effect on the AC breakdown voltages in detail. In the breakdown tests, the initial voltage step was 35 kV/hour and subsequent voltage steps were 5 kV/half hour (Method I). Several samples exhibited the AC breakdown voltage of 35 kV which is the initial voltage step. To clarify the causes of these, we changed the initial voltage step to 5 kV/half hour (Method II). In this case, the AC breakdown voltages increased and the lowest breakdown voltage was 75 kV. We suppose that this increase is largely caused by moisture condensing at the bow-tie trees during the breakdown voltage test. Furthermore, we used the pre-breakdown partial discharge detection method to identify the starting point of the breakdown. The electrical trees start from the bow-tie trees which have been touched by the inner or the outer semi-conductive shields. In order to verify that the increase of the AC breakdown voltage is caused by the moisture condensing at the bow-tie trees, we raised the water immersion temperature to 60°C and 85°C. We then carried out the AC breakdown tests (using Method I) and the pulse breakdown tests respectively. From these results, we propose that the field relaxation is the main cause of higher AC breakdown voltage using Method II