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Development rules of inter-turn partial discharge defects on oil-paper insulation

In power transformers, the main reason why insulation faults occur is that partial discharge (PD) produces insulation deterioration at the weakest point of the insulation structure and ultimately lead to insulation breakdown. The insulation defects exist in the form of electrical trees or carbon channels reduce the effective insulating distance. Take the opaqueness of pressboard into consideration, the carbon channels generated by PD can't be easily observed. Therefore, in this paper, tests based on inter-turn partial discharge model are conducted to study the development rules of oil-paper insulation material defects. Constant voltage application was adopted in this series of tests. High voltage of 40 kV is applied on the inter-turn electrode specimens for different time in different groups of tests. After test, insulation tapes are stripped from inter-turn electrodes layer by layer so as to observe the carbon channels clearly. Results indicate that the development process of the carbon channel can be roughly divided into three stages. Analysis of discharge feature reveals two different shapes of q-φ scatter diagrams, the shape of seeds and the shape of inverted tacks. The shape shown in scatter diagram could be used as effective criteria to diagnose the inter-turn insulation defects produced by partial discharge at later stage.