Breakdown characteristics of typical model in transformer oil under AC and DC mixed voltage

In the HVDC transmission system, the valve side insulation of the converter transformer endures the AC and DC voltage simultaneously, and that could cause insulation failures in the converter transformer. Unfortunately, there is no method to simulate its working condition, so AC and DC test voltage are respectively imposed on the transformer during the factory test. There is a lack of research on the influences of AC and DC mixed voltage on the oil-pressboard insulation characteristics. In this paper a needle-plate electrode model is designed, and different values of AC and DC mixed voltage are imposed on the oil surrounded needle-plate electrode model to study the differences of breakdown characteristics. The test reveals different effects of AC and DC voltage component on the insulation model breakdown characteristics. The results indicate that, the AC breakdown voltage value decreases linearly as the DC pre-voltage value increases and vice versa. In addition, different pre-applied voltages affect the test results remarkably, that is, the breakdown voltage value under AC pre-voltage is higher than that under DC pre-voltage.