Distribution laws of partial discharge and surface flashover of insulation cushion blocks for transformers

With the rapid development of HVDC, the converter transformer, which is a key component of HVDC, often has insulation failures. The oil-pressboard insulation structure is the main insulation of the converter transformer, and the insulation performance of the insulation cushion block is a key parameter required in the insulation design. Therefore, it is necessary to study the discharge characteristics of insulation cushion block. In this paper, the partial discharge initiation characteristics and surface flashover characteristics of three kinds of insulation cushion blocks produced by three manufacturers under AC voltage were tested. The test results show that under the parallel electric field component, the initial field strength of partial discharge of rectangular and T-shaped single slot cushion blocks is relatively close, both of which are about 1.7kV/mm. The minimum initial field strength of the rectangular T-shaped double slot cushion blocks is 1.3kv/mm, and the maximum difference between the three manufacturers is 21.2%. Under the vertical electric field component, the starting characteristic trend of partial discharge is the same as that under the parallel component, in which the rectangular and T-shaped single slot cushion blocks are between 4.63kV/mm and 5.54kV/mm. The rectangular T-shaped double slot cushion block has an average of 4.16kV/mm, which is still the lowest, with a maximum difference of 20.7%. For the surface flashover characteristics, the creepage flashover field strengths of the three kinds of cushion blocks are not significantly different, which are 2.3kV/mm, 2.32kV/mm and 2.19kV/m respectively.