In this paper

the breakdown point of discharge is found by oil chromatograph analysis and core suspension inspection

and the construction defects of lacking insulation pad between neutral point and low voltage lead are determined by comparing the design drawings

the charge simulation method and the finite element method are separately used to build the model and calculate the field intensity distribution between the leads. The results show that the field strength between electrodes is up to 200 kV/cm

the breakdown probability is large

but the maximum field strength is not more than 40 kV/cm after standard installation

and the insulation margin is large. At last

the preventive measures are put forward

which is beneficial to the quality control of equipment and the analysis of insulation examination.