Analysis on Failure causes of bushing used in 40.5kV metal enclosed air insulation switchgear

40.5kV metal enclosed air insulation switchgear is a very important electrical device for electrical power system and industry customer. According to the main insulation dielectric used in switchgear, the switchgear can be classified into air-insulated and gas insulated devices. In fact, in the air-insulated switchgear, additional to the air used as insulation dielectric, also there are others insulation parts were applied to strengthening the insulation capability of phase to phase and phase to ground. Among them, the bushing manufactured from organic insulation material is one of the most critical insulation parts in air insulation switchgears. After almost 20 years' operation experiences, we can find that insulation failures resulted in bushing is very frequent in operation field. The main cause can be verified into three types: 1) bad design can lead to concentrated electrical stress around the mounting plate of bushing, consequently, high electrical stress results in the partial discharge in bushing, finally, insulation aging due to partial discharge lead to the insulation failures.;2) some flaws occur in the process of product manufacturing, these defect can cause the partial discharge, after a long operation time under discharge condition, insulation failure is also possible to appear in bushing; 3) severe operation environment is also an important fact that change the electrical field distribution around bushing and lead to the insulation failures.