In order to find the cause of the abnormality occurred on the capacitor bank when the vacuum circuit breaker（VCB） switching off in a 110 kV substation in Foshan

the primary connection diagram of capacitor bank

the operation principle of the installation

and the data measured by the operation persons are analysed and judged comprehensively.The reason is founded out such abnormal phenomenon is caused the fault of the VCB.The correct operating procedures are taken to disconnect the faulted equipment from the network. The fault location is determined by visual inspection of the VCB and electrical testing

which is coincided with analysis result

and the hidden danger of fault disappeared.The fault caused by wrong operating when defects lie in equipment is avoided.