For a fire accident of 20 kV capacitor device operating in 110 kV substation

it is found throughout such process as operation record

fault record

test and calculation analysis of upper substation as well as fault development procedure that the internal insulation breakdown is happened in the capacitor due to inappropriate operation mode

enlargement and even resonance by the harmonic current of the capacitor bank because of neglecting transformer in light load operation

and the developed into three-phase short circuit. Insufficient operation load of new substation in early operation stage is likely to cause light load or no load of main transformer. At such operation

shunt capacitor device with series reactance at 5% and below shall not be switched on so to prevent capacitor from damage due to harmonics. It is suggested that the harmonic operation status identification and protection of shunt capacitor device shall be improved by AVC system etc.