In view of the rare successful early warning cases of online monitoring devices for domestic capacitor banks in recent years

it is pointed out by analyzing the cooperative mechanism of external fuse and relay protection of 10 kV capacitor bank without inner fusesthat the early warning function can be achieved by monitoring the sudden change of electrical quantity at breakdown of single element of capacitor at theoretical condition

yet in engineering practice

since the actual setting value of protection is much lower than the theoretical value

the breakdown of a single element in capacitor can usually trigger the relay protection.Various kinds of early warning methods based on mutation will lose time margin.It is necessary to adjust the protection setting value appropriately and allow the capacitor bank to run with illness for a short termin order to play warning effect of online monitoring devices.