On circuits exposed to lightning it is recommended that lightning arresters be provided on all delta-connected capacitors either housed or hanger type large or small banks.
Likewise arresters are recommended for all wye-connected capacitor banks where the neutral is ungrounded. Where the capacitor bank is switched, it is best practice to provide arresters on the capacitor side of the circuit breaker.
A capacitor bank connected in wye with the neutral grounded has the ability of sloping off the front and reducing the crest of traveling waves, so that it affords added lightning protection to the capacitor bank itself and to transformers and other adjacent equipment.
Thus there is some question as to whether or not arresters are needed. In addition, for those surges where arresters are required there is also some hazard to the arrester because the capacitor discharges through the arrester when the arrester operates.
When the capacitors are connected to a bus with transformers and other circuits, arresters are required to protect this other equipment whenever the capacitor bank is disconnected. The arresters are therefore available and in service at all times.
Where the capacitor bank is the only load on a transformer winding the arresters can be omitted if the transformer is removed from service when all capacitors are disconnected. Where the capacitors are supplied from a third winding of the transformer, arresters may be required on this winding if all of the capacitors are to be out of service at times.
From a surge-protection point of view for greatest safety to the arresters, wye-connected capacitor banks should be operated ungrounded. For best surge protection of the capacitors, the neutral should be grounded and arresters provided.
There are other problems with capacitor banks, however, which make the wye-grounded bank undesirable. The grounded-neutral bank provides a path for the third or residual harmonics, thereby increasing the probability of communication interference; if a capacitor unit becomes shorted, where there is a single unit between line and neutral, the fault current can exceed the ability of the fuse to clear before the capacitor unit is ruptured.
Lightning arresters protecting high-voltage capacitor banks above 15 kv are subjected to switching surges, when the capacitors are switched, whether or not the capacitor bank neutral is grounded. With restriking across breaker contacts, which may occur, the arresters may be damaged.