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.
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