why should we place 9KV lightning arrestors in 11KV
substation ?
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Answer Posted / d. bhowmick
All power system devices are designed with certain
tolarence limits.This is because it is always desirable
that the equipments/devices can withstand transient
voltage/current rise without any damage up to a certain
preset limit.
For an 11 KV line, the effective phase to ground voltage is
6.35 KV. That is, an LA associated with this line(between
the phases & ground)is supposed to operate when the system
voltage exceeds 6.35 KV. However, in many cases the system
voltage may momentarily rise above this value owing to
switching action or induction-that can easily be tolarated
by the associated equipments(having their individual
tolarances). Under this circumstance, it is desirable that
the LA donot unnecessarily operate especially when the rise
in voltage is within the tolance limit of the associated
equipments. To enable this, LAs with slightly higher value
of voltage rating are provided for a system operating at a
certain system voltage. Clearly, for a 6.35 KV(L-G)system
associated with a 9 KV LA, no damage is expected to be
caused if the system voltage rises by a maximum of 2.65 KV.
Beyond this, the LAs will come into action.
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