Medium voltage switchgear application guide

Medium voltage switchgear application guide
1 - TRANSFORMER 1 - 1 Switchgear to be used
A fused switch-disconnector, fused contactor and circuit breaker are
usually used for operating and protecting high power transformers.
type power transformer controlled by
MV/LV P ≤ 2 MVA dry or immersed type fused switch-disconnector
transformer
or circuit breaker
or fused contactor
2 MVA < P ≤ 4 MVA dry or immersed type fused switch-disconnector
or circuit breaker
or fused contactor
MV/MV P ≤ 4 MVA dry type circuit breaker
transformer immersed type circuit breaker
P > 4 MVA immersed type circuit breaker
The switch and contactor make sure the transformer is switched on and off
during normal and overload running.
network The fuse short-circuit limits and breaks power. short-circuit currents generated by the upstream
The as short-circuit breaker currents. can make, withstand and break operating currents as well
The protection system (CT, VT, relay, release…) automatically causes tripping
when a fault is detected.
1 - 2 Device characteristics
The device must be able to:
■ withstand and operate the continuous operating current and
eventual overloads,
■ break the fault current at the connection point; on the transformer’s
secondary terminals,
■ withstand short-circuit switching and no-load transformer
switching peaks,
■ break the no-load currents without excessive overvoltage
(downstream open).
The control and/or protection device must be located upstream of the
transformer.
Transformer faults cannot be picked up by downstream protection alone.
A downstream control mechanism cannot isolate the fault transformer.
date
11/94
- B•3•2 -
revised
12/95
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