Page 227 - Offshore Electrical Engineering Manual

214 CHAPTER 2 Transformer Faults and Protection Devices

Where the LV switchboard has a bus section switch, a third operating alternative
is available where each section is fed from one transformer. This has the benefit that
one transformer fault will only require the reinstatement of one LV bus section and
will not disrupt supplies to the bus section fed by the healthy transformer.

INTERTRIPPING
A faulty transformer must be isolated by the circuit breakers on both the primary and
secondary sides, wherever the fault is sensed, for two reasons.
First, a fault sensed by protection only on the secondary circuit breaker would
leave the transformer energised in a dangerous state. Second, a fault sensed by pro-
tection only on the primary circuit breaker could allow the transformer to be back fed
from the secondary circuit, again leaving the transformer energised in a dangerous
state.
A scheme where the primary and secondary circuit breaker trip circuits are linked
is known as intertripping. Momentary paralleling and intertripping schemes are often
combined, and the scheme may have additional facilities, such as those required for
the emergency back feeding of a main switchboard air compressor when main gen-
eration is not available.