Page 90 - Offshore Electrical Engineering Manual

Switchboard Construction 77

of this between fault inception and contact opening. Generator ac decrement must
also be taken into account. Providing the switchgear fault make rating is adequate;
problems with fault break ratings may be overcome by delaying the circuit breaker
opening until the fault current has decayed to a value within the rating. The use of
bus bar reactors is not recommended due to offshore weight and space limitations.
Because of the high prospective fault currents, it is likely that any large motors sup-
plied directly from this switchboard will require circuit breaker rather than fused
contactor switching. To avoid shutting down generators or other vital equipment to
carry out maintenance on the switchboard, a duplicate busbar switchboard may be
considered. This is not often specified, however, because of the extra complexity,
cost, weight and space involved. If it is likely that further generators will be required
to be installed, due to a later operational phase such as artificial lift, then the switch-
board will require to be rated for this future load and fault rated for the future pro-
spective fault current capacity of the expanded system. Sufficient spare equipped
circuit breakers should be provided for the expansion.

LARGE DRIVE SWITCHBOARDS 3.3–6.6 KV
Development of motor controlgear at up to 6.6 kV has resulted in very compact units
where relatively low load currents are switched by vacuum contactors protected from
short circuit faults by suitable HRC fuses. For incoming and outgoing distribution,
circuit breaker cubicles are provided, the whole forming a composite switchboard of
low weight and compact dimensions. The prospective fault level on this switchboard
can be regulated to some extent by adjusting the reactance of the supply transformer
windings. Therefore, motor control is usually by fused contactor rather than circuit
breaker.

UTILITY SERVICES AND PRODUCTION SWITCHBOARDS
Because of the interdependence of various systems on an offshore installation, as can
be seen by the examples in PART 1 Chapter 2, the low voltage switchboards must be
considered as just as vital as their medium voltage neighbours. Maintenance of cir-
cuits for such supplies as machinery auxiliaries and hazardous area ventilation must
be given high priority. An example of a generator lube oil auxiliary system is given
in PART 2 Chapter 4.

EMERGENCY SWITCHBOARDS
The function of the emergency switchboard is described in PART 1 Chapter 1.
It is beneficial to provide synchronising facilities for the switchboard’s associ-
ated emergency generator. The generator has automatic start facilities which will
initiate a start following a main generation failure, provided the start signal is not
inhibited by one of the safety systems. The synchronising facility gives a conve-
nient means of routine load testing for the generator, and allows for changing over