Page 74 - Offshore Electrical Engineering Manual

Governors 61

The basic criteria for establishing electrical supplies to generator lube oil pumps
and other vital auxiliaries are as follows:

1. Although weight and space limitations usually prevent auxiliary switchboards
being fed from separate generator transformers, as one would expect to find in
a power station onshore, the switchboard or individual motor starters should be
electrically as close to the generator as possible.
2. Each complete set of electrically driven auxiliaries should be supplied from one
switchboard, so that only one supply is required to ensure that the particular set
of auxiliaries is available.
3. Any standby auxiliaries should be fed from another switchboard, which obtains
its supply from the generator via a different electrical route.
4. If possible, the complete set of auxiliaries should be duplicated in order that a fault on
one switchboard or its incoming supply will not lead to a shutdown of the generator.
5. Loss or temporary disconnection of supplies to the emergency DC lube oil pumps
should be adequately displayed on the generator control panel annunciator so that
the operator is aware that loss of AC power and, consequently the failure of the
AC-driven lube oil pumps, may lead to damage to the generator as it runs down.

GOVERNORS
Sophisticated electronic governors able to provide reliable service coupled with high
performance have been available for some time. However, there are a few points worth
considering when selecting a suitable governor. Firstly, it is advisable to obtain as many
of the governor’s control system parameters as possible from the governor manufac-
turer. These will be required if the power system is to be computer simulated before
the project design stage is complete, which is usually the case. Any governor-engine-
alternator system will have a finite response time. It also has a certain amount of stored
energy which can be extracted from the system by sacrificing some speed, and this
energy may be used while the governor is responding to a load increase to maintain the
system. Apart from large power system disturbances such as large motors starting or
short-circuit faults, which need to be studied by dynamic simulation, any cyclic load
should be considered carefully. The following anecdote illustrates the problem. A radio
transmitting station was built in an isolated location without access to any external
electricity supply, depending entirely on two diesel generators for electrical power. The
station was successfully commissioned with the exception that the supply frequency
became unstable and the generators shut down during each broadcast of the time pips.
The problem was traced to the cyclic loading imposed by the series of pips being at a
frequency at or close to the natural frequency of the governor mechanical linkages. In
this case the problem was overcome by substantially increasing the mass of the engine
flywheel. The only cyclic loads likely to be experienced offshore are those produced by
heaters which use thyristor integral cycle firing controllers. The phase angle controlled
rectifiers and variable-frequency invertors used on the drilling rigs are unlikely to pro-
duce the necessary supply frequency subharmonics.