Page 282 - Offshore Electrical Engineering Manual

Generator Controls 269

engraved ‘LOW OIL LEVEL’. The logic of the annunciator should also indicate to
the operator which fault occurred first. This ‘first up’ facility is usually provided by
making the window of the first fault flash on and off or for it to remain steady and
any later faults to flash. The occurrence of any alarm or shutdown will be accompa-
nied by an audible alarm which will be silenced when an ‘accept’ button is pressed
on the panel. There should also be a lamp test button which while pressed causes all
the lamps on the matrix to light up. Once the fault has been cleared, the annuncia-
tor can be cleared of all fault and failure indications by pressing the reset button.
Annunciator windows should be segregated into groups dealing with different types
and severities of fault. Faults which cause an immediate shutdown must be indicated
by windows of a different colour and be well segregated, usually by being placed on
the lower lines of the matrix. Recommended colours are amber for faults which do
not cause an immediate shutdown and red for those that do.

LOAD SHARING SYSTEMS
The operator’s workload may be lightened by introducing facilities which automati-
cally control the sharing of two or more generators running in parallel. The principle
of a typical load sharing system is described in the following section.
An astatic control system is set up between the AVRs of each generator. This
operates by comparing a direct current (DC) reference voltage with DC voltages
derived from respective AVR control signals. The power sharing is accomplished by
monitoring the output power of each machine and feeding this to an associated power
comparator unit. Each machine power comparator also receives a signal from a fre-
quency controller unit. This compares the actual supply frequency with a frequency
reference so that the resulting comparator output signal is a function of the generator
output modified by any frequency error. The outputs of each machine power com-
parator are linked to the next, forming a comparator loop which provides each asso-
ciated governor with an appropriate power mismatch signal. The mismatch signal is
used to drive governor raise/lower relays. The resulting system, once set up correctly,
will provide good load sharing of both real and reactive power over a wide range of
system loads.

POWER MANAGEMENT SYSTEMS
These are usually integrated with other systems in the installation or vessel in order
to, for example,

1. avoid loss of production; by automatically tripping, nonessential large loads
such as water injection pumps, which may be load shed automatically, both to
maintain main generation stability and keep production up and running;
2. maintain dynamic positioning during the loss of a ship’s generator.

With the advent of microprocessors, much more of the power generation sys-
tem can be automated. To avoid embarrassing failures, microprocessors must be