Page 222 - Offshore Electrical Engineering Manual

Introduction 209

machines of rating 500 kW and larger, although this threshold will vary with the
degree of criticality of the supply. A stabilising resistor must be provided in series
with the relay to prevent CT saturation effects causing the relay to operate in faulty
conditions. The method of calculating the resistance value is given in an example at
the end of the chapter.

OVER-/UNDERVOLTAGE PROTECTION

This facility is often provided as part of the generator’s automatic voltage regulator
(AVR), but if this is not the case, it must be included in the generator control panel
or switchboard. If fed from a separate voltage transformer (VT), it may duplicate
the AVR facility, so that failure of one set of VT fuses cannot lead to loss of voltage
control and/or monitoring.

OVER-/UNDERFREQUENCY PROTECTION
This facility is often provided by the prime mover governor, but as it is more criti-
cal on a generator isolated from a grid system, it is often duplicated electrically.
This ensures that the circuit breaker trips before damage is done to motors or driven
machinery, even if the prime mover overruns because of gas ingestion at the air
intake (see PART 2 Chapter 4).

UNBALANCED LOADING AND NEGATIVE PHASE
SEQUENCE PROTECTION
If the platform load is unbalanced, there will be a negative phase sequence com-
ponent in the generator load current which, if excessive and over a long period,
will cause overheating of the rotor. On the larger offshore generator packages, it is
therefore necessary to fit negative phase sequence protection, which after a timed
period corresponding to the thermal characteristic of the rotor, will trip the genera-
tor. As the source of negative sequence current is external to the generators and will
lead to the tripping of all the generators if not removed, it is normal to provide an
alarm at onset of the problem so that operators may have time to find the offending
load before production shutdown occurs. A suitable setting value should be sought
from the generator manufacturers in each case, but a value no greater than 30% is
recommended.

ROTOR FAULTS
The exciter output current on brushless machines is monitored and should a rotating
diode go short circuit, an alarm will be annunciated on the generator control panel.
This facility is usually included in the AVR circuitry, and therefore, it is not likely to
be included in the discrete protection relay suite.