Page 52 - Offshore Electrical Engineering Manual

Parallel Operation of Generators 39

3. The stator and rotor heating limits – this is an alternator design limit associated
with the type of insulation and the avoidance of damage.
Considering the rotor, the major cause of losses in the rotor is the field wind-
ing resistive loss. If the field current is above a certain level, the I R losses will
2
be too high, possibly causing insulation failure in the field winding. To manage
thermal conditions, field current is limited; therefore, the induced excitation in
the armature winding will be limited.
Considering the stator, excessive armature currents will cause the stator
temperature to rise beyond a safe level.
4. The underexcitation limit – this is an electromagnetic limit; underexcitation will
weaken the field, causing instability and leading to loss of synchronism.

It is vital to operate the generator within the locus of the diagram in Fig. 2.2.3 to
maintain a reliable supply and to avoid reduction in the operational life or damage
to the generator.

PARALLEL OPERATION OF GENERATORS

When an AC generator is operating alone (often called ‘Island’ mode) the power
factor is determined only by the load, and altering the excitation only changes the
voltage (although changing the voltage will itself change the load and the fault level).
Similarly, increasing the power input from the prime mover will increase the genera-
tor speed and hence the frequency.
When the offshore power system has more than one generator running on it in
parallel, the power and reactive kilovolt-ampere contributions from each generator
may be adjusted separately and it is possible to adjust the power factor of individual
machines in this way.
A set of paralleling conditions must be met to avoid failure to synchronise and
possible damage to both generators and switchgear:

1. The two generators must have the same root mean square line voltages.
2. The phase sequence must be the same in the two generators.
3. The two ‘a’ phases must have the same phase angles (assuming phases are a, b
and c).
4. The frequency of the oncoming generator must be slightly higher than the fre-
quency of the running system.

If the sequence in which the phase voltages peak in the two generators is dif-
ferent, then two pairs of voltages are 120 degrees out of phase and only one pair of
voltages (the a phase) is in phase. If the generators are connected in this configura-
tion, large currents will flow in phases b and c, causing damage to both machines and
possibly the synchronising circuit breaker.
This would be a commissioning issue and corrected by swapping the connections
on any two of the three phases on one of the generators.