Page 455 - Handbook of Electrical Engineering

444 HANDBOOK OF ELECTRICAL ENGINEERING

set point. Such a scheme can be easily incorporated into the PMS because all the measurements and
output signals exist if a load-sharing scheme is present. The master integral controller must be slow
acting relative to the time response of the governor control action. For a 50 Hz power system, for
example, the master set point frequency could be set at 50 Hz or some other frequency to suit the
operating conditions.

If it is necessary to switch off the isochronous controller when the plant is heavily loaded, then
the PMS should calculate what the individual governor set points should be for a system frequency to
suit the overall droop. It should send these signals shortly after the isochronous controller is switched
off. This feature will prevent the system frequency rising too high when the plant is lightly loaded. As
the frequency rises each motor will run at a higher speed and thus consume a little more power and
current. If a motor normally runs close to its nameplate rating, rather than at its originally designed
load, then it is possible that the higher current could activate the overcurrent protection of the motor.
After some length of time such a motor may trip. Alternatively the PMS could allow the frequency
to rise to a predetermined maximum value before the corrective isochronous signals are sent to the
governor set points. This would give the benefit of maintaining a good frequency for most operating
levels of the plant.

16.3.8 Reactive Power Sharing for Generators

As explained in sub-section 2.5.3 for speed governing it is possible to compensate for the mismatching
of reactive power delivered by each generator in a similar manner. If this is achieved together with
reducing the mismatch of active power, then each generator will be operating at the same current.
This will eliminate the possibility of an overcurrent occurring at one of the generators when a high
load factor exists. The method described in sub-section 2.5.3 can be used with frequency being
replaced by busbar voltage, and the set points to be adjusted will be those of the automatic voltage
regulators (AVRs).

16.3.9 Isochronous Control of Busbar Voltage

Likewise as explained in sub-section 2.5.3 it is possible to superimpose an integral controller on to
the set point controls of the individual AVRs. The master set point will be the busbar voltage. The
master controller must be slow acting relative to the time response of the AVR control action, so as
to avoid the possibility of oscillatory or even unstable operation.
Safeguarding against over-frequency was explained in sub-section 16.3.7. The same concept
can be used to safeguard against overvoltage at light loads.

The sharing of active and reactive power and the isochronous control of the system frequency
and main busbar voltage can be displayed on the VDU using a diagram similar to that shown in
Figure 16.2.

16.3.10 Condition Monitoring of the Gas Turbines

The power output from a gas turbine is greatly influenced by changes in ambient temperature and
the state of cleanliness of the combustion equipment and power turbine blades.