Page 32 - Handbook of Electrical Engineering

ESTIMATION OF PLANT ELECTRICAL LOAD 11

Table 1.10. (continued)

Power Initial Number of Generator rating at Turbine ISO ratings Final load
consumption load factor installed site conditions for a site amb. temp factor
◦
(MW) P l (%) generators (MW) of 40 C(MW) (%)
F i N P g P iso40 F o
40 80.0 6 10.0 11.8 100.0
60 66.7 4 30.0 35.3 100.0
60 72.7 4 27.5 32.4 109.1
60 80.0 4 25.0 29.4 120.0
60 66.7 5 22.5 26.5 88.9
60 75.0 5 20.0 23.5 100.0
60 83.3 5 18.0 21.2 111.1
60 66.7 6 18.0 21.2 83.3
60 75.0 6 16.0 18.8 93.8
60 85.7 6 14.0 16.5 107.1
80 66.7 4 40.0 47.1 100.0
80 76.2 4 35.0 41.2 114.3
80 66.7 5 30.0 35.3 88.9
80 72.7 5 27.5 32.4 97.0
80 80.0 5 25.0 29.4 106.7
80 88.9 5 22.5 26.5 118.5
80 71.1 6 22.5 26.5 88.9
80 80.0 6 20.0 23.5 100.0
80 88.9 6 18.0 21.2 111.1
100 83.3 4 40.0 47.1 125.0
100 71.4 5 35.0 41.2 95.2
100 83.3 5 30.0 35.3 111.1
100 66.7 6 30.0 35.3 83.3
100 72.7 6 27.5 32.4 91.9
100 80.0 6 25.0 29.4 100.0
100 88.9 6 22.5 26.5 111.1

In practice if F i is too high the operator of the plant will become nervous and will often
switch into service the spare generator. If F i is too low then there will be too many generators in
service and it should be possible to withdraw one. Gas turbines have poor fuel economy when they
are lightly loaded.
High values of F o should be avoided because of the risk of cascade tripping by the gas turbines.
The margin of overload that a gas turbine can tolerate is relatively small and varies with the turbine
design. The higher the normal combustion temperature within the turbine, the lower the tolerance
is usually found to be available. A high overload will also be accompanied by a significant fall in
electrical system frequency, caused by the slowing down of the power turbine and the relatively
long time taken by the speed governing system to respond. Many power systems that use gas-turbine
generators are provided with underfrequency and overfrequency protective relays, and these may
be set to trip the generator when a high overload occurs. The initial rate of decline in frequency is
determined by the moment of inertia of the power turbine, plus the generator rotor, and the magnitude
of the power change seen at the terminals of the generator. See Reference 1. This subject is discussed
and illustrated in sub-section 12.2.10 and Appendix D.