Handbook of Electrical Engineering For Practitioners in the Oil, Gas ...

Powerconsumption(MW) P lInitialload factor(%)F iNumber ofinstalledgeneratorsNTable 1.10.ESTIMATION OF PLANT ELECTRICAL LOAD 11(continued)Generator rating atsite conditions(MW)P gTurbine ISO ratingsfor a site amb. tempof 40 ◦ C(MW)P iso40Final loadfactor(%)F o40 80.0 6 10.0 11.8 100.060 66.7 4 30.0 35.3 100.060 72.7 4 27.5 32.4 109.160 80.0 4 25.0 29.4 120.060 66.7 5 22.5 26.5 88.960 75.0 5 20.0 23.5 100.060 83.3 5 18.0 21.2 111.160 66.7 6 18.0 21.2 83.360 75.0 6 16.0 18.8 93.860 85.7 6 14.0 16.5 107.180 66.7 4 40.0 47.1 100.080 76.2 4 35.0 41.2 114.380 66.7 5 30.0 35.3 88.980 72.7 5 27.5 32.4 97.080 80.0 5 25.0 29.4 106.780 88.9 5 22.5 26.5 118.580 71.1 6 22.5 26.5 88.980 80.0 6 20.0 23.5 100.080 88.9 6 18.0 21.2 111.1100 83.3 4 40.0 47.1 125.0100 71.4 5 35.0 41.2 95.2100 83.3 5 30.0 35.3 111.1100 66.7 6 30.0 35.3 83.3100 72.7 6 27.5 32.4 91.9100 80.0 6 25.0 29.4 100.0100 88.9 6 22.5 26.5 111.1In practice if F i is too high the operator of the plant will become nervous and will oftenswitch into service the spare generator. If F i is too low then there will be too many generators inservice and it should be possible to withdraw one. Gas turbines have poor fuel economy when theyare 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 turbinedesign. The higher the normal combustion temperature within the turbine, the lower the toleranceis usually found to be available. A high overload will also be accompanied by a significant fall inelectrical system frequency, caused by the slowing down of the power turbine and the relativelylong time taken by the speed governing system to respond. Many power systems that use gas-turbinegenerators are provided with underfrequency and overfrequency protective relays, and these maybe set to trip the generator when a high overload occurs. The initial rate of decline in frequency isdetermined by the moment of inertia of the power turbine, plus the generator rotor, and the magnitudeof the power change seen at the terminals of the generator. See Reference 1. This subject is discussedand illustrated in sub-section 12.2.10 and Appendix D.