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Figure 19-4 shows the behavior of the gas turbine for changes in frequency as a stand-alone and also for changes as part of a combined cycle plant. Thefigure shows changes in the Gas Turbine plant (GT)， the Steam Turbine plant (ST) and the Gas Turbine (GTC) and the steam turbine (STC) as part of a steam turbine plant. In a Combined Cycle PowerPlant， the falling
100
5
–100 FREQUENCY DROOP (%)
frequency is usually taken up by the GTC， by a fast change in increasing theload， since the steam turbine cannot respond fast enough. For an increasingfrequency， the gas turbine and the steam turbine both can respond，thus， asshown in the figure， the gas turbine (60% load) and the steam turbine (40% load) take their appropriate change in load.
The startup and shutdown of a typical gas turbine is shown in figures 19-5and19-6， respectively. The time and percentages are approximate values and will vary depending upon the turbine design.
The gas turbine during the start-up is on an auxiliarydrive， initially it is brought to a speed of about 1200-1500 .PM when ignition takes place and the turbine speed and temperature rise very rapidly. The bleed valves are open to prevent the compressor from surging. As the speed reaches about2300-2500rpm， the turbine is declutched from its start-up motor， the firstset of bleed valves areclosed， and then as the turbine has reached near fullspeed， the second set of bleed valves are closed. If the turbine is a two orthree shaft turbine as is the case with aero-derivative turbines， the power turbine shaft will ..break loose.. at a speed of about 60% of the rated speed of the turbine.
The turbine temperature，flow， and speed increases in a very short time of about three to five minutes to the full rated parameters. There is usually a short period of time where the temperature may overshoot. If supplementaryfiring or steam injection for power augmentation is part of the plantsystem， these should be turned on only after the gas turbine has reached full flow.The injection of steam for power augmentation， if done before fullload， could cause the gas turbine compressor to surge.
The shutdown of a gas turbine first requires the shutdown of the steam injec-tion and then the opening of the bleed valves to prevent the compressor from
120 100 80 60 40 20 0
Firing Temperature  LoadSpeed 
Figure 19-5. . typical startup cur.e for a gas turbine.

0 24 6 810 12
Time in Minutes
120
100
80
60
40
20
0
Power  Flow  Firing Temperature Speed 
Figure 19-6. . typical s.utdown cur.e for a gas turbine.

0 2 4 6 8101214 Time in Minutes
surging as the speed is reduced. The gas turbine， especially for frame type units， must be put on a turning gear to ensure that the turbine rotor does not bow. The lubrication systems must be on so that the lubrication can cool of thevarious components， this usually takes about 30-60 minutes.
Startu. Se.uence
.ne of the major functions of the combined control-protection system is to perform the startup sequence. This sequence ensures that all subsystems of thegas turbine perform satisfactorily， and the turbine does not heat too rapidly or overheat during startup. The exact sequence will vary for each manufacturer.sengine， and the owner.s and operator.s manual should be consulted for details.
The gas turbine control is designed for remote operations to start fromrest， accelerate to synchronousspeed， automatically synchronize with thesystem， and be loaded in accordance with the start selector button depressed. The control is designed to automatically supervise and check as the unit proceeds through the starting sequence to load condition. A typical startup sequence for a large gas turbine follows:
Startingpreparations. The steps necessary to prepare the services and apparatus for a typical startup are as follows:
　
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