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The protective system is independent of the control system and providesprotection from over-speed， over-temperature，vibration， loss offlame， and loss of lubrication. The over-speed protection system generally has a trans-ducer mounted on the accessory gear orshaft， and trips the gas turbine at approximately 10% of maximum design speed. The over-temperature sys-tem has thermocouples similar to the normal temperature controls with a similar redundant system. The flame detection system consists of at least two ultraviolet flame detectors to sense a flame in the combustion cans.
In gas turbines with multiplecans， the detectors are mounted in cans not equipped with spark plugs to assure flame propagation between cans duringstartup. .nce the unit isrunning， more than one indicator must indicatea loss of flame to trip the machine， although the loss of flame in only one can is indicated on the annunciator panel.
.ibration protection can be based on either of the three measurementmodes.acceleration， velocity， or displacement.but velocity is frequently used to provide constant trip levels throughout the operating speed range. Due to the problemsencountered by velocimeters， manymanufacturers，especially in aero-engines， have started using accelerometers. Two transdu-cers are normally located on the gas turbine with additional transducers on the driven component. .ibration monitors are set to provide a warning atone vibration level with a trip at a higher level. .ormally， the control systemis designed to provide a warning in the event of an open-circuit， ground， or short circuit.
The gas turbine control loop controls the Inlet Guide .anes (IG.) and the Gas Turbine Inlet Temperature (TIT). The TIT is defined as the temperatureat the inlet of the first stage turbine nozzle. Presently， in 99% of the units， theinlet temperature is controlled by an algorithm， which relates the turbineexhaust temperature， or the turbine temperature after the gasifier turbine，the compressor pressure ratio， the compressor exit temperature， and the air mass flow to the turbine inlet temperature. .ew technologies are being developed to measure the TIT directly by the use of pyrometers and otherspecialized probes， which could last in these harsh environments. The TIT iscontrolled by the fuel flow and theIG.， which controls the total air mass flow to the gas turbine. In a Combined Cycle Power Plant application， the turbine exhaust temperature is maintained at or near a constant down to about 40% of the load.
80
60
GT>C 40
20
ST 0
–5 –4 –3 –2 –1 0 1 2 3 4
–20
STC
–40
GT&ST GTC
–60
–80
Figure 19-4. Droop cur.es for Combined Cycle Power Plants.
All power plants are synchronized to the overall grid and thus the opera-tion of the plant at the given frequency is very important. The grid cannotstand many fluctuations of the plant frequency. Itis， therefore， very import-ant to operate the plant at its assigned frequency， which is 60 Hz in the United States and the Americas as well as many countries in the mideast. Europe and most of Asia are operating at a 50 Hz frequency. If there is afrequency change， this must be taken care of in seconds.
Frequency response will be needed outside a dead band of +/-0.1 Hz. The dead band is essential for stable operation of a plant otherwise the plant could oscillate and plant failures have occurred due to a lack of a dead band. Frequency droop is a major problem in plants due to machinery degrada-tion. The standard droop setting is about5%， which means that a grid frequency drop of 5% would cause an increase of the load by 100%. Gas turbines can easily take swings of 20-30% but large swings cause changes infiring temperature， which places a large strain on the hot section of the turbine. Gas turbines are rated for peak operation to about 10-15% of their base load. It is therefore suggested that the gas turbine be operated at about 95% of the base load so that there is room for adjustment.
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