TIME
Figure 19-.. Startup c.aracteristics of a gas turbine.
Drive Level
receives all its inputs from the D-CSsystem， and from the steam and gas turbine controllers. The signals are checked initially for their accuracy and then a full machinery performance analysis is provided. The new perform-ance curves produced by the condition monitoring system are then providedto the optimization system. The optimization system， usually used wheremultiple turbines areused， receives the load and then sends a signal to theD-CSsystem， which in turn sends the signal to the gas turbine for the best settings of the gas turbine to meet the load.
The gas turbine has a number of systems， it controls such as the:
1. Lubrication skid. The gas turbine lubrication skid is usually independ-ent of the steam turbine skid as the lubrication oil is usually synthetic due to the high temperatures in the gas turbine. Another reason is due to water contamination of the lubrication oil from the steam turbine. It is advisable to have the lubrication system be totally independent. The gas turbine lubrication skid would report to the gas turbine controller.Since the lubrication system is also used for providing cooling， it is usually operated for about 20 minutes after the gas turbine is shutdown.The lubrication skid contains at least three pumps， two pumps in whicheach can provide the head required and a third pump， which is usually recommended to be a DC drive for emergency use. These pumps and their control fall under the drive level hierarchy.
2. The uel skid. This could contain a gas compressor if the fuel gas pressure is low and a knockout drum for any liquid contamination that the gas may have. The requirement of fuel gas pressure is that it should be operated at a minimum of 50-70 psi (3.5-4.83 Bar) above the compressor discharge pressure. The compressor and its motordrive fall under the drive level hierarchy. In the case of liquidfuels，the skid may also contain a fuel treatmentplant， which would havecentrifuges， electrostatic precipitators， fuel additivepumps， and otherequipment. These could be directly controlled by the D-CSsystem， which would then report its readiness to the gas turbine controller.
The control system requires inputs for speed determination， temperaturecontrol， flamedetection， and vibration. The speed monitoring system receives an input from magnetic transducers in the form of an AC voltage with a frequency proportional to the rotational speed of the shaft. A fre-quency-to-voltage converter provides a voltage proportioned to speed， which is then compared to a set value. If the measured voltage is different from thereferencevoltage， a speed change is made. Typically， the desired speed can be manually set to a range between 80% and 105% of design speed.
The temperature control receives its signals from a series of thermocouples mounted in the exhaust. The thermocouples are normally iron-constantan or chromel-alumel fully enclosed in magnesium oxide sheaths to prevent erosion. The thermocouples are frequently mounted with one for each combustion can. The output of the thermocouples is generally averaged into two independent systems with half of the thermocouples in each group. The output of the two systems is compared and used for decisions requiring a temperature input. This redundancy protects the system against tripping if a thermocouple fails.
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