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Gas turbine instrumentation has expanded in the past few years from simple control systems to more complex diagnostic and monitoring systems that are designed to avert major catastrophes and operate a unit at its peak performance.
Control Systems
All gas turbines are provided with a control system by the manufacturer. The control system has three fundamental functions: startup and shutdownsequencing， steady-state control when the unit is in operation， and protec-tion of the gas turbine.
Control systems can be an open loop or closed loop system. The open-loop system positions the manipulated variable either manually or on aprogrammed basis， without using any process measurements. A closed loopcontrol system isone， which receives one or more measured process vari-ables and then uses it to move the manipulated variable to control a device. Most combined cycle power plants have a closed loop control system.
Closed loop systems include either a feedback or feedforward， controlloop or both to control the plant. In a feedback controlloop， the controlled variable is compared to a set point. The difference between the controlled variable to the set point is the deviation for the controller to act on to minimize the deviation. A feedforward control system uses the measured load or set point to position the manipulated variable in such a manner to minimize any resulting deviation.
In many cases the feedforward control is usually combined with a feed-back system to eliminate any offset resulting from inaccurate measurements and calculations. The feedback controller can either bias or multiply the feedforward calculation.
A controller has tuning parameters related to proportional， integrated，derivative，lag， deadtime， and sampling functions. A negative control loopwill oscillate if the controller gain is toohigh， but if it is too low it will be ineffective. The controller must be properly related to the process para-meters to ensure close-loop stability while still providing effective control. This is accomplished first by the proper selection of control modes to satisfythe requirements of the process， and second by the appropriate tuning of those modes. Figure 19-1 shows a typical block diagram for forward and feedback control.
Computers have been used in the new systems to replace analog PIDcontrollers， either by setting set points， or lower level set points in super-visory control， or by driving valves in direct digital control. Single-station digital controllers perform PIDcontrol in one or twoloops， including
Feedforward

Load
Controller
Set Point Error

Power Plant
Feedback
Controlled
Controller

Variable

Feedback Loop
Figure 19-1. Forward and feed back control loop.
computing functions such as mathematical operations， with digital logic andalarms. D-CS provide all the functions， with the digital processor shared among many control loops. A high level computer may be introduced toprovide condition monitoring， optimization， and maintenance scheduling.
The gas turbine control systems are fully automated， and ensure the save and proper startup of the gas turbine. The gas turbine control system is complex and has a number of safety interlocks to ensure the safe startup of the turbine.
The startup speed and temperature acceleration curves as shown in Figure 19-2 are one such safety measure. If the temperature or speed are notreached in a certain time span fromignition， the turbine will be shutdown. In the early days when these acceleration and temperature curves were notused， thefuel， which was not ignited， was carried from the combustor andthen deposited at the first or second turbinenozzle， where the fuel com-busted which resulted in the burnout of the turbine nozzles. After an aborted start the turbine must be fully purged of any fuel before the next start isattempted. To achieve the purge of any fuel residual from the turbine， there must be about seven times the turbine volume of air that must be exhausted before combustion is once again attempted.
　
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