19 - Control Systems and Instrumentation

Abstract

All gas turbines are provided with a control system by the manufacturer. The control system has three fundamental functions: start-up and shutdown sequencing, steady control when the unit is in operation, and protection of the gas turbine. Closed-loop systems include either a feedback or feed-forward, control loop, or both to control the plant. In a feedback control loop, the controlled variable is compared with a set point. The difference between the controlled variable and the set point is the deviation for the controller to act and minimize the deviation. Computers have been used in the new systems to replace analog PID controllers, either by setting set points or lower level set points in supervisory control or by driving valves in direct digital control. The control system requires inputs for speed determination, temperature control, flame detection, 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. Temperature measurement is important to gas turbine performance. Exhaust gas temperature should be monitored to avoid overheating turbine components. Most gas turbines are equipped with a series of thermocouples in their exhausts.