FUNDAMENTALS OF GAS TURBINE CONTROLP2M FTUISeptember 2008

INTRODUCTION

Control System is designed to fulfill all gas turbine control requirements. These include control of liquid, gas or both fuels in accordance with the requirements of the speed, load control under part-load conditions, temperature control under maximum capability conditions or during startup conditions. In addition, inlet guide vanes and water or steam injection are controlled to meet emissions and operating requirements. basic system.

INTRODUCTION

If emissions control uses Dry Low NOx techniques, fuel staging and combustion mode are controlled by the system, which also monitors the process. Sequencing of the auxiliaries to allow fully automated startup, shutdown and cool down are also handled by the Control System. Turbine protection against adverse operating situations and annunciation of abnormal conditions are incorporated into the

INSTRUMENTATION

VIBRATION MEASUREMENT PRESSURE MEASUREMENT TEMPERATURE MEASUREMENT

Typical Instrumentation (MinimumRequirements)

1. Accelerometera.At machine inlet bearing case, verticalb.At the machine discharge bearing case,

verticalc.At machine inlet bearing case, axial

2. Process pressurea.Pressure drop across filterb.Pressure at compressor and turbine inletc.Pressure at compressor and turbine

discharge

Typical Instrumentation (MinimumRequirements

3. Process temperaturea.Temperature at compressor and turbine inletb.Temperature at compressor and turbine

discharge

4. Machine speeda.Machine speed of all shafts

5. Thrust-bearing temperaturea.Thermocouples or resistance temperature

elements embedded in front and rear thrustbearing

VIBRATION MEASUREMENT

Machine vibration is monitored using: Displacement probes Velocity pickup detectors Accelerometers (measurement of

acceleration)

PRESSURE MEASUREMENT

Pressure transducers to be used

the locations of pressure and temperature probes in a typical gas turbine

TEMPERATURE MEASUREMENT Temperature detectors such as

thermocouples (used for high-temperature measurement)

resistive thermal detectors (RTDs)

TEMPERATURE MEASUREMENT Turbine exhaust temperature Redundant RTDs are embedded in the

babbitt (white metal) of the bearing to monitor the oil temperature in the bearings

The compressor inlet and discharge temperatures are measured to evaluate the compressor performance

Thermocouples

Thermocouples provide transducers used for measuring temperatures from 330 to 5000°F (201 to 2760°C). Figure 17.3 shows the useful range of each type of thermocouples. They operate by producing a voltage proportional to the temperature difference between two junctions of dissimilar metals. Thermocouples measure this voltage to deter-mine the temperature difference. The temperature at one of the junctions is known. Thus, the temperature at the other junction can be determined. Since they produce a voltage, there is no need for an external power supply

Resistive Thermal Detectors

Resistive thermal detectors (RTDs) determine the temperature by measuring the change in resistance of an element due to a change in temperature. Platinum is normally used in RTDs due to its mechanical and electrical stability. Platinum RTDs are used for measuring temperatures from 454 to 1832°F (270 to 1000°C). The RTD requires an electrical current source to operate. Its accuracy is within ±0.02°F (± 0.01°C)

CONTROL SYSTEM FUNCTIONS Gas Turbine Control System performs

many functions including fuel,air and emissions control; sequencing of turbinefuel and auxiliaries for startup, shutdown and cooldown; synchronization and voltage match-ing of the generator and system; monitoring ofall turbine, control and auxiliary functions; andprotection against unsafe and adverse operatingconditions.

CONTROL SYSTEM

The control system of a gas turbine performs the following functions Provides speed and temperature control in

the machine Control the unit during normal operation Provide protection to the gas turbine Perform start-up and shutdown sequence of

events

Speed Control

Magnetic transducers measure the speed of the shaft at a toothed wheel mounted on the shaft. The transducers provide an output in the form of AC voltage having a frequency proportional to the rotational speed of the shaft. A frequency-to-voltage converter is used to provide a voltage proportional to the speed. This measured value of the speed is then com-pared to the desired value of the speed (speed set point). The difference between these two values is called the error. If there is an error, the control system will adjust the opening of the fuel valve to eliminate it. It relies on a proportional-integral-derivative (PID) algorithm(mathematical expression) to eliminate the error within minimal time and without instabilities (oscillations in the speed).

Temperature Control

A series of thermocouples mounted around the periphery at the exhaust of the turbine pro-vides an input to the control system. They are normally made from iron-constantan or chromel-alumel fully enclosed in magnesium oxide sheaths to prevent erosion. Two thermocouples are frequently mounted for each combustion can. The redundancy improves the reliability of the control system. The output of the thermocouples is generally averaged. The control system compares this measured value of the turbine exhaust temperature with the desired value of set point. The difference between these values is called the temperature error. If the temperature error is different from zero, the control system will adjust the opening of the fuel valve to eliminate it. It relies also on PID algorithm to eliminate the error within minimal time and without instabilities.

Protective Systems

The protective systems provide protection during the following events: Overspeed Overtemperature Vibration Loss of flame Loss of lubrication

START-UP SEQUENCE

The gas turbine control system performs the start-up sequence. It consists of ensuring that all subsystems of the gas turbine perform satisfactorily, and the turbine rotor temperature does not increase too rapidly or overheat during start-up. The control system is designed to start the unit remotely, accelerate it to operating speed, synchronize it automatically with the grid, and increase the load to the desired setting.

OPERATION AND MAINTENANCE The operator interface is comprised of a

VGA color graphics monitor (or LCD), keyboard and printer. The functions available on the operator inter-face as follows