Training on GT Controls

Introduction to Control Philosophy

A Gas Turbine Consists of

1. Starting means2. Auxiliaries3. Compressor4. Combustion System5. Turbine 6. Exhaust7. Load

Introduction to Control Philosophy

Control systemCommunicates with the turbine toMeasure, adjust the parameters

The Main Inputs areAir & Fueland the Outputs areLoad & Exhaust Gases.

It also protects the turbine from abnormal operations

Gas Turbine Controls

1. Basic Control Parameters of GT

2. Minimum Gate Concept of

Six Control Loop

• Start up

• Speed/Load

• Temperature

• Acceleration

• Manual

• Shut down

Main

Auxiliary

Simplified control schematic

Example of Minimum FSR of Six Control Loops on HMI Screen

Min Value

Detailed Control Schematic

We will use this as referenceas we go in detail of the course

Startup Control

Start up control controls the amount of fuel to the GT from zero speed to Full Speed No Load

FSRSU is minimum of the six control loops

Speed control schematic

• The Speed Control Loop ensures the correct speed as required by the system

• The Load Control Loop maintains the power out put. (The load control can be either Isochronous or Droop, depending on whether the system is connected to Grid or Isolated)

This serves both Speed & Load functions and so called Speed/Load Controller

FSRN is the minimum of Six Control Loops

Speed Control Characteristics

Droop speed control is a proportional control,changing FSR in proportion to the difference betweenactual turbine speed and the speed reference (TNH & TNR)

Isochronous Controller is a closed loop controller that maintains the Turbine Speed to the reference speed by regulating FSR during the isolated operation.

Droop Control Curve

Exhaust Temperature control

Temperature Controller ensures that the Turbine internals are protected from over heat and Optimum power is produced.

The firing temperature is difficult tomeasure and hence the Controller uses the exhaust thermocouplesas reference which is directly proportional to Firing Temperature

Tf = Tx (Pcd/Pa)k

Exhaust/Firing Temperature Relation

The firing temperature remains constant even with increase in MW, FSR & CPR ratio with less exhaust temperatures

Curve comparing the load

at different ambient

Temperature control scheme

FSRT is the minimum of the Six Control Loops

Exhaust Temp. control set points

Exhaust temp control reference set point TTRXB calculated based on CPD

The back up exhaust temp reference is based on FSR in case of CPD measurement failure

TTKn_C

TTKn_I

FSR BIAS

CPD BIAS

TTKn_STTKn_M

TTKn_KIsothermal

Exh

aust

Tem

per

atu

reCPD FSR

Protection of Turbine internals and optimum power

Acceleration control

Acceleration Control functions during sudden Load Changes and Start Up

TNH

0.35 %/sec

100%0%

0.10 %/sec

40% 50% 75% 95%

Manual control

Manual Control Loop Can be used to limit fuel to prevent over firing and over riding active control

Shut Down control

Shut down control loop reduces the fuel at a predetermined during shut down to reduce thermal stresses

FSRSD is the minimum of Six Control Loops

IGV Control

IGV Control Loop

• Controls air fuel ratio

• Prevents Compressor

Pulsations

IGV Control

Maintains higher Exhaust Temperature at partial loadsfor Combined Cycle Operation

IGV Schedule

IGV scheduling is requiredto ensure the protection fromPulsation/Stall by excessive opening at lower speeds/loads and negative pressures at partial loads by less opening

IGV Control

Maintains higher Exhaust Temperature at partial loads for Combined Cycle Operation

IGV control reference

The exhaust temp. variesfor simple and combined cycle operations.IGV temperature control neverexceeds the base temperature control set point

Exh

aust

Tem

per

atu

reCPD

Isothermal

CPD BIASCPD BIASCPD BIAS

Base temp control set point

CC IGV temp control set point

SC control set point

Electro Hydraulic Servo valve

Servo valve is the interface between the electrical and mechanical systems

Servo valve controls the direction and rate of motion of a hydraulic actuator based on the input current to the servo.

Servo valve contains three electrically isolated coils on the torque motor.

A null-bias spring positions the servo so that actuator goes to the fail safe position when ALL power and/or control signal is lost.

Liquid Fuel Control Scheme

As the speed sensors in the FD send a signal to controller, it compares with demand signal and an error signal is created.

The amplifier in the controller will change the signal to drive the servo Valve of the Bypass valve.

Amplifier

Liquid Fuel Control

The VSVO card receives the PR signals from 77FD – 1,2 &3 and Outputs Analog Signal, the fuel necessary to meet the demand.

The VSVO Card modulates Servo Valve based on FQROUT & FQ1

Gas Fuel Control Scheme

Fuel Gas Control System consists of Stop ratio Valve, Gas Control Valve Controls

Gas Stop Ratio Valve Scheme

• SRV is a dual function Valve, as a stop Valve and to hold desired P2 Pressure

• Control loop consists of both Pressure and Position Control Loops

• P2 is Controlled by pressure loop as a function of TNH with a gain & offset, the signal FPRG

• Position Loop takes feed back from Valve Position FSGR

The VSVO Card modulates Servo Valve based on FPRG, FPG & FSGR

Gas Control Valve Scheme

The position of the gas control valve is proportional to FSR2 which represents called for gas fuel flow

The VSVO Card modulates Servo Valve based on FSROUT & FSG

Fuel Splitter Scheme

• Transfer from one fuel to the other on command.

• Allow time for filling the lines with the type of fuel to which turbine operation is being transferred.

IGV Control scheme

The Inlet Guide Vanes (IGVs) modulate during

• Acceleration of the gas turbine to rated speed • Loading and unloading • Deceleration of the gas turbine

The VSVO Card modulates Servo Valve based on CSRGV & CSGV

VSVO

IGV Schedule

IGV modulation maintains

• Proper flows and pressures, and thus stresses, in the compressor• Maintains a minimum pressure drop across the fuel nozzles• High exhaust temperatures at low loads during combined cycle operation

GT Protections

• GT is not allowed to start until permissives are set correctly

• GT Trips during operation if operating limits exceeded

Protection System ensures that GT is safe from dangerous operating conditions

Duel Fuel Trip oil scheme

• The Trip Oil System is the primary interference between the master protection circuits and the fuel flow and VIGV position control devices