Basically it is a SISO system with governor acting as feedback element and speed changer acting as a reference input. While modeling we get two inputs deltaPL and deltaPref . From that view it is a MISO system. 

Dear Abisoye Blessing,

The turbine control system is one of the key control loops in the dynamic performance of steam power generation units

The speed control of a turbine with a governor is essential, as turbines need to be run up slowly to prevent damage and some applications (such as the generation of alternating current electricity) require precise speed control. Uncontrolled acceleration of the turbine rotor can lead to an over speed trip, which causes the nozzle valves that control the flow of steam to the turbine to close. If this fails then the turbine may continue accelerating until it breaks apart, often catastrophically. Turbines are expensive to make, requiring precision manufacture and special quality materials.

The Turbine Speed control by following parameters,

1.       Direct speed control rpm by giving speed reference.

2.       Extraction control

3.       Woodward governor control,

4.       Start-up device through amplifier HP, LP secondary oil pressure variation.

The modern digital turbine control system (TCS) is designed to control the main steam flow to the steam turbine in all operational conditions by means of the turbine throttle, governor, admission, and or extraction control valves. The control functions are conducted through the use of simplex or redundant electronic functions incorporated within the TCS software and hardware, integrated through the use of servo coil actuators.

The principle features of the TCS system are summarized as follows:

1.       Speed Control.

2.       Inlet/Admission Pressure Control-two channel selection which facilitates initial pressure (boiler follow mode) and limit pressure (turbine follow mode) functions.

3.       Load Control-via either a load setpoint command from a DCS, or can be configured to receive MW input for utilization of the load control function developed within the system, or can be configured stand-alone as a MW or Speed Droop function.

4.       Turbine Stress Influence.

5.       Frequency Influence.

6.       ADS (Automatic Dispatch System) Influence

7.       Valve Lift Control

Regards,

Prem Baboo

My previous answer is for decoupled LFC and AVR. If they are not decoupled then you get one more input from excitation system. 

I want to say thank you to everyone for answers to my question.

I want to ask how to get the materials for comprehensive mathematical modelling of steam turbine system.

Hi,

Try the following

Modelling of Steam Turbine and

its Governing System

http://www.sari-energy.org/PageFiles/What_We_Do/activities/CEB_Power_Systems_Simulation_Training,_Colombo,_Sri_Lanka/Course_ppts/Lecture_27_Model_Steam_Turbine_Gov_System.pdf

Mathematical Modelling and Simulation of the Behaviour of the Steam Turbine

http://www.sciencedirect.com/science/article/pii/S2212017313007408

you can see this lecture too: Lec-23 Dynamic Modeling of Steam turbines and Governors

https://www.youtube.com/watch?v=VtCkvN2kJSk

Good luck

Maki Habib