TURBINE GOVERNING SYSTEM
Abstract----
Steam turbine control systems are being designed with today’s technology to operate a turbine in a safe and reliable manner. There are many considerations to be taken when choosing a controller for steam turbine applications. Many benefits may be realized by choosing the proper steam turbine control system, whether it is a mechanical, an electrical or a programmable controller system. This paper presents the basic concepts of the steam turbine control system and develops the fundamentals of the speed control. Using MATLAB/Simulink software facilities, have been simulated the speed variations, of the steam turbine-load unit, connected with a mechanical-hydraulic system, than the speed deviations of the steam turbine-load unit, to different load deviations, proportional and proportional-integrative control algorithms.
1. Introduction-----
Steam turbines are energy conversion machines. They extract energy from the steam and convert it to torque, which rotates the shaft of the turbine. The amount of energy that t m is a function of its temperature and pressure
There are two categories of steam turbine controls as follow: safety (protection) system and process system.
The turbine safety systems are intended to eliminate/minimize the possibility of damage to the machine or the hazard to operators. It protects the turbine from over speeding, monitors all critical turbine parameters, and trips the turbine if a condition exists that could cause equipment damage. The main safety control element on a turbine is the steam supply valve. This safety valve can be a separate on/off valve or the shut-off function can be incorporated into the controls of the steam supply valve that is used for speed control. The process systems control the operation of the steam turbine, so as to follow the load in a stable and efficient manner. The steam turbine is controlled by the governor, which can be mechanical-hydraulic (developed from James Watt’s original flyball governor) and/or electrical. They all include a pilot valve, or controller, which modulates the turbine’s inlet valve in order to keep the shaft speed on set point. An electro-hydraulic control system use electronic circuits, is more flexibility, but the overall requirements are similar. A digital electro-hydraulic control system, use a digital controller and a lot of functions can be implemented through software. Even the advanced controls usually operate the high-pressure and low-pressure valves through the existing electro-hydraulic controls of the turbine. A typical governor model for steam turbines has two main sections, the governor and steam control valve, whose output is effective control valve area in response to speed deviation of the machine, and a section modelling the turbine, whose input is steam flow and output is mechanical power applied to the rotor.
In
this block diagram, steam flow to the turbine is the product of valve area and
throttle pressure. Thus, if throttle pressure drops due to an increased demand
in steam flow, the valve area must further increase to maintain the same flow
as compared with the simplified model.
2. Fundamentals of speed steam turbine control
A
steam turbine control system is a closed loop system. The simplest application
is one in which a turbine is used to operate a rotor to constant speed (Fig.
2). The controller senses the shaft speed, compares the actual speed with the
desired set point. If there is a difference between the actual speed and
desired speed, the controller sends a signal to the actuator operating the
steam valve, which will adjust the speed until the two are again balanced
[1,2,3,5]. Variations in load, caused by shaft loading or variations in
pressure supply, affect the balance between the energy supplied to the turbine
from the steam system and the work removed from the turbine’s shaft. If more
energy is available than is being used, the shaft will speed up. The controller
will detect this increase in speed and act to eliminate it. Its means of doing
so is to reduce the energy supplied to the turbine by closing the supply valve.
If the net change in the energy balance were negative, the shaft would slow down and the controller would respond by opening the supply valve.
- droop control or proportional-only control (P), defined as a decrease in speed with an increase in load, produces a change in valve position proportional to the signal between the speed set point and the actual speed; the main disadvantage of proportional-only control is that it cannot completely eliminate the error caused by a change in load.
- isochronous control, defined as no decrease in speed with an increase in load, maintain the speed of shaft rotation constant regardless of load; in order to eliminate the error and to minimize the overshoot, it is accomplished with proportional and integrative (PI) controllers, usually in conjunction with derivative (D) controller, resulting the steam PID type controllers. Depending on the control required and the number of valves, a steam turbine may employ several control loops, each of which requires a feedback loop.
Depending on configuration, the steam turbine is equipped with high pressure valves (HPV), re-heater valves (RHV), and consist of high pressure (HP), medium pressure (MP) and low pressure (LP) sections (Fig. 3) [15]. As a requirement for any turbine - generator unit, the speed/load control function is achieved through the control of the HPV (adjust the speed/load set point to control admission of steam to the turbine through HPV pos). In addition to the HPV, a second valve is required. It controls the steam flow rate that is extracted from the HP section of the turbine and is sent to the MP, LP sections and crossover pipe (CP). The integrity of the turbine depends on the possibility to limit the speed, involving the reheat valve position (RHV pos). The re-heater stores a large amount of steam so, the HPVs control is not enough to limit the overspeed. The overspeed control involves fast control of the HPV and RHV, because the RHV controls about 60% to 80% of the total power (steam flow to MP and LP sections). The output of the speed sensor is compared with the speed set point and the error signal is used to control the HPV and RHV. The power servos are used to amplify the energy levels necessary to move the steam valves.
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