GOVERNING OF STEAM TURBINE
Topic Covered in Part-1
Need of Governing System
Function of governing System
Types of Governing System
1.) Need of Governing System
- To control speed
- To maintain speed
2.) Functions of Governing System
- To maintain speed
- To regulate the load as per the frequency demand when working in a grid network.
- To Ensure desired percentage of load distribution
- To maintain desired speed in isolated operation of the set.
- To regulate/control the load as per the frequency demand when working in a grid network.
- To insure desired percentage of load distribution between sets running in parallel in a grid network to maintain frequency (the quality of out put).
- To control the initial run up and synchronization of the machine
- To control the turbine load when running in parallel with other generating sets.
- To contain the speed rise within acceptable limits in case of the unit getting disconnected from the load
- To provide protective gears to ensure the safe operation of the turbine
- To regulate the speed of the Turbine at various loads in a isolating set
- To control steam valve positions in response of operator or automatic control system.
- To bring turbine speed smoothly from baring gear to synchronization speed under the control of operator action.
- To synchronize Turbo-generator unit with grid under control of operator or automatic control system.
- Safe and healthy loading of the TG Set
- To prevent and protect the turbine from Transient speed rise/over speeding.
- To allow on line testing of various safety protections
- Protection of Turbine—To trip TG set in the event of emergency by actuation of protections
Thus the individual TG governing system imply a need to
Withstand a full load rejection safely
Provide appropriate contributions to system frequency control.
Safe Rolling, Synchronizing, Loading And Unloading Of The Turbine.
Maintaining Load/Throttle Steam Pr. / Speed as per requirement
SAFE ISLANDING OPERATION in Case Of Grid Disturbance
PROTECTING THE TURBINE in Case Of Emergency
3.) TYPES OF GOVERNING
- TYPES OF GOVERNING-I
- TYPES OF GOVERNING-II
- TYPES OF GOVERNING-III
1.) TYPES OF GOVERNING-I
- Nozzle Governing
- Throttle governing
- Bypass Governing
Governing is effected by varying the amount of steam which is further done by changing the positions of control valves. Any Throttling of Steam will lead to losses . In governing system along with the change of quantity of steam the quality of the steam also changes. Therefore Throttling should be minimum.
Comparison between nozzle and throttle Governing
- To control load, the turbine can be either NOZZLE CONTROLLED or THROTTLE CONTROLLED with the unit in both cases operating at constant steam pressure or variable pressure or a combination of two.
Throttle Governing System
In throttle controlled turbines, steam flow is controlled by opening and closing of all the control valves simultaneously to the extent required by load and admitting the steam to the group of nozzles located on the entire periphery.
Nozzle Governing System
In nozzle controlled turbines, steam flow is controlled by sequential opening or closing of control valves allowing steam to flow to associated nozzle groups.
- It is employed in small capacity turbines running on high pressure conditions and with small blading dimensions.
- Here the loading up to appx 80% ( Economic loading ) is met by normal control valves feeding the First stage. For higher loading , to supply more steam which is not possible due to small blading dimensions ( can lead to operational problems) the extra quantity of the steam is fed to the intermediate section of turbine bypassing the initial high pressure stages.
2.) TYPES OF GOVERNING-II
- Constant Pressure Mode: Here pressure upstream of control valves is kept constant and change is made by changing the position of control valves.
- Variable Pressure mode: Here control valves are in full open position and pressure upstream of control valves varies proportionately with the load requirement.
Response of Constant pressure Mode is much faster than Variable pressure mode, but Constant pressure leads to more losses.
3.) TYPE OF GOVERNING -III
- Mechanical: Transducer is mechanical centrifugal speed governor which actuates controls valves through mechanical linkages.
- Hydraulic: Here speed transducer is a centrifugal pump whose discharge pressure is proportional to square of speed. This signal is sent to hydraulic converter which generates a signal which is proportional to valve opening/Closure is required. Before applying it to control valves servomotor this signal is suitably modified.
- Electro Hydraulic: Here transducer can be electrical or Electronic. The generated signal after processed electronically and electrically is fed to a Electro- hydraulic converter which converts electrical signal into Hydraulic signal. Hydraulic signal before applying to control valve servomotor is suitably amplified.
- Any Combination of Above Three: Such As Hydro mechanical: Here transducer is a centrifugal speed governor .It is connected to a Hydraulic system where the signal is amplified so that Control valves servomotor can be actuated.
* Commonly Used System ELECTRO-HYDRAULIC GOVERNING (EHG)
- In mechanical governing system, speed transducer is mechanical centrifugal type speed governor, which actuates control valves through mechanical linkages.
- Now-a-days purely mechanical governing systems are not used for utility turbines.
PRINCIPLE OF MECHANICAL GOVERNING
2.) Hydraulic Governor
- A hydraulic Governor for a turbine consists of a centrifugal pump driven from the main shaft which supplies oil to a cylinder containing a spring loaded piston or bellows. The pressure is proportional to the square of the speed, so the position of the piston or bellows is a function of speed.
- In hydraulic governing system , speed transducer is a centrifugal pump , whose discharge pressure is proportional to square of speed.
- This signal is sent to hydraulic converter / transformer which generates signal proportional to valve opening / closure required.
- Before applying the signal to control valve servomotors, the same is suitably amplified.
3.) Electro-Hydraulic Governor (EHG)
- This system provides very good combination of electrical measuring & signal processing and hydraulic controls.
- It offers many advantages over other three types of governing systems and is popular in large steam turbine units due to growing automation of turbine and generator sets.
4.) Any Combination
- Common design incorporates a permanent type shaft driven permanent magnet generator producing an a.c. wave having frequency of shaft speed. This wave is fed into an electric filter that resonates at a frequency corresponding to rated speed and the output is rectified to produce a d.c signal proportional to difference between rated and actual speed. This signal is amplified to form required speed correction signal.
- Another design utilizes digital signals from a shaft driven square wave generator, the output is compared with a reference square wave signal generated by an oscillator. The output of the comparator is rectified and amplified to form required control signal.
DIGITAL ELECTRO HYDRAULIC SYSTEM
With continuous development of computer technology, microprocessor based DCS control has been widely used.
150 MW Digital electro-hydraulic control system jointly developed by Dongfang steam turbine works and SYMPHONY