Steam turbine Governing system is a heart system of steam turbine to maintain synchronous speed of the turbine under changing load/grid; it helps in raising the speed of the turbine and synchronizing it to the grid power system. Now days, the governing system for a steam turbine is equipped with electro-hydraulic governor: fully backed up by the hydraulic governor. The measuring, processing in the form electrical signal provides advantages such as flexibility, dynamic stability and simple representation of complicated functional systems. The integration of electrical and hydraulic system is an excellent combination with following advantages:
- Exact load frequency droop with high sensitivity.
- Prevents over speeding of the turbine during load through off.
- Fine adjustments of speed of the steam turbine during on-loan conditions.
Components of Steam turbine governing system:
The main components of governing system and brief description of them is as follows
Main trip Values:
The main trip values are the main trip gear components in steam turbine protection circuit. The control oil flows from the remote solenoid values to the main trip valves. The trip oil from main trip valves forms two paths called as Trip oil and Auxiliary Trip oil. The Trip oil is supplied to HP and IP stop valves. The Auxiliary trip oil is supplied to other hydraulic trip devices like over speed trip device, low vacuum trip device etc which forms a closed path. If these trip devices operate these will drain the auxiliary trip oil from the main trip valve and causes it to operate.
Start up and Load Limiting Device:
The start up and load limiting device is an important component of turbine governing system and its functions are as follows:
- To reset the stop valves after tripping
- To Open the Stop values during starting
- To release the control valves and operates the control values during starting of the turbine.
The start-up and load limiting device consists of pilot value with a plunger which can be operated by means of hand wheel or a motor from remote. The start up device mechanically acts up on hydraulic governor with a lower break mechanism. Before starting the turbine the pilot valve is kept in o% position to release the start up oil which resets the stop valves.
Once it is brought to 0% and after ensuring the reset position of Stop valves, the pilot valve is moved to raise the start-up device. When the pilot valve i.e. starting device is raised, the start-up oil and auxiliary start-up oils are drained which then opens the stop valves (HP ESV opens at 45% and IP stop vales opens at 56% position of starting device). Further raising the start-up device releases he blocking spring of hydraulic device to stop the auxiliary secondary oil draining by closing them.
Hydraulic Speeder Gear:
Hydraulic Speeder gear is a speed adjustment device which operates on the difference of spring force and feedback primary oil pressure. The spring tension can be adjusted by means of a hand wheel or by an electrical motor. This hydraulic speeder gear spring is limited by the start up load limiting device. The control oil acts on Auxiliary follow up position valves to stop the draining auxiliary secondary oil. This auxiliary secondary oil is further applied to hydraulic amplifier to produce secondary oil which controls control valve position.
Load Shedding Relay Solenoid:
Pair of solenoid valves is placed in HP and IP secondary oil pressure line which are operated through an electrical pulse generated by the load shedding relay. These are placed to prevent the over speeding of the turbine during major load through off. The Negative power spike is captured by the load shedding relay and it generates a pulse with a width proportional to the load decreased. The electrical pulse opens the solenoid valves in two lines causes to drain the secondary oil pressure. As a result the HP control valves and IP control valves will close instantaneously. So that the speed of the turbine is in design limits. Once the pulse is completed or de-energized the solenoid valves again recloses and mean time HP and IP secondary oil pressure build-ups according to the new Steam turbine load.
The HP and IP stop valve servo motors receives the trip oil through their associated valves called test valves. The test valves have two port opening, one is for trip oil and second one is start-up oil. The trip oil is supplied beneath to the stop valves which are normally Open position of the valve. This is for the purpose of resetting the valves after tripping.
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When the start up oil pressure builds up in the test valve, it forces the plunger against the spring force which intern pressurises the trip oil beneath the test valve. This trip oil is accumulated at the top of the stop valve forcing the level of stop valve down. Once the start up oil pressure reduces then it releases the spring and the trip oil top of the stop valves drains, same time the trip oil pressure beneath the stop valve pressurises and raises the liver which is opening of the stop valve.
Electro Hydraulic Controller:
It is a heart component of modern governing system where the sensing and processing is done electronically and this electrical signal is converted to hydraulic signal through Electronic to Hydraulic converter. The main functions of Electro Hydraulic controller is as follows:
- Speed control of the turbine during start-up up o synchronization and during load through of conditions.
- Turbine load controller once it is synchronized to the grid power supply.
- Various other control like steam generator pressure control if needed.
- Integrated Operation with turbine stress elevator to trip the turbine in case of any stress violations.
- ATRS system can be easily integrated with electro hydraulic controller.