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.
Test Valves
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.
Operation:
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.
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