Notes on Governing System of KWU Steam Turbine: By K. Venkata Rao, Chief Engineer (Retired) - APGENCO General: The purpose of the Governing System is to regulate the turbine speed till the unit is synchronized and then to regulate the load on the unit as per the Load Limiter setting after the unit synchronization, these two functions being called as Speed Governing & Load Governing characteristics. Before knowing how the Governing System is regulating the unit behavior, we should know about the normal steam flow path, about the specific valves that are to be operated by the Governing System and their location in the steam flow path. The Control Oil which is supplied by the AOP/MOP plays a vital roll in the operation of the governing system which assumes different names and pressures depending up on its function as per the governing logic. Steam flow path : Main Steam from the boiler enters the HP Turbine through the Emergency Stop Valves and the HPT Control Valves. The CRH steam from the HPT exhaust flows to the reheater in the boiler and the HRH steam delivered by the reheater flows to the IP Turbine through the IVs & IPT Control Valves. From the IPT exhaust the steam flows to the LP Turbine, directly through either cross over or cross around pipes without any valves in between the IPT exhaust and the LPT steam admission point. From the LPT exhaust the steam flows to the condenser 1
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Notes on Governing System of KWU Steam Turbine:
By K. Venkata Rao,
Chief Engineer (Retired) - APGENCO
General:
The purpose of the Governing System is to regulate the turbine speed till the unit is synchronized and
then to regulate the load on the unit as per the Load Limiter setting after the unit synchronization, these
two functions being called as Speed Governing & Load Governing characteristics.
Before knowing how the Governing System is regulating the unit behavior, we should know about the
normal steam flow path, about the specific valves that are to be operated by the Governing System and
their location in the steam flow path. The Control Oil which is supplied by the AOP/MOP plays a vital
roll in the operation of the governing system which assumes different names and pressures depending
up on its function as per the governing logic.
Steam flow path:
Main Steam from the boiler enters the HP Turbine through the Emergency Stop Valves and the HPT
Control Valves. The CRH steam from the HPT exhaust flows to the reheater in the boiler and the HRH
steam delivered by the reheater flows to the IP Turbine through the IVs & IPT Control Valves. From the
IPT exhaust the steam flows to the LP Turbine, directly through either cross over or cross around pipes
without any valves in between the IPT exhaust and the LPT steam admission point. From the LPT
exhaust the steam flows to the condenser where it is cooled as condensate by the CW System and
collected in the condenser hot well.
About the operation of ESVs, IVs, and the HPCVs & IPCVs of a Steam turbine:
The HP Turbine Stop Valves called as ESVs & the IP Turbine Stop valves called as Interceptor
Valves will either open by 100% or close fully to 0% opening by the action of the servo motor
assembly mounted on each Stop Valve Body when open or close command is initiated.
The HP control Valves and the IP Control Valves will be operated by a common servo motor
called Control Valve Servo Motor (CVSM). A teethed lever with the teeth on its lower side
extending to both sides and connected to the crank levers of the HPCVs & IPCVs will be attached
to the piston of the CVSM. Whenever the CVSM is operated to regulate the opening of the
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Control Valves the Control Valves of both HPT & IPT will be actuated simultaneously to the
required % opening to stabilize the turbine speed as well as load on the unit.
The condensate so collected in the hot well is pumped to the boiler after preheating in the LP Heaters
and HP Heaters.
Functions of Turbine Governing System:
Thus it can be understood that the Stop Valves and Control valves of the HP Turbine and IP Turbine
are to be opened to roll the turbine, and the Control Valves opening is to be varied to regulate the
turbine speed till the unit is synchronized & also to regulate the steam admission to the turbine,
during load raising & load variations to suit the grid demand. All these operations are achieved by the
Turbine Governing System installed.
The working medium of the Governing System Components is the Control Oil supplied by the AOP
initially during the turbine start up and finally by the MOP after rolling the turbine to its rated speed.
Hence the Governing System is called the Hydraulic Governing system. Further the turbine will be
tripped through the governing system only on the initiation of the Turbine / Unit Lock out Relay. After
the developments taken place the Electro Hydraulic Governing System is also installed along with the
Conventional Hydraulic Governing System.
For tripping the turbine on the initiation of the Turbine/Unit Lock out Relay, trip command will be sent
to the Turbine Trip Solenoid Valves and consequent to the actuation of these solenoid valves the ESVs
will close first and then the control valves will close and the Generator Circuit Breaker will be tripped,
thus isolating the unit from the grid. After the unit tripping the unit will coast down to the barring
speeds of 180 rpm and continue to run at this speed where the jacking oil pumps are available to keep
the machine floating on jacking oil pressure.
If the TG set is provided with Barring Gear Motor the turbine will be allowed to come to stand still, after
which the barring gear will be engaged manually and the Barring Gear Motor will be switched on from
the local control panel. The barring speeds for the turbines provided with the barring gear mechanism
will be 3.0 rpm.
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Types of Governing system :
There are two types of Governing Systems called as the Hydraulic Governor and the Electro Hydraulic
Governor. The Hydraulic Governing system will act as a back up for the Electro Hydraulic Governing
System. Function of both types of Governors is the same, the difference in both the systems being only
that:
The Hydraulic Governor operates based on the Auxiliary Secondary Oil signal input received
from the Speeder Gear.
The Electro Hydraulic Governor operates based on the 0-10 Volts supply received from the
ATRS.
All the components of the governing system are common for both systems except that there are
dedicated follow up valves for each system. Hence the HP Secondary Oil & IP Secondary Oil will
be supplied by both sets of Follow up Valves. The main difference between both the systems
being only the input signal whether it is hydraulic or electrical.
Both the governing Systems get the Control Oil supply from the common internal System.
The dedicated follow up valves of each Governing System will modulate as per the signal oil
supplied by the respective Governing Systems and supply the HP secondary oil and IP secondary
oil to operate the respective Stop Valves & Control valves.
The combination of the Starting Device and the Speeder Gear is called the Hydraulic Governor
which provides the input hydraulic signal to the hydraulic converter which in turn delivers the
signal to the follow up valves after amplification of the input hydraulic signal.
Main functions of the Governing System:
The Stop Valves will be opened to 100% opening or closed fully to 0% opening.
The main steam flow or steam admission to the HP Turbine will be regulated by varying the %
opening of the Control Valves for rolling the turbine and also for loading the Turbine.
After synchronization, the load on the generator will be picked up by admitting sufficient
quantity of steam into the turbine to match the load demand.
If the connected load has exceeded the generation, the turbine speed will fall momentarily
which will be brought to normal rated speed by increasing the opening of the Control Valves and
thus admitting more quantity of main steam into the HP Turbine till then the turbine speed is
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brought to the rated speed thus stabilizing the unit at the new level of load demand. The control
valves opening will be increased by the Governing System.
Similarly if the demand on the generator has decreased, the turbine speed will rise momentarily
which will be brought to the normal rated speed by decreasing the opening of the Control
Valves and thus admitting less quantity of main steam into the HP Turbine till then the turbine
speed is brought to the rated speed thus stabilizing the unit at the new level of load demand.
The control valves opening will be decreased by the Governing System.
In case of unit tripping, the turbine trip command will be sent to the Remote Trip Solenoid
Valves of the Governing system by the Turbine / Unit Lock out Relay, consequent to which the
turbine Stop Valves and Control Valve will close in sequence and the generator circuit breaker
also trips thus isolating the TG Set from the grid.
The T.G. Set will coast down and will run at barring speed of 180 rpm till the next unit start up.
Important Components of the KWU Steam Turbine Governing System and their functions:
Sl.
No
Description of
component Quantity Function
1Remote Trip
Solenoids2
When any turbine trip gets initiated, the solenoids get energized and
the Control Oil gets drained through these valves and (actuates)
trips the MTV.
2 Main Trip valves 2
These Valves are also called as the main trip gear. Turbine tripping
takes place through this device. While the unit is in service or under
non trip condition these MTVs will establish Trip Oil & Auxiliary Trip
Oil which will be used for resetting & opening the Stop Valves and
also for producing the HP Secondary Oil & IP secondary Oils.
3 Starting Device 1
As the name of this device denotes, this device is used for resetting
and opening the Stop Valves and Main Trip Valves & Hydraulic Trip
Devices and prepare the Governing System ready to start the
Turbine. This device can be operated either manually by hand wheel
or from remote (MCR) through the motorized actuator.
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4 Speeder Gear 1
This device in combination with the Starting Device forms the
Hydraulic Governor and provides the input hydraulic signal to the
hydraulic converter. This device can be operated either manually by
hand wheel or from remote (MCR) through the motorized actuator.
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Hydraulic &
Electro
Hydraulic
Converter
1 + 1
Amplifies and converts both the hydraulic & electrical signals into
hydraulic signals called the HP Secondary & IP Secondary Oils for the
operation of HP & IP Control Valves.
6Hydraulic Trip
Device1
This device provides the mandatory protections of the turbine by
draining the Auxiliary Trip Oil through it. This device is reset by the
Auxiliary Start up Oil.
Description of various oils in the Governing System:
Sl.
No:Name of the oil Description
1 Control OilIt is the oil taken from the AOP / MOP discharge header fro the Governing
System.
2Trip Oil & Auxiliary
Trip Oil
These two oil circuits are established through the Main Trip Valves under
reset condition.
3
Start up Oil &
Auxiliary Start up
Oil
These two oils are established to their maximum pressure through the
Starting Device at 0% position and their pressures start falling when the
position of the Starting Device is raised.
4Auxiliary
Secondary Oil
This oil circuit is nothing but the output oil of the Hydraulic Governor and
the input oil to the hydraulic converter.
5 Secondary Oil Out put oil of the Hydraulic/Electro Hydraulic converters.
6 Test OilThis oil circuit will be established to test the Over Speed Trip Device while
the turbine is running at rated speed.
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Description of various oils in the Turbine Governing System, their source, pressures and their function:
Sl
No
Name of the
Oil
Its source &
pressure Purpose/Function of the oil
1 Control Oil
AOP/MOP
(7.0 – 8.5 Kg/cm
sq)
To establish the Trip Oil, Aux. Trip Oil, Start up Oil & Aux.
Start up Oil, Aux. Secondary Oil, Secondary Oil Primary Oil
etc.
2 Trip Oil
Main Trip Valve
(4.8 – 6.0 Kg/cm
sq)
To Reset, Open & Trip the Stop Valves
3Auxiliary Trip
Oil
Main Trip Valve
(4.8 – 6.0 Kg/cm
sq)
To trip (actuate) the Hydraulic Trip device.
4 Start up OilStarting Device
(5.5 Kg/cm sq)To assist resetting & opening the stop Valves.
5Auxiliary Start
up Oil
Starting Device
(5.5 Kg/cm sq)To reset the Main Trip Valve & the Hydraulic Trip device.
6Auxiliary
Secondary Oil
Speeder Gear (2.3
– 4.8 kg/cm sq)It is the input signal oil to the Hydraulic Converter.
7Secondary Oil
(HP & IP)
Hydraulic / Electro
Hydraulic
Converter
(0 – 6 Kg/cm sq)
To govern the HP & IP Control Valves opening.
8 Primary OilPrimary Oil Pump
(0 – 2.3 kg/cm sq)
It gives the feed back signal for the Hydraulic Governing
System.
9 Test Oil
Over Speed Test
Device (2.0 kg/cm
sq)
To test the Over Speed Test Device.
10 DrainsAll drains in the
system
By draining the various governing oils from a particular
section, the particular component of the Governing System
will be activated to discharge its function.
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Block Diagram of Governing System: (Slide – 7 of the Power Point Presentation)
The following are the various components of the Governing System: