To regulate the speed of the Turbine at various
loads in a isolating set
To regulate the load as per the frequency demand
when working in a grid network.
To prevent and protect the turbine from over
To allow on line testing of various safety
To enable the tripping of TG set in the event of
actuation of protections.
Safe and healthy loading of the TG Set
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
The Valves in a set opens or closes
consequentially or in a sequence
It is a Partial Arc admission turbine except at
It is good for Low load or variable load
May have operational Problem at partial
All the valves in a set opens or closes
simultaneously / together
It is a Full arc Admission Turbine
It is most suitable for full load or based
No operational Problems
It is employed in small capacity turbines running on
high pressure conditions and with small blading
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.
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
Response of Constant pressure Mode is much
faster than Variable pressure mode, but Constant
pressure leads to more losses.
Mechanical: Transducer is mechanical centrifugal
speed governor which actuates controls valves
through mechanical linkages.
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.
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
Electro Hydraulic : Here transducer can be electrical or
Electronic. The generated signal after processed
electronically and electrically is fed to a Electrohydraulic converter which converts electrical signal into
Hydraulic signal. Hydraulic signal before applying to
control valve servomotor is suitably amplified
With continuous development of computer
technology, microprocessor based DCS control has
been widely used.
This 150 MW Digital electro-hydraulic control
system jointly developed by Dongfang steam
turbine works and SYMPHONY
Regulation % of Turbine is defined as
(No load Speed - Full Load speed) X 100 %
( Nominal Speed)
It varies from 2.5% to 5%
Normally it is 4.5 %
Turbine having less regulation will be more
sensitive in the grid and hence will share more load
and vice versa
Normally base load plant has high regulation and
Peak load plant has small regulation.
Turbine latching means the process to make the protection system of
steam turbine entering into security mode.
Allowable conditions for turbine latching
Steam turbine is tripped.
All main steam valves are full closed.
When condition is satisfied, allow to latching. After DEH receives the
order of turbine latching, relay is energized to close, so resetting
1YV is energized for switching into flowing.
Turbine lube oil enters into emergency tripping device, to push levers
The MSV & RSV oil supply solenoid energized 15YV,16YV,21YV and
The return oil of HP safety oil to the oil tank is shut off, PS1, PS2, and
PS3 will send out signals, and the pressure of HP safety oil is built up.
At the same time, HP tripping solenoid valve 5YV,6YV, 7YV, 8YV are
This turbine's mode of start is HIP START
In 150MW Unit, two HP MSVs and four HP CVs are used for
controlling HP steam admission, and two RSVs and two IP CVs
are used for controlling IP steam admission. Hydraulic actuator
mechanisms are used for driving the above CVs to ensure rapid
action and high positioning accuracy.
The working speed of ST is 3000r/min. The speed of ST will
vary with the load in grid. After the speed measurement loop
of ST regulation system measures the actual speed of ST and
compares it with rated speed (3000 r/min), the opening of HP and
IP CV and ICV will be controlled.
In order to ensure safe operation of ST, the hydraulic system is
also fitted with several sets of redundant protection assembly:
Emergency tripping device and testing solenoid valve HP and LP
tripping solenoid valves
Over speed limiting solenoid valves
Automatic Judgment of Thermal State
Start-up of ST is a process during which ST and rotor are heated. In order
to reduce the thermal stress during start-up procedure, different starting
curves are adopted for different initial temperature.
DEH will automatically identify the thermal state of unit according to
the temperature (T) of inner upper wall of HP inner casing at regulation
system each time closing switch. In case the temperature of upper wall
is bad, it will automatically be replaced by the one of lower wall.
Excessive hot state
2 Start-up mode
The start-up mode of the unit-HIP combined start
Before grid-connection of steam turbine-generator units, DEH is speed closed loop
isochronous control system. Its set point is a demand speed. The difference between the
demand speed and actual speed, after calculated with PID regulator, by controlling the
opening of oil servo motor by means of servo system, enable the actual speed to follow the
variation of the given speed.
After target speed is set, the given speed will approach to the target speed at a specified
raising speed rate. When it enters into the critical speed zone, the raising speed rate will be
automatically changed into 400r/min/min to pass through the critical speed zone quickly.
During raising speed, the steam turbine normally needs to be warmed up at moderate speed
or high speed, in order to reduce thermal stress.
Except that operators set target speed through OIS, DEH may automatically set target
speed under following conditions:
When steam turbine is just latched, the target is current speed
When breaker open, the target is 3000r/min;
When steam turbine is tripped, the target is zero.
When the target exceeds the upper limit, it will be changed into 3060 or 3360r/min;
When the unit is at ATC mode , the target speed is determined by ATC
During synchronization, the target will change with synchronization increasing or
decreasing signals (change rate：60r/min/min); If the target is set by mistake at the
critical zone, it shall be changed as a specified critical value.
Grid-connection & increasing load and normal control load
After the units come to constant speed, with the interface with auto synchronization
device, DEH can receive increase or decrease signals from the auto synchronization device,
and control the speed of units to realize a quick synchronization.
In case one of following conditions occurs, the auto synchronization mode will
Exit Speed < 2985 or > 3015r/min;
Grid-connection with initial load
When all grid-connection conditions are
met, DEH will immediately increase the set value,
to have the generator operated with initial load to avoid reverse power.
Since the load feedback is not put in use at the beginning of grid-connection, the pressure
of main steam shall be used to correct the set value to be increased.
Set value ＝original value＋3＋f（P0） At the beginning of synchronization, the
target is equal to this set value.
If set by operators, the load rate is within (0~100) MW/min;
Under self starting mode, the load rate is within (1.5~30) MW/min, with step length of
When switching over single valve/sequence valve or switching over valves, load rate is
When certain fault occurs with steam turbine-generator unit, you may quickly
reduce the opening of valve and release partial load, to prevent from deterioration of
fault. After RUNBACK function is put in use, when DEH receives the signal of opening
and putting in RUNBACK, the total valve position reference value will decrease at a
relevant change rate based on original value, till the signal of opening and putting in
RUNBACK disappears, or the RUNBACK function is switch off, or the reference value
is reduced to the lower limit at corresponding level. At the same time, the target and
set point i.e. equivalent to total valve position reference, will decrease accordingly.
The conditions for switching off RUNBACK:
The load is less than 25%;
Steam turbine tripped;
Based on different faults, RUNBACK may be classified as three levels, which will be
triggered by on-off input signals from No.1#, 2# and 3# RUNBCK switches.
When RUNBACK is acted, it will automatically switch off Load feedback or main
steam pressure feedback, and exit CCS mode.
Load limitation is high load limitation and low load limitation ， it’s limitation value is
setted by operator in OIS default high load limitation value is 145MW，default low load
limitation value is0MW.
High load limitation
If it is not desired to have too high load on the steam turbine-generator unit for certain
period for some reasons, the operators may set high limitation value within (low limitation
value ~165 MW), so that the load point of steam turbine is always lower than the limited
value. The high load limitation value mustn’t be lower than the low load limitation value.
When the actual load is higher than high load limitation value high load
limitation is actived cut off load feedback automatically exit CCS mode.
Low load limitation
If it is not desired to have too low load on the steam turbine-generator unit for certain
period for some reasons, the operators may set low limitation value within (2MW~ high
limitation value), so that DEH set point Is always higher than the value relating to the
Valve position limitation
If it is not desired to have too high opening of valves on the steam
turbine-generator unit for certain period for some reasons, the operators may set valve
position limitation value within (0~120) %. The load reference value or this limitation
value, whichever is smaller, shall be taken as the set value for DEH total valve position.
In order to prevent valve position from jumping, the valve position limitation value is
provided with change restrictions, and the change rate is 1%/second
turbine generating units are intermediate reheat unit
system ones, namely, one boiler operates with one steam turbine,
therefore, in order to accomplish secondary frequency regulation,
load set point of ST control system is not only properly changed but
also the load set point of boiler control system is properly changed so
that sufficient energy and safe operation of boiler can be ensured, this
is called coordinated control
operator can press "CCS" button in the "AUTO CONTROL"
graphic to make
it set 'IN's, in other words, the function of CCS control is enabled El.
2 Under the CCS control, the function of load control can' t be input,
the unit' s target load value is determined by CCS, the load rate is
100MW/min .The operator is prohibited to operate high, low load
the value is set to 165MW and.OMW automatically. (Note: the value
of load rate and load high, low limitation is determined in the field)
the boiler system can not maintain main steam pressure due to certain fault, the decrease of main steam
pressure can be slowed down by reducing steam flow by means of closing down the opening of control valve, so as
to assist in stabilizing the combustion in boiler
The conditions for switch off TPC mode
, Pressure signal failed , High/low load limitation acted , RUNBACK acted
System is at “MANUAL” mode.
The conditions for putting TPC in use
In auto mode
Main steam pressure higher than 90% rated value; Main steam pressure signal is normal Main steam pressure
limitation power-up default value is 10MPa, and the operators
may set this limitation value within (3~13) MPa when switch off TPC mode.
During TPC mode being put in use, if the main steam pressure is lower than the
specified value, the main steam pressure limitation will be acted. When it is acted, the
set point will decrease at a change rate of 1%/second based on the point where the action
is started. At the same time, the target and set point i.e. equivalent to total valve position
reference, will decrease accordingly. If the main steam pressure rises again above the
limitation value, the set point will stop decreasing. If the main steam pressure failed to
return back, the set point will stop decreasing when the total valve position reference is
less than 20%.
When main steam pressure limitation is acted, automatically switch off Load feedback and main steam pressure
feedback, and quit CCS mode
The method that is used to prevent the speed of steam turbine from jumping into steam
turbine tripping speed is called as overspeed limitation.
time constant of rotor of large capacity steam turbine is small and the
time constant of volume of steam cylinder, when load rejection takes place, the speed of
steam turbine will fly swiftly upward, if it is controlled with the speed feed back of the
system only, the highest speed may be over 110%, thus emergency tripping of steam
turbine may occur. the overspeed limitation solenoid valve should be acted quickly, HP&IP
control valves are closed, and the target speed and the given speed should be changed into
3000r/min. After a period of time, the overspeed limitation solenoid valve is de-energized,
the control valves are controlled by the servo valves again, speed closed-loop control is
restored, and finally the speed of the steam turbine is stabilized at 3000r/min. in this way,
the synchronization can be carried out quickly after the accident is eliminated.
overspeed occurs, the life of the steam turbine will be influenced negatively. Except the
speed can exceed 103% speed only during the overspeed test of steam turbine, it is allowed
to exceed 103% speed at any time (103% means network frequency up to 50.5Hz).
When not in overspeed test, once the speed exceeds 105%, act up the overspeed limitation
solenoid valve, close HP&IP control valves. After the speed is lower than 103%, the
overspeed limitation solenoid valve is de-energized, and the control valves are controlled by
the servo valves again.
Note: when performing overspeed test, the function of 103% overspeed limitation is
If the speed of steam turbine is too high, the steam turbine may be damaged
due to action of centrifugal stress. In order to prevent from overspeed of
steam turbine, the DEH system is provided with overspeed limitation
function, but if it is failed to control the speed, and the preset speed is
exceeded, turbine tripping should be carried out immediately, to close all
main steam valves and control valves.
For safety and reliability, many overspeed protections are set in the system:
DEH electric overspeed protection 110% Mechanical over speed protection
Mechanical over speed energized HP trip test module solenoid
valve5YV,6YV,7YV,8YV, HPCV oil supply solenoid 9YV to 12YV, IPCV oil
supply solenoid 17YV, 18YV and HP trip solenoid 27YV, 28YV
Electrical trip energized electrical trip solenoid 3YV, HPCV & MSV oil
supply solenoid 9YV to 14YV, IPCV & RSV oil supply solenoid 17YV to
20YV and HP trip solenoid 27YV, 28YV
startup, in order to reduce the thermal stress of HP cylinder, startup in
single valve mode shall be used normally. During normal operation, sequence
valve mode shall be used normally, in order to reduce the throttling loss of HP
Normally, the single valve mode is mandatory when the load is < 30%. While it
is started at hot state and extreme hot state, due to small thermal stress and high
steam parameter, the opening of valve will be too small if it is started in single
valve mode, which will result in instable control, so it can be switched into startup
in sequence valve mode.
Set value for CV = single valve coefficient X set value for single valve + (1-single
valve coefficient) X set value for sequence valve
Under single valve mode, single valve coefficient is 1. After the operators send
out the order of switching to sequence valve mode, it will take 10 minutes for
single valve coefficient to become from 1 to 0, and finally keep as 0, which is
known as sequence valve mode. During the switchover of valve control modes,
the main steam pressure feedback or Load Back shall be put in use normally to
maintain the load constant.
In order to guarantee that the fly loop of emergency overspeed governor can
quickly fly out once the overspeed of units occurs to trip the steam turbine, the
flyball should be tested for free movement.
During such a movement test, oil may be sprayed in the fly loop to increase
centrifugal force, so that it can fly out. The flyout of flyball due to oil injection
test shall not result in tripping. In order to improve the reliability, test isolation
valve is added.
Allowable conditions for oil injection test:
Test soft button in allowable position;
The speed within (2985~3015) r/min;
HP expansion differention is lower than 3mm.
During the test, 4YV energized, 2YV energized, oil is sprayed into emergency
tripper, fly ring strikes out, and ZS2 sends signals, 2YV is de-energized, after a
while, 1YV latch automatically,when latched, the isolation valve is de-energized,
which means a successful test.
After the steam turbine is installed for the first time or after an overhaul,
the precision of overspeed protection action must be verified, and each
loop of overspeed protection shall be tested for verification. When doing
overspeed protection test, set the target speed of DEH as 3360r/min,
and slowly increase it to the speed of steam turbine. When the action
speed of one loop of overspeed protection under test is met, this loop of
overspeed protection will be acted to trip the steam turbine. For
this reason, overspeed test is also called as raising speed test. DEH can
automatically record steam turbine tripping speed and the highest speed.
a) DEH electric overspeed test
The test can be carried out by setting target value as 3360r/min and the
rate as 180r/min/min on OIS.
b) Mechanical overspeed test
In OIS, set target speed to 3360r/min ，the turbine’s speed raise from
3000 r/min speed that fly ring fling out tripped turbine. the speed that fly
ring fling out should be between at 110%～111% .
During normal operation of the unit, action test for HP and IP main
and HP and IP regulating valve can be carried out regularly to prevent
valve from blocking. Full travel action test can be carried out to HP and
IP main steam valve and regulating valve.
Allowable conditions for action test of valve:
All main steam valves fully opened.
Load larger than 65 MW but less than 135MW;
Not in CCS mode
DEH can control the unit to conduct MSV leak test and CV leak test in
case ST idles at rated speed and boiler steam pressure meets certain
Allowable conditions for valve leak test:
Speed is higher than2990r/min；
The breaker is off；
HP tripping solenoid valves can be tested on OIS
Under normal conditions, 5YV, 6YV, 7YV and 8YV solenoid valves are energized,
and pressure switches PS4 & PS5 are in normal positions. During test,
de-energized, 6YV, 7YV and 8YV are energized, and PS5 sends signals, which
successful test of 5YV. 7YV may be tested in the same way with 5YV.
6YV is de-energized, 5YV, 7YV and 8YV are energized, and PS4 sends signals,
which means a successful test of 6YV. 8YV may be tested in the same way with
If the solenoid valve is acted, and pressure switch signal is not correct
seconds, the test can be considered failed
Loading and unloading piston