FIELD OF THE INVENTION:-
This invention relates to an Electro-hydraulic Governor (EHG) for Steam
turbines in power plant operation. In particular, the invention relates to
EHG with improved control and automation of EHG operation.
BACKGROUND OF THE INVENTION:-
Steam turbines were originally equipped with hydro-mechanical
governing system for the control and operation of 210 MW Steam
Turbines. A set of mechanical and hydraulic signals with proper
consideration of all the process parameters govern the operation of steam
turbine. All the operations like starting and loading of the turbine sets
are performed manually by operating speeder gear hand wheel located in
the field or by operating speeded gear motor remotely from unit control
panel. Hydro mechanical governor also serves to prevent the undesired
over-speeds following sudden loss of load. The control action is of the
proportional type with steady state overall speed regulation of 4 ± 1%
with fixed droop.
In view of state of art technologies and steam turbines with conventional
control methods, it was necessary to develop improved control
equipment. The development involves an Electro-hydraulic governing
system along with Digital Control System (DCS) for the turbine controls.
Using DCS techniques, apart from EHG, other systems can also be
developed like Automatic Turbine Run up System (ATRs) etc.
Presently the design incorporating the modified Electro-hydraulic
governing equipment utilizing the existing governing system is not
available. The existing governing scheme is improved to incorporate

the changes suiting to the Electro-hydraulic system keeping in mind the
existing space with redundant measurement. The development work
would involve design and development of EHC along with proportional
valve, design of additional new equipment for speed/ position
measurement.
In the existing 200/210 MW steam turbines, Hydro-Mechanical
governing system has been foreseen to maintain the speed/ load at
desired set point during start up and shut down and normal operation.
Control strategies were implemented for turbine pre warming, speed
ramp, synchronization, initial loading and throttle to nozzle transfer of
turbine speed and load controller.
The Hydro mechanical system has the following features:
1) This system is a collection of mechanical links and lever, hydraulic
circuit and control valve servo motors for governing the control valves. In
normal operation, system provides a means it starting and stopping the
turbine and controlling speed and power output.
2) Acceleration from speeder gear to rated speed is strictly a manual
operation. There is no acceleration control and control of loading rates.

3) Back-up control to arrest drop in main steam pressure.
4) Electrical speed measurement is not utilized for governing.
5) All the operations of starting and loading of the set are performed by
manually operating the speeded gear wheel located at front pedestal of
the turbine or from unit control room.

Objects OF THE INVENTION:-
An object of the invention is to propose an improved governing system for
more precise control of speed.
Another object of the invention is to provide a scheme to aid to restricted
governor mode of operation (RGMO) requirements.
Another object of the invention is to enable easier modification (through
DCS) in droop setting.
Yet another object of the invention is to avoid operator intervention.
DETAILED DESCRIPTION OF ACCOMPANYING DRAWINGS:-
Figure 1 shows control block diagram.
Figure 2 shows improved governing scheme block diagram.
DETAILED DESCRIPTION OF A PREFERED EMOBODIMENT OF THE
INVENTION:-
Fig.1 shows an electro-hydraulic governing system essentially that
consists the following mode of operation
1) Speed Control Loop
2) Load control Loop
3) Pressure Control Loop
One of these three loops acts selectively on the subordinate valve lift
control loop. The valve lift controller actuates the proportional valve of

the electro-hydraulic converter, which acts as an interface between the
electrical and the hydraulic systems. For initial rolling of the turbine,
speed controller is in action. Once the set reaches 3000 rpm,
synchronization of the set is done. Pressure controller has been
envisaged mainly to replace the function of Initial Steam Pressure
Unloading Gear (ISPUG), which is partial closure of Control Valves in
case of sudden pressure reduction to reduce the load.
Electro-hydraulic governing system has been developed for operation of
control valves (of both HPT and IPT) only through the existing common
servomotor provided for these valve. The opening/closing of Emergency
Stop Valve (ESVs) and Interceptive Valve (IVs) two existing servomotors
each is realized through operation of speeder gear which is operated
through a hand wheel or from UCP/DCS. The speeder gear actuates
Load/Speed Control Pilot valve which in turn opens/closes Emergency
Stop Valve (ESV) and Interceptive Valve (IV) through their respective
servomotors. These are provided in the original hydro mechanical design.
Closing of ESVs and IVs may be achieved by operation of speeded gear,
turbine shut down switch or additional trip solenoid valves (2 no’s) Once
the ESVs and IVs are opened, turbine controller in conjunction with EHC
along with Pilot Valve (PV) takes over for opening of Control Valves (with
a speed set point fed through Operator Work Station). The output of the
Proportionate Valve (PV) acts on servomotor (for 4 no. HPCVs and 4 no.
IPCVS) of the control valves directly. The conditions required for taking
over of EHC from mechanical governing are (AUTO mode)
(a) Turbine speed ≥ 1200 rpm
(b) Steam/ Metal Parameters at required value.
Interface between the electronic controller and the hydraulic valve
positioning system has been conceptualized through a proportional valve.
The proportional valve controls the flow of control oil above/below the

piston of electro-hydraulic converter.
Redundancy in speed measurement, Redundancy in load measurement
for load control, Frequency influence, and Redundancy in trip solenoids
are introduced.

We claim:-
1. An electro-hydraulic governing system for operation of control valves
(of both HPT and IPT) of steam turbines only through the common
servomotors of hydro mechanical electro governing system ,
each of which is realized through torque transmitting means, which is
responsible for opening and closing of Emergency Stop Valve (ESVs) and
Interceptive Valve (IVs), wherein the said governing system operates
through functioning of three control means, namely
Speed control means
Load control means
Pressure control means.
2. An electro-hydraulic governing system as claimed in claim 1, wherein
at least one of the said three control means acts selectively to establish
an interface between electrical and hydraulic system.
3. An electro-hydraulic governing system as claimed in claim 1, wherein
the pressure control means operates for partial closure of control valve,
in case a sudden pressure reduction is detected.