FIELD OF THE INVENTION
The present invention relates to governing devices for Medium rating Steam
turbines used in Thermal and Nuclear plants. More particularly, the invention
relates to a Microprocessor based Electro-Hydraulic Governor including a backup
Mechanical Governor for Steam Turbines.
BACKGROUND OF THE INVENTION
The prior art Steam turbines as per AEI design are provided with Flyball-
mechanical governors. The mechanical governors operate via centrifugal weight
fly balls which move the sleeve up/down corresponding to rotational speed of the
turbine. However, control of technically-advanced transmission grid presently in
use, is quite difficult with the existing mechanical Governing system, which
involves poor accuracy, linearity and stability as compared to the Electronic
Governor. The mechanical governing systems comprise a plurality of moving
components and susceptible to quick wear and tear leading to lower operational
efficiency, and low reliability.
The response to fault in mechanical governors is slow as compared to Electro-
Hydraulic Governor and hence control is inefficient, and inaccurate causing
Energy losses. Mechanical vacuum unloading including low steam pressure
unloading response is quite sluggish, inaccurate and un-reliable, therefore these
are normally being kept isolated.
Mechanical governors start functioning only at and above 2600/2700 RPM speed.
CIES (Combined Interceptor and Emergency Stop) valves control the turbine
speed.

Turbine can operate only on Speed mode and cannot run on Load mode.
Operator has to take corrective action frequency to change speeder position for
any frequency change. Control is rigid and its PID functions cannot be changed.
Therefore it is very difficult to modify and improve the operation of mechanical
governors.
Due to limitations stated above including other inherent disadvantageous
features of mechanical governors in the existing/old TG (Turbine-Generator),
leads to the disadvantageous activities for example, hunting of the machine to
determine wear and tear of the governing components, replacement of worn out
parts and frequent down-time.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a microprocessor based
Electro-Hydraulic Governor for medium rating, process steam power generation
plants having mechanical governor, particularly, used in Thermal and Nuclear
plants.
Another object of the invention is to propose a microprocessor based Electro-
Hydraulic Governor for medium rating, process steam power generation plants
having mechanical governor, particularly, used in Thermal and Nuclear plants,
which enables the power plants to operate with existing mechanical governor
supported by the electro-hydraulic governor.
A still another object of the invention is to propose a microprocessor based
Electro-Hydraulic Governor for medium rating, process steam power generation
plants having mechanical governor, particularly, used in Thermal and Nuclear
plants, which provides a 100% back-up to the existing mechanical governors in
case EHG system malfunctions.

A further object of the invention is to configure a microprocessor based Electro-
Hydraulic Governor for medium rating, process steam power generation plants
having mechanical governor, particularly, used in Thermal and Nuclear plants,
which provides optimum, reliable, and safe control with minimum modification to
the existing mechanical governors.
SUMMARY OF THE INVENTION
Accordingly, there is provided a programmable microprocessor based Dual Hot
Redundant Electro-Hydraulic governing control system for safe and high
performance of the utilities receiving power from electrical grid. The inventive
system is enabled to provide:
• Automation in rolling of machine (better control of speed of machine,
provision of fast ramp for critical speed zone, link with DDCMIS and other
supervisory controls).
• Automation in running of the machine.

- Accurate control of speed of machine.
- Accurate control of load on machine on load mode.
- Fast response to the changes in the grid.
- Enabling operation on Free Governing Mode.
• Protection of machine
- Protection for low vacuum
- Protection for low load operation.
- Protection for malfunctioning of the electrical/electronic parts.
According to the configuration of the inventive system, two Sensitive oil
pressures for each Governing valve are generated. A first oil pressure is

generated by respective I/H (Electro-Hydraulic) converter (9) based on EHG (10)
output signal, and second oil pressure is a generated by a Mech. Gov. System
(3). Under Normal operation the EHG (10) will be governing and simultaneously
supplying a positive bias signal (8) to a LLG motor (6), such that HP sensitive oil
generated by the Mech. Gov. system (3) is at a higher pressure than that
generated the EHG. Both these sensitive oil pressures will be compared at a
Minimum Selector Unit (7) and the lower of said oil pressures (which under
normal operating conditions will be from the EHG) will be applied to a Governing
Valve Relay Gear for positioning the Governing valves.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
Figure 1 - Block Diagram of the programmeable micro-processor based
Electro-hydraulic governor system according to the invention.
Figure 2 - Detailed Process flow diagram depicting one embodiment of the
inventive EHG system in operation mode controlling a
turbine/generator unit.
Figure 2 - Detailed Process flow diagram depicting an alternative
embodiment of the inventive EHG system in operation mode
controlling a turbine/generator unit.
DETAILED DESCRIPTION OF THE INVENTION
According to the inventive concept as implemented, the EHG system essentially
operates under the following control loops-
- Speed control loop, Load control loop, Pressure control loop

The Speed Control Loop realizes the following main operations-
- Start up shut down of turbine
- Synchronization of the generator
- Provides a minimum load operation
- Operation of TG set in complete power range.

- Load control loop is effective only when the unit is synchronized and minimum
block load is achieved. In load control mode, the EHG controls the load against a
target load set by operator and during complete load mode operation speed
mode will closely follow the turbine speed and reference speed set point with
bias to allow the EHG to bump less transfer from load mode to speed mode.
- Under NO-LOAD condition the speed mode controls is always operative to
provide the normal frequency governing action.
- While tripping EHG will spontaneously shift from load control mode to speed
control mode and changes its speed reference set point at and below rated RPM
to avoid over speed trip condition.
- The EHG provides four 4 to 20 mA analogue outputs for turbine
Governor valves control. I/H converters are installed in the HP sensitive oil
supply to the HP governor valve servomotor and the other converters are
installed in the IP sensitive oil supply to the IP governor valve. The I/H
converters modulate the HP and IP sensitive oil pressures and, hence the
positions of the governor valves.

CONTROL OPERATIONS
(A) GOVERNING VALVES
- The EHG processed electronic signals are introduced at the hydraulic
circuit through Electro-hydraulic converters. HP Sensitive oil pressure
generated by the HP I/H converters corresponding to EHG output signal,
is fed to the HP minimum selector unit.
- In the HP minimum selector unit both the HP sensitive oil pressures i.e.
one generated by the mechanical governor and another generated by the
HP I/H converters is compared and the minimum pressure is applied to
the HP governing valve relay gear for positioning the HP Governing valves.
- While rolling and while the turbine operating on load, the EHG ensures
and transmit positive bias signal/command to position the LLG motor
(Load Limiting Gear) such that HP sensitive oil pressure generated by the
mechanical governor is maintained at 5 to 10% higher than the HP
sensitive oil pressure generated by the EHG system through the I/H
converter. This way the EHG exercises a total control of the turbine in
normal operating conditions.
- In case malfunctioning of the EHG system, the system transmits a
maximum current signal (with alarm) to the HP I/H converter to generate
HP sensitive oil with high pressure, (higher pressure than the HP sensitive
oil pressure being generated by the mechanical governor). Due to this the
unit governing control smoothly gets shifted from the EHG control to the
mechanical governing control.

- As the IP Gov. Valves remains in full open position throughout the
Turbine rolling period and when the unit is on load, and the valves start
closing only when turbine sped rises above 3090 rpm. The EHG also
generates IP sensitive oil through its I/H converter such that IP Gov.
valves remain in full open position and the valves start closing only at a
speed above 3090 rpm.
- In IP minimum selector unit compares both IP sensitive oil pressures i.e.
one generated by mechanical governor and another generated by IP I/H
converters and the minimum pressure is applied to the IP governing valve
relay gear for positioning the IP Governing valves.
(B)TURBINE RUNUP (TURBINE ROLLING)
- Turbine run up is performed by the EHG speed controller, through
Selectable Auto run up / manual run up, which constitutes checking and
confirming that all start Pre-requisites are full-filled,
- The speed controller compares the speed reference generated by speed
reference limiter circuit and actual speed and accordingly provides an
output for the valve lift controller,
- Run up starts by controlled fraction opening of CIES valve and EHG
following the turbine speed rise by holding the HP Gov. Valves just
sufficient open. However, it is possible for the EHG to take over control of
run up using the HP Gov. Valves at a predetermined speed 500 RPM or
less depending on the effectiveness of these valves at low speeds.

- The EHG Turbine run-up task is very flexible and versatile function,
which allows target values and required acceleration rates can be set and
adjusted throughout the run-up phase. Both parameter have default
values, can be changed at any time within predefined limits.
- EHG can monitor critical speed bands and can accelerate the turbine
speed at higher rate between critical speed bands.
- dn/dt monitoring avoid rolling of turbine with insufficient rate at or near
critical speeds.
- Once EHG takes over speed control CIES valves opened up to 8-10% till
turbine speed reaches its rated RPM. CIES valves to be opened fully just
before synchronizing.
(C)CHANGEOVER FROM ELECTRO-HYDRAULIC GOVERNING TO MEC.
GOVERNING
As shown in Figure-1, under Normal operation, EHG (10) governs the
turbine, and simultaneously transmits a positive bias signal to LLG (6), such
that HP sensitive oil generated by the Mech. Gov. system (3) is at a higher
pressure than that by the EHG. Both these sensitive oil pressure are
compared at the Minimum Selector Unit (7) and the minimum pressure
applied for positioning the Governing Valves.
But in case of faults or malfunction of the EHG (10), value of the signal
generated by the EHG increases to a maximum, and at the minimum selector
gate the signal from the Mech. Gov. (3) is used for positioning of the

governing valves. This ensures that there is a bump less transfer of the
control from the EHG to the Mech. Governing in case of faults.
As shown in figure-2, the system comprises four I/H Converter, Two for HP
Governing valve and another two for IP governing valves. The I/H converters
modulate the sensitive oil pressure which in turns regulate the position of
Governing valves.
As shown in figure-3, the EHG system comprises two I/H Converters, One for
both the HP Governing valve and another one for both the IP governing
valves. This process is utilized where a single I/H Converter is enabled to
cater the supply of signal on oil pressure for two valves simultaneously.
(D)TURBINE UNLOADING
(i) LOW MAIN STEAM PRESSURE UNLOADER:
This function operates while the turbine is on load, and EHG
monitors the main steam pressure. If the main steam pressure falls
below a set level the EHG starts unloading the turbine at a preset
rate with a operator alarm until the main steam pressure recovers.
(ii) LOW VACUUM UNLOADING:
This function operates while the turbine is on load, and monitors the
Condenser Vacuum. If the vacuum drops below a set level, the
turbine is unloaded at a preset rate until the vacuum recovers.

(E) TURBINE OVERSPEED PROTECTION: - As overspeed protection is very
critical and hence it is proposed to retain the existing mechanical safety
governor and overspeed limiting gear system.
- In the event the speed governor fails to control the speed, the mechanical
safety governor over speed trip actuates to shut down the machine.
The advantages of the Electro-HYdraulic system are
(a) it is fast, accurate and easily adjustable,
(b) it allows great flexibility of system arrangement,
(c) it can be easily tested without running the machine, and
(d) it gives a high degree of repeatability.


WE CLAIM
1. An electro-hydraulic governing system for medium rating steam turbine sets
having mechanical governors, comprising:
- a mechanical governor; and
- a programmable micro-processor based electro-hydraulic governor,
the system is configured to
- generated a first sensitive oil pressure signal for a first electro-hydraulic
(I/H) Converter based on output signal from the electro-hydraulic
governor;
- generate a second sensitive oil pressure signal for a second electro-
hydraulic converter from the mechanical governor;
- transmit under normal operating condition a positive bias signal by the
electro-hydraulic governor to a load-limiting gear motor such that the
second sensitive oil pressure signal generated by the mechanical governor
is at a higher pressure than that of the first sensitive oil pressure signal;
- compare both the first and sensitive oil pressure signals in a minimum
selector unit, and
- position the governing valves by applying the lower of said two oil
pressure signals to be a governing valve relay gear.
2. An electro-hydraulic governing system for medium rating steam turbine sets
having mechanical governors, as substantially described and illustrated
herein with reference to the accompanying drawings.

The present invention relates to an electro-hydraulic governing system for
medium rating steam turbine sets having mechanical governors, comprising a
mechanical governor; and a programmable micro-processor based electro-hydraulic
governor, the system is configured to generated a first sensitive oil
pressure signal for a first electro-hydraulic (I/H) Converter based on output
signal from the electro-hydraulic governor; generate a second sensitive oil
pressure signal for a second electro-hydraulic converter from the mechanical
governor; transmit under normal operating condition a positive bias signal by the
electro-hydraulic governor to a load-limiting gear motor such that the second
sensitive oil pressure signal generated by the mechanical governor is at a higher
pressure than that of the first sensitive oil pressure signal; compare both the first
and sensitive oil pressure signals in a minimum selector unit, and position the
governing valves by applying the lower of said two oil pressure signals to be a
governing valve relay gear.