TG Governing System
- NEED OF GOVERNING
In a thermal power plant the steam produced in the boiler drives the turbine and the torque produced by the turbine is utilized to generate electricity.
In India the frequency of power supply network is 50 Hz. Frequency stability is one of the parameters, which determines the quality of power.
To produce electricity at 50 HZ, Turbine (coupled with two pole AC generator) is supposed to rotate at a speed of 3000 rpm.
Turbine-Generator Unit requires a control & regulatory mechanism to enable TG unit to produce quality power with safe operation. To ensure the quality of power this
mechanism should tend to maintain turbine speed in the narrow range so as to stabilize the grid frequency, which keeps on fluctuating due to variation of demand load.
B. DEFINITION OF GOVERNING SYSTEM
To govern means to control and regulate
certain parameters to achieve expected functional requirements.
In the context of thermal power plant,
Governing system of steam turbine can be defined as a control and regulatory
mechanism whose objective is to enable turbine generator unit to produce
quality power with safe operation and to achieve functional
requirements of turbo-generator operation by controlling and regulating steam
turbine
input parameters viz. inlet pressure and mass flow rate of steam.
C. FUNCTIONS OF STEAM TURBINE GOVERNING SYSTEM
The main functions of the governing system of a steam turbine generator unit
used for
electricity generation via an extensive power network are:
1. To control steam valve positions in response to signals from operator or
from separate automatic control system.
2. To bring turbine speed smoothly from
baring gear speed to synchronizing speed under the control of operator action.
3. To stabilize the speed in isolated
operation of the set.
4. To synchronize turbine-generator unit with the grid under control of
operator or an automatic control system.
5. To enable turbine-generator unit to
produce quality power by matching the power generated to that demanded by
responding to network frequency changes.
6. To insure desired percentage of load distribution between sets running in
parallel to maintain the quality of output i.e. frequency.
7. To contain speed rise within
permissible limits if the unit gets disconnected from the load (Unit trip).
8. To provide protection to the turbine
under any abnormal or emergency situation by shutting off the machine through
sudden closure of stop valve via protection system.
D. CLASSIFICATION OF GOVERNING
SYSTEM
Depending upon steam admission method, speed sensing transducer and
operating mechanism governing system of steam turbine can be classified into
following
category.
CLASSIFICATION ON THE BASIS OF STEAM ADMISSION
METHOD
On the basis of steam admission method governing system of utility steam is
classified
into the following types
1) Nozzle governing method
2) Throttle governing method
1) Nozzle Governing
In this method nozzles for steam inlet are made up in sets. Each sets are being
controlled by a separate valve. With decrease of load, required number of nozzles
may be shut off and same may be opened to regain the load. All valve may be operated
by a single servomotor or by separate servomotor Provided individually and control
valves open in a pre-arranged sequence to control turbine load.
This method is restricted to the first stage of turbine, the nozzle are in the
other stages remains constant. It is suitable for simple impulse turbines and
for large units which have an impulse stage as first stage followed by
impulse-reaction stages. This method is
mostly adopted in LMV design turbines. This method is more efficient than
throttle governing method at part load operations.
2) Throttle Governing
In this method the mass flow rate of steam is controlled by throttling the steam
by simultaneous operation of all control valves by which 1st stage pressure at turbine
inlet can be regulated to the extent of required load.
In throttle governing the mass flow of
steam is reduced whenever there is a reduction of load compared to economic or
design load and vice-versa. Whenever load decreases, shaft speed tends to
increase then steam inlet valves are partially closed to admit less steam
to the turbine and to produce less power to match the demand. Due to
restriction of passage in the valve, steam is throttled.
In throttling, enthalpy of steam remains
constant but the available energy for doing work reduces. This reduces the
efficiency of the turbine at part load. This method is mechanically simple
resulting in less initial cost but thermodynamically less efficient than nozzle
governing at part loads. Throttle governing is adopted in KWU turbines.
E. CLASSIFICATION ON THE BASIS OF
STEAM PRESSURE
On the basis of steam pressure before admission in turbine governing system of
utility
turbines is classified in following types
1) Constant Pressure Governing System
In constant pressure mode, the steam pressure at upstream of control valves is
kept
constant though out entire load range.
2) Variable Pressure Governing System
In variable pressure mode, the control valves are kept wide open and the steam
pressure
at upstream of control valves vary proportionately with the load over a
definite load
range (generally over 60% load).
As the turbo-generator sets operating in frequency control must react quickly
even to
moderate load changes, turbine control valves must be continuously throttled by
a
limited amount. This ensures that instant reserve output is made available.
Boiler can
not respond so quickly as to fulfill the necessary pressure changes.
In nozzle governing or throttle governing both the methods, the unit may be
operating
at constant steam pressure or variable steam pressure before stop valves or a
combination of two (i.e. in constant pressure mode over a certain load range
and
variable pressure mode over a remaining load range).
F. CLASSIFICATION ON THE
BASIS OF SPEED SENSING DEVICE AND OPERATING MECHANISM
On the basis of speed sensing device and operating mechanism used governing
system
can be classified into following types.
(1) Mechanical Governing System
(2) Hydro-Mechanical Governing System
(3) Hydraulic Governing System
(4) Electro-Hydraulic Governing System
1) Mechanical Governing System
In mechanical governing system speed transducer is a mechanical centrifugal fly
ball type speed governor, which actuates control valves through mechanical
linkages. Hence in this type of governing both speed sensing device and
operating device are purely
mechanical type. Now-a-days purely mechanical governing systems are not used
for
utility turbines.
2) Hydro – Mechanical Governing
System
In hydro mechanical governing system speed transducer is usually mechanical centrifugal
fly ball type governor. It is connected to hydraulic system either
hydraulically or mechanically. In the hydraulic system this signal is amplified
so that control valve’s servomotors can be actuated. Hydro-mechanical governing
system is used in LMW turbine in which speed sensing transducer is usually
mechanical
centrifugal type governor (Watt governor) and operating mechanism is hydraulic
type.
3) Hydraulic Governing System
In hydraulic governing system speed transducer is a centrifugal pump whose
discharge is proportional to square of speed. This signal is sent to hydraulic
converter/transformer which generates a signal which is proportional to valve
opening as desired. Before applying it to control valves servomotors this
signal is suitably amplified.
Hydraulic Governing System is used in KWU turbine in which speed transducer is
a centrifugal pump whose discharge is proportional to square of speed and know
as primary oil.
4) Electro-Hydraulic Governing System
In electro hydraulic governing speed sensing transducers are
electrical/electronic type.
The acquisitioned signals are processed electronically and processed signal is
given to electro-hydraulic converter, which convert electrical signal to
hydraulic signal. Hydraulic signal before application to control valves
servomotors is suitably amplified.
The electro hydraulic governing offers certain advantages over above three
types of governing systems and is becoming popular for units rating larger than
200 MW. Due to large interconnected systems and growing automation of turbine
generator sets, governing system has to meet many additional requirements. The
combined advantages of electrical measuring, signal processing and hydraulic controls
provide a very good combination in electro hydraulic governing.
Following are the important characteristics of electro-hydraulic governing
(a) Exact steady state regulation with high sensitivity
(b) Safe load shedding by avoiding any speeding up along the steady state
regulation characteristic
(c) Possibility to adjust steady state regulation in fine steps even during
operation.
KWU Turbine is equipped with Electro-Hydraulic governing system for normal operation
along with hydraulic governing system as a back up.
