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Dr. K.V. Vidyanandan
AGM & Sr. Faculty Member (PMI)
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TSR and Dead-band
Governor Droop
Importance of Inertia
Need of Load Frequency Controls
Introduction to Governing System
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Frequency deviation from nominal value (50Hz) represents
mismatch between generation (supply) and load (demand).
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When there is an increase in system load, due to the inherent
mechanical delay, the governor cannot act instantaneously.
In order to meet the extra demand, kinetic energy of the
rotating mass is released and there by machine speed reduces.
The deviation in speed (∆ω) is used to activate the governor to
open the CV further and the increased steam flow arrests the
speed drop.
The magnitude of deceleration depends upon the quantity of
the power mismatch and the inertia of the turbine-generator.
WHY SPEED DROPS AS LOAD INCREASES ?
The steady state speed after the load increase will be less thanthe nominal value.
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For satisfactory operation of the power system, the frequency
should remain close to nominal value (50/60 Hz)
This will ensure constancy of speed of induction motors
Variation in frequency will affect the performance of equipments
and can interfere with system protection schemes
Considerable drop in frequency will result in high magnetising
currents in motors and transformers
NEED OF CONSTANT FREQUENCY
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Most of the turbine generators must have frequency regulated to
avoid mechanical resonances. If a rotating machine spins at or near
one of its resonant modes, mechanical vibration damage can occur.
Manufacturers design their machine's resonant frequencies to be
far away from the intended frequency of operation, so this is nottypically a concern unless frequency deviates more than 5%.
IMPACTS OF FREQUENCY
DEVIATION IN POWER PLANTS
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LOAD IMPACTS
Poor power system frequency control can degrade power quality.
Most industrial processes which require a high precision reduce
their risk by using variable frequency drives (VFD).
Most VFDs are insensitive to supply frequency, and they precisely
regulates the output frequency. Thus systems using VFD are
insensitive to small deviations in power system frequency.
Converters used to rectify the AC source are not frequency
sensitive in the range of ±5%.
IMPACTS OF FREQUENCY DEVIATION
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Frequency provides an indication of the interconnected system’s
generation-load balance.
It is instantly available everywhere within the interconnection
without the need for additional communications.
This facilitates dispersed, autonomous response to system
casualties by generators and loads.
Assuming that all control systems such as AGC and speed
governors are working correctly, a low system frequency is
indicative of a low generation reserve.
FREQUENCY AS A
SYSTEM HEALTH INDICATOR
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Primary source of electrical energy supplied by utilities are the
Kinetic Energy of water and steam
Prime movers convert the kinetic energy into shaft work
This is in turn converted into electrical energy by Synchronous
Generators
Prime mover governing systems provide a means of controlling
power generated and frequency, a function generally called as Load
Frequency Control (LFC) or Automatic Generation Control (AGC).
PROCESS OF ENERGY CONVERSION
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Functions of AGC are:
To maintain power balance in the system.
To make sure that operating limits are not exceeded in the
Generators
Tie-lines
To maintain the system frequency constant under all conditions
AUTOMATIC GENERATION CONTROL
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COMPONENTS OF AGC
Primary control
– Immediate (automatic) action to sudden change of load,
for example, reaction to frequency change.
Secondary control
– To bring tie-line flows to scheduled.
– Corrective actions are done by operators.
Economic dispatch
– Make sure that the units are scheduled in the most
economical way.Dr. K.V. [email protected]
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FUNDAMENTAL QUESTIONS
Is it always necessary that
load variation results in frequency variation..?
Under what situation load variation results in
frequency variation..?
Why frequency variation occur with load change..?
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BLOCK DIAGRAM OF
GENERATION AND CONTROL SYSTEM
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The engineering of Governing System is a combination of
Mechanical-Hydraulic System
Electrical Power System
Control System
GOVERNING SYSTEM
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The main functions of governing system for steam turbines are:
Speed (frequency) and load (power) control:
→ mainly through HPCV
Overspeed control:→ mainly through the IV
Overspeed trip:→ through HPSV and IPSV
Start-up and shutdown control
GOVERNOR FUNCTIONS
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All governors must have five fundamental features :
1. A way to set the desired speed
2. A way to sense actual speed
3. A way to compare the actual speed to the desired speed
4. A way for the governor to change steam flow to the turbine
5. A way to stabilize the rotor speed after a load change
FEATURES OF SPEED GOVERNOR
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Turbine
ActualSpeed
DesiredSpeed
SteamSupply
Governor
Deviation
Operator
Negative(Balancing)Feedback
Sensor Actuator
Controller
Feedback Loop
GOVERNOR ACTION
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When turbine speed deviates from the set value, governor
action modulates the control valve to regulate the steam flow.
TURBINE
CONTROL
VALVE
GEN
STEAM
GOVERNOR ACTION
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HYDRO POWER GENERATION
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HYDRO TURBINE GOVERNOR
In hydro turbines, due to water inertia, a change in gate position
produces an initial turbine power change which is opposite to
that sought. This is because flow will not establish immediately.
For stable control performance, a large transient (temporary
droop) with a long resetting time is required.
A gain reduction compensation retards the gate movement until
the water flow and power output have time to catch up.
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TYPES OF TURBINE GOVERNORS
Mechanical
Electro Mechanical
Hydraulic
Electro Hydraulic
MECHANICALSpeed transducer is mechanical centrifugal type speed governor,
which directly actuates control valves through mechanical linkages.
ELECTRO MECHANICALMechanical centrifugal type speed governor is connected to
hydraulic system either mechanically or hydraulically.
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TYPES OF TURBINE GOVERNORS
HYDRAULIC
Speed transducer is a centrifugal pump whose discharge pressure is
a function of machine speed. This signal is sent to a hydraulic
converter, to generate a high power hydraulic signal for the
operation of different control valves or gate.
ELECTRO HYDRAULIC
Electronic Transducer is used for measuring the machine speed.This signal is processed electronically and then sends to an Electro
Hydraulic Converter for converting the electronic signals into
proportional hydraulic signals for the operation of control valves. Dr. K.V. [email protected]
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METHODS OF GOVERNOR CONTROL
Throttle Governing
Nozzle Governing
By-pass Governing
THROTTLE GOVERNING
In this method of governing, steam is throttled to a suitable pressure
using one or more sets of throttle valves (control valves). All these
valves operate simultaneously and the throttle control is achieved
usually by controlling the steam admission to the HP cylinder. Arc of
steam admission in this governing method is 3600.
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METHODS OF GOVERNOR CONTROL
NOZZLE GOVERNING Various groups of Nozzles with suitable isolating valves are used
for regulating the steam flow to the turbine. These nozzles are
grouped in 2, 3, 4 or more and their operation is donesequentially. These nozzle groups achieve control by regulating
the steam flow to the first stage of HP cylinder. Arc of steam
admission in this method is < 1800
.
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Hydraulic turbines can be modelled by a first-order model if
water hammer (wave) and surge effects are neglected.
Second-order models for hydraulic turbines with water hammer
effect in the penstock are considered
The speed controller of the hydro turbine governor has a
permanent droop and a transient droop.
Reversible hydraulic machines are used for pump-storage plants.
Optimal pumping speed: 12-20% above optimal turbine speed.
This needs variable-speed operation, thus power electronics.
HYDRO TURBINE GOVERNOR FEATURES
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The main parts of a ball-head hydro-mechanical governor are:
Speeder Spring
Thrust Bearing
Flyweights
Pilot Valve
Servo (Power) Piston
Drive Shaft
GOVERNOR COMPONENTS
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Speeder spring is used to set desired speed.
Applying more force down on the speeder spring causes
the governor to increase steam.
This initial force is usually set by the operator for the
“reference” speed.
It can be set by a screw adjustment, a knob, a lever, an
electric motor, air pressure, or solenoids, depending on the
specific governor.
SPEEDER SPRING
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ISOCHRONOUS GOVERNOR
The ability to return to the original speed (constant speed) after
a change in load is called isochronous speed control
Governor components include comparator and integrator
Works only when unit supplies to an isolated load or only one
unit in a multi-machine system need to respond to load change
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For a 5% droop, a 5% increase in frequency causes change in
turbine output from 100% to 0%
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The ratio of speed deviation
(∆ω) or frequency deviation
(∆f) to the change in valve
position (∆Y) or power
output (∆P) is equal to the
droop parameter R.
Unit of R: Hz/MW
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Droop (or speed regulation)
permits a machine to share load
with other machines in an
interconnected system.
Typical values of droop for
steam turbines vary from 2.5 to
8% and generally set at 5%.
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Governor Droop, Speed Regulation or Speed Error are the
common terms used in describing a turbine’s response tochanges in system frequency (or speed).
New grid code requires all units above 10 MW capacity should
have an operating governor with droop.
Droop distributes frequency regulation to all generators in
the interconnected network.
Recommended droop settings for thermal units: 4 to 5% witha maximum dead band of ± 0.036 Hz.
Minimum value should not be less than 2.5% to maintain
stability in a speed-droop governor. Dr. K.V. [email protected]
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WHY DROOP IS NECESSARY..?
With droop, an increase in load will result in reduction in speed
reference and the problem of instability will not occur.
An increase in load will cause the turbine to slow down. The
governor will respond by increasing the steam until the speed hasreturned to the original value. Due to the combined effects of
inertia and power lag, the speed will continue to increase beyond
the setting, causing a speed overshoot.
The governor again will respond to decrease speed to correct for
the overshoot. It will over-correct the speed in the reverse
direction causing undershoot. This overcorrection of speed in both
directions (instability) will amplify until the turbine trips out on
over speed.
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LOAD SHARING BETWEEN UNITS
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During total load rejection in a unit, speed of the turbine-
generator shoots up temporarily before settling down to
steady state value.
This temporary speed rise is called transient speed rise
and is expressed as percentage speed rise of ratedspeed on full load throw off.
Typical Value of TSR : 5-7%
TRANSIENT SPEED RISE
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TRANSIENT SPEED RISE
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Due to inherent inertia of the components of
governing elements and friction present in
governing system, certain motion of governing
system is lost before corrective signal can actuatethe control valves.
The lost of the motion is called DEAD BAND of thesystem and is expressed as percentage of rated
speed.
GOVERNOR DEAD BAND
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EE E F C
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SPEED GOVERNOR FUNCTIONING
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HYDRAULIC SPEED GOVERNOR
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HYDRAULIC SPEED GOVERNOR
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SPEED MEASUREMENT
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SPEED MEASUREMENT
(HYDRAULIC)
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A TYPICAL
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A TYPICAL
ELECTRO-HYDRAULIC CONVERTER
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A TYPICAL OVER SPEED TRIP
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In order to ensure the power balance in the system, three types
of load-frequency controls are commonly used. These are:
Primary Control, Secondary Control and Tertiary Control
Primary control maintains power supply-demand balance by
using proportional control action.
Secondary control restores the frequency after every supply-
demand mismatch using integral action.
Tertiary control ensures economic allocation of secondary
control reserve.
LOAD-FREQUENCY CONTROLS
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Inherent control of the prime mover
Fast in Response (< 1 min)
With increase in speed, steam or water flow reduces
With decrease in speed, steam or water flow increases
Control parameter is Droop
Droop vary between 2.5 - 8%
Results in static frequency error after load change
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Supplementary Control
Relatively Slow Response (~ few min.)
Comes into service after the Primary Control
Restores the frequency back to nominal value after a load change
Reset control action is provided
Error acts on Speeder Motor to shift the Droop line up or down
Controllers commonly used are: Integral and PI
Dr. K.V. Vidyanandan
TIME SPAN OF PRIMARY SECONDARY
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TIME SPAN OF PRIMARY, SECONDARY
AND TERTIARY CONTROL
Dr. K.V. Vidyanandan
GOVERNING OIL SYSTEM (KWU)
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GOVERNING OIL SYSTEM (KWU)
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Dr. K.V. VidyanandanAGM & Sr. Faculty Member (PMI)
NTPC Ltd., Noida.
Comments and suggestions are welcome
Further Reading
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Further Reading
Title
Author(s)
Publisher
1. Electric Energy Systems
Theory: An Introduction
O.I Elgerd McGraw-Hill, 1983
2. Power System
Control and Stability
P. M. Anderson
A. Fouad
Wiley, 2003
3. Power SystemStability and Control Prabha Kundur McGraw Hill, 1994
4. Steam Turbines: Design,
Applications and Rerating
H.P Bloch
M.P Singh
McGraw Hill, 2009
5. Robust Power SystemFrequency Control
Hassan Bevrani Springer, 2009
6. Load-frequency Control
and Performance Policy 1
UCTE 2004
Dr K V Vidyanandan