governing - general.pdf

8/18/2019 Governing - general.pdf

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Dr. K.V. Vidyanandan

AGM & Sr. Faculty Member (PMI)

[email protected]


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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).

Dr. K.V. [email protected]


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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


EE E F C


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SPEED GOVERNOR FUNCTIONING


HYDRAULIC SPEED GOVERNOR


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HYDRAULIC SPEED GOVERNOR


SPEED MEASUREMENT


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SPEED MEASUREMENT

(HYDRAULIC)


A TYPICAL


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A TYPICAL

ELECTRO-HYDRAULIC CONVERTER



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A TYPICAL OVER SPEED TRIP


[email protected]


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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


[email protected]


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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


[email protected]


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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


[email protected]

TIME SPAN OF PRIMARY SECONDARY


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TIME SPAN OF PRIMARY, SECONDARY

AND TERTIARY CONTROL


[email protected]

GOVERNING OIL SYSTEM (KWU)


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GOVERNING OIL SYSTEM (KWU)


[email protected]


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Dr. K.V. VidyanandanAGM & Sr. Faculty Member (PMI)

NTPC Ltd., Noida.

[email protected]

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

governing - general.pdf

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