automatic generation control

RVS College of Engineering and Technology

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AUTOMATIC GENERATION CONTROL

Presented by:Birju Besra


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•Automatic generation control An Introduction•The need of Automatic Generation Control•Reasons for limiting frequency deviations•Automatic generation control (Block diagram)•Automatic voltage regulator(AVR)•Automatic load frequency control(ALFC)•Exciter modelling•Generator modeling•Speed governing system•Future work


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In an electric power system, automatic generation control (AGC) is a system for adjusting the power output of multiple generators at different power plants, in response to changes in the load. Since a power grid requires that generation and load closely balance moment by moment, frequent adjustments to the output of generators are necessary. The balance can be judged by measuring the system frequency; if it is increasing, more power is being generated than used, which causes all the machines in the system to accelerate. If the system frequency is decreasing, more load is on the system than the instantaneous generation can provide, which causes all generators to slow down.


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As our development has increased, there has been a higher demand of electrical power loads both on industrial and domestic scale. As the number increases, it is also imperative to manage load properly since a failure to do so results in frequency fluctuation and voltage drops.An effective regulatory strategy is available in the form of•Automatic Voltage Regulator Systems (AVR) and•Automatic Load Frequency Control (ALFC)The main function of ALFC system is to assess and rectify the power and frequency while that of AVR system is to regulate voltage and reactive power.

The Need of Automatic Generation Control


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REASONS FOR LIMITING FREQUENCY DEVIATIONS

There are few reasons as to why there should be strict limitations on frequency deviations and keeping the system frequency constant. They are as follows:

•The three phase a.c. motors running speed are directly proportional to the frequency. So the variation of system frequency will directly affect the motor performance.

•The blades of the steam turbine and the water turbines are designed to operate at a particular speed and the frequency variations will cause change in the speed. This will lead to excessive vibration and cause damage to the turbine blades.

•The frequency error may produce havoc in the digital storage and retrieval process.


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AUTOMATIC GENERATION CONTROL (BLOCK DIAGRAM)


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The exciter is the main component in the AVR loop. It delivers the DC power to the generator field. It must have adequate power capacity and sufficient speed of response. The basic role of the AVR is to provide constancy of the generator terminal voltage during normal and slow changes in the load.

Automatic Voltage Regulator(AVR)


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| | | |refV V e

Exciter Modelling:Mathematical modelling of the exciter and its controls follows:

andR Av K e

where is amplifier gainAKLaplace Transformation of these two equations yields

| | ( ) | | ( ) ( )refV s V s e s and ( )

( )R

A Av sG Ke s

where is amplifier transfer functionAG

In reality the amplifier will have a delay that can be represented by a time constant AT and its transfer function will then be of the form

( )( ) 1R A

AA

v s KGe s sT


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If and represent respectively the resistance and inductance of the exciter field we have for voltage equilibrium in the latter

eR eL

( )eR e e e

d iv R i Ldt

Measured across the main field the exciter produces armature volts per ampere of field current , that is, we have the proportionality

1Kei

1f ev K i

Upon Laplace Transformation of the last two equations and elimination of we obtain the transfer function of the exciter

ei( )( ) 1f e

eR e

v s KGv s sT

where 1e

e

KKR

e

ee

LT

R

and

The time constants and have values in the ranges 0.02-0.10 and 0.5-1.0 seconds respectively.

AT eT


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| |refV e

1A

A

KsT

RV

1e

e

KsT

fV

1e

e

KKR

ee

e

LTR

Linear model of the comparator-amplifier-exciter portion of AVR loop

V

Amplifier ExciterComparator


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Generator Modeling:

We need to close the loop in fig. by establishing the missing dynamic link between the field voltage and the generator terminal voltage equals the internal emf minus the voltage drop across the internal impedance it is clear that the relationship between and depends on the generator loading. The simplest possible relationship exists at low or zero loading in which case V equals approximately the internal emf .

| |refV e1

A

A

KsT

RV

1e

e

KsT

fV

1e

e

KKR

ee

e

LT

R

1f

do

KsT

V

V

1R

R

KT s

Amplifier Exciter

Generator

Sensor


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ffdo

f

LT

R

where

doT open circuit time constant

ffL rotor winding self inductance

fR rotor winding resistance

doT typically have values in the range 5-10 seconds


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MATLAB Simlation of AVR

Amplifier KA=10 TA=0.1Exciter Ke=1 Te=0.4Generator Kf=1 Tdo=0.2Sensor Kr=2 Tr=0.05


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AUTOMATIC LOAD FREQUENCY CONTROL(ALFC)

The basic role of ALFC is :•To maintain desired megawatt output of a generator unit matching with the changing load•To assist in controlling the frequency of larger interconnection•To keep the net interchange power between pool members at the predetermined value

The ALFC loop will maintain control only during normal (small and slow) changes in load and frequency. It is typically unable to provide adequate control during emergency situations, when large megawatt imbalance occur. Then more drastic emergency control must be applied.


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SPEED GOVERNI`NG SYSTEM


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The system consists of the following components:I. Fly ball speed governorII. Hydraulic amplifierIII. Linkage mechanismIV. Speed changer

I. Fly ball speed governor:This is the heart of the system which senses the change in speed frequency. As the speed increases the fly balls move outwards and the point B on linkage mechanism moves downwards. The reverse happens when the speed decreases


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II.Hydraulic amplifier:It comprises a pilot valve and main piston arrangement. Low power level pilot valve movement is converted into high power level piston valve movement. This is necessary in order to open or close the steam valve against high pressure steam.

III.Linkage mechanism:ABC is a rigid link pivoted at B and CDE is another rigid link pivoted at D. This link mechanism provides a movement to the control valve in proportion to change in speed. It also provides a feedback from the steam valve movement


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IV. Speed changer:It provides a steady state power output setting for the turbine. Its downward movement opens the upper pilot valve so that more steam is admitted to the turbine under steady conditions. The e reverse happens for upward movement of speed changer


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1sg

sg

KT s 1

t

t

KT s 1

ps

ps

KT s

1R

Speed Governer

Turbine Generator Load

Sensor

Modeling of Automatic Load Frequency Control

( )cP s( )EY s ( ) ( )t gP s P s

( )DP s

( )F s


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MATLAB Simlation of ALFC

Speed Governor Ksg=1 Tsg=0.2Turbine Kt=1 Tt=0.5Generator Load Kps=1 Tps=10Sensor R=0.05


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