EE533 POWER OPERATIONS Automatic Generation Control

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EE533 POWER OPERATIONSAutomatic Generation
Control Satish J. Ranade Fall 05
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Automatic Generation Control

• The first step in system operation is to ensure
  generation load balance
• This translates into maintaining system frequency
• In classical operation (regulated)
• each utility defines a control area
• Controls its generation to help maintain SYSTEM
  frequency
• Controls its generation to meet load and
  interchange

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Automatic Generation Control
Control Area
Net Interchange from area
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Automatic Generation Control

• General Objective ( Classical)
• Control MW Generation to
• Maintain(Regulate) Frequency
• -- assist entire system irrespective of
  cause
• Regulate Contractual Interchange
• Technical Criteria set by National Electric
  Reliability Council(NERC)

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Automatic Generation Control

• Local Governor Control
• -- responds at generator level to correct
  frequency deviation. Does not attempt to restore
  all the way to 60 Hz
• Load Frequency Control
• --real-time control from Control centers
  controls generation to restore frequency to 60 Hz
  and interchange to contracted amount
• Economic dispatch
• -- Reallocates generation to minimize cost

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Automatic Generation Control

• To understand AGC we will look at
• Effect of small load-generation imbalances in
  terms of frequency and power flow changes
• Governor action and how a governor controls
  frequency
• Classical approach to restoring frequency and
  interchange using Area Control Error concepts
• Economics of generator scheduling and economic
  allocation of generation
• Later we will look at changes brought about by
  restructuring

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Generator Turbine Governor Behavior

• Generation (Mechanical Power) Load (Electrical
  Power) Imbalance results in change in machine
  speed, frequency and power flow
• Machine electro-mechanical dynamics is described
  by swing equation
• For a single machine serving a load ( in per unit)

Pm-Pe M d?/dt
Pm mechanical power Pe electrical power ?
speed(frequency) M Inertia
On time-scale of electromechanical dynamics Pe
Pl where Pl is the load
Pm-Pl M d?/dt
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Generator Turbine Governor Behavior

• We will look at how governors work and derive a
  model that shows how frequency changes because of
  load generation imbalance
• We will begin with a single generator and load
  and then generalize to large system

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Generator Turbine Governor Behavior
Pm-Pl M d?/dt
Majority of imbalances encountered in normal
operation are small ( as compared to a fault at
the terminals!) Customary to use small-signal
linearized models
Pmo ? Pm- Plo ? Pl M d (? o? ?) / dt
Pmo, Plo and ? o represent the initial operation
point where PmoPlo ? Pm, ? Pl ,? ?are (small)
deviations from the operating point
? Pm- ? Pl M d (? ?) / dt
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Generator Turbine Governor Behavior
Pe
? Pm- ? Pl M d (? ?) / dt
In modeling, analysis and simulation we often use
Laplace domain block diagrams ( dF/dtgt sF(s)
for zero initial conditions)
? Pm(s)- ? Pl(s) sM ? ?(s) gt ? ?(s) ?
Pm(s)- ? Pl(s)/ sM
? Pm(s)
1/(Ms)
? ?(s)
? Pl(s)
A sustained load generation imbalanced would
lead to a continuous change in frequency!!!!!!!
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Generator Turbine Governor Behavior
Pe
? Pm- ? Pl M d (? ?) / dt
In modeling, analysis and simulation we often use
Laplace domain block diagrams ( dF/dtgt sF(s)
for zero initial conditions)
? Pm(s)- ? Pl(s) sM ? ?(s) gt ? ?(s) ?
Pm(s)- ? Pl(s)/ sM
? Pm(s)
1/(MsD)
? ?(s)
? Pl(s) -
A sustained load generation imbalanced would
lead to a continuous change in frequency!!!!!!!
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Generator Turbine Governor Behavior
Load response to frequency change For Rotating
components of load the real power increases
with frequency
Pe
? Pl(s) ? Pl(s) D? ?(s) ? Pl(s) now is an
incipient load change ( a motor starts) D? ?(s)
represents the response that the additional load
causes frequency to drop, all motors slow down,
and so load drops as D? ?(s)
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Generator Turbine Governor Behavior
Load response to frequency change For Rotating
components of load the real power increases
with frequency
Pe
? Pm(s)- ? Pl(s)-D ? ?(s) sM ? ?(s) gt ?
?(s) ? Pm(s)- ? Pl(s)/ (MsD)
? Pm(s)
1/(MsD)
? ?(s)
? Pl(s) -
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Generator Turbine Governor Behavior
Load response to frequency change For Rotating
components of load the real power increases
with frequency
Pe
? Pm(s)
1/(MsD)
? ?(s)
? Pl(s) -
Lets say ? Pm0 ? Pl P u(t) or ? Pl(s)
P/s ? ?(s) - ? PL(s)/(MSD) - P /
s(MSD) ? ?(t) - (P/D) ( 1 e tM/D)
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Generator Turbine Governor Behavior
Load response to frequency change For Rotating
components of load the real power increases
with frequency
Pe
? Pm(s)
1/(MsD)
? ?(s)
? Pl(s) -
Lets say ? Pm0 ? Pl P u(t) or ? Pl(s)
P/s ? ?(s) - ? PL(s)/(MSD) - P /
s(MSD) ? ?(t) - (P/D) ( 1 e tM/D)
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Generator Turbine Governor Behavior
Load response to frequency change For Rotating
components of load the real power increases
with frequency
Pe
The offsetting load change arrests frequency
change frequency settles to ? ? -P/D Could
derive this through final value theorem or energy
balance P original load increase load
drop due to frequency drop -D ? ?
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Generator Turbine Governor BehaviorModel with
governor
For M 6, D 1 and Load change P1 pu Steady
stae frequency drop ?? -P/D -1 pu
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Generator Turbine Governor Behavior
The Governor
A 1 pu frequency drop is unrealistic and speed
governing is needed!
Measures speed(frequency) and adjusts valves to
change generation Frequency drops gt Raise
generation
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Generator Turbine Governor Behavior
Pe
Pl
Pm
Speed
Governor
Desired Generation
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Generator Turbine Governor Behavior
Emphasis A low tech gadget that is Autonomous
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Generator Turbine Governor Behavior
Detailed and complex models for Governors exist
and are used in long-term dynamic
simulations Simplest model
Droop
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Generator Turbine Governor BehaviorResponse
Steady state error
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Generator Turbine Governor BehaviorSteady State
Response
Using energy balance
? Pl - D ?? - (1/R) ??
0 Load Load Generation Change
Response Change from Governor
Steady state error
?? - ? Pl /( D1/R) Typical R 0.05 pu ( 5
factory set) For ?P 1 , D 1, R0.05
?? 1/21 - 0.0476 pu
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Generator Turbine Governor BehaviorSteady state
response Role of Pref
?Pm ? Pref (1/R) ? ?
? Pref is used to change generation from the
control center through SCADA At nominal
frequency (? ?0) unit Would generate Pref0 ?
Pref
? ?
0
?Pm
? Pref
? Pref
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Generator Turbine Governor Behavior-Isochronous
governor
Uses Integral control and restores Frequency
error to zero Use in standalone or islanded
cases
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Generator Turbine Governor Summary
No Governor
Iso restores exactly
Standard Governor
Std leaves small error
No unacceptable deviation
Isochronous
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Multiple Generators and Areas
Ptie?
Pe1
Pe2
jX
Area 1 or Gen 1 Tie Line Area 2 or Gen 2
Now look at two generators or areas connected by
a line or network If load changes in any area
how do frequencies and line power Ptie
change? We will want to restore both to nominal
value A simple model for the line is just a
series inductive reactance
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Multiple Generators and Areas
Ptie?
Pe1
Pe2
jX
Area 1 or Gen 1 Tie Line Area 2 or Gen 2
A simple model for the line is just a series
inductive reactance. Let us also assume voltage
magnitudes are nominal ( 1pu) From simplified
transmission line models Ptie (1/X)
sin(d1- d2) We also know that d d1
/dt ?1 d d2 /dt ?2 Combine these with
swing equations
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Multiple Generators and Areas
Ptie?
Pe1
Pe2
jX
Area 1 or Gen 1 Tie Line Area 2 or Gen 2
Pm1- Pl1 D1? ?1-Ptie M1d ?1/dt Pm2- Pl2
D2? ?2-Ptie M2 d ?2/dt Ptie (1/X) sin(d1-
d2) d d1 /dt ?1 d d2 /dt ?2
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Multiple Generators and Areas
Ptie?
Pe1
Pe2
jX
Area 1 or Gen 1 Tie Line Area 2 or Gen 2
? Pm1- ? Pl1 D1? ?1- ? Ptie M1d ? ?1/dt
? Pm2- ? Pl2 D2? ?2- ? Ptie M2d ?
?2/dt Ptie (1/X) (? d1- ? d2) sin xx for
small x d ? d1 /dt ? ?1 d ? d2 /dt
? ?2
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Multiple Generators and Areas
Ptie?
Pe1
Pe2
jX
Area 1 or Gen 1 Tie Line Area 2 or Gen 2
In s domain with zero initial conditions ?
Pm1(s)- ? Pl1(s) D1 ? ?1(s)- ? Ptie(s) M1 s?
?1(s) ? Pm2(s)- ? Pl2(s) D2 ? ?2(s)- ? Ptie(s)
M2 s? ?2(s) Ptie(s) (1/X) (? d1(s)- ?
d2(s)) ? d1(s) ? ?1(s)/s d2 (s) ?
?2(s)/s
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Multiple Generators and Areas
Ptie?
Pe1
Pe2
jX
Area 1 or Gen 1 Tie Line Area 2 or Gen 2
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Multiple Generators and Areas
Ptie?
Pe1
Pe2
jX
Area 1 or Gen 1 Tie Line Area 2 or Gen 2
Qualitative Response Load increase in area
1 Area 1 frequency drops Area 1 voltage phase
angle fall behind are 2 Ptie decreases
(stabilizes Area 1 frequency, drags down area
2) Area 2 frequency drops Both governors raise
generation Steady state achieved at a lower
frequency and Ptie Area 1 assists Area 2 in
meeting the load increase frequency drop is lower
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Multiple Generators and Areas
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Multiple Generators and Areas
??2
??1
Frequency Error
Interchange Error
?ptie decreases from 1 to 2
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Multiple Generators and Areas
Phase angle difference
?ptie decreases from 1 to 2
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Multiple Generators and Areas
Ptie?
Pe1
Pe2
jX
Area 1 or Gen 1 Tie Line Area 2 or Gen 2
Steady state gt return to synchronism at some
frequency ?? ??1 ??2
? Pl1 - D1 ?? - (1/R1) ?? ?Ptie
0 Load
Load Generation Interchange Change Response
Change from Governor ? Pl2 - D2 ??
- (1/R2) ?? - ?Ptie 0
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Multiple Generators and Areas
Ptie?
Pe1
Pe2
jX
Area 1 or Gen 1 Tie Line Area 2 or Gen 2
Steady state gt return to synchronism at some
frequency ?? -( ?Pl1 ?Pl2)/( D1D2 1/R1
1/R2) ?Ptie -? Pl1 (D11/R1)( ?Pl1
?Pl2)/( D1D2 1/R1 1/R2) ?Pm1 - (1/R1)(
?Pl1 ?Pl2)/( D1D2 1/R1 1/R2) ?Pm2 -
(1/R2)( ?Pl1 ?Pl2)/( D1D2 1/R1 1/R2)
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Multiple Generators and Areas
Ptie?
Pe1
Pe2
jX
Area 1 or Gen 1 Tie Line Area 2 or Gen 2
Identical Areas R1R20.05 D1D21 ?Pl1 1
?Pl20 ?? -( ?Pl1 ?Pl2)/( D1D2 1/R1
1/R2) -0.0238 pu Corrected! ?Ptie
-0.5 pu ?Pm1 0.5pu ?Pm2 .5 pu
--------- Concept of Assist
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Multiple Generators and Areas
Load sharing (also see Example 11.2 Glover and
Sarma) Area 1 R10.05 1000 MVA Base Area 2 R2
0.05 500 MVA base D1D20 ?Pl1 1 ?Pl20 On
1000 MVA Base R2 R2Sbase new/Sbase old0.1 ??
-( ?Pl1 ?Pl2)/( D1D2 1/R1 1/R2) .03333
pu ?Ptie -0.5 pu ?Pm1 0.666 pu 2/3 of
load change ?Pm2 0.333 pu 1/3 of load
change If droop is same on own area( or machine)
base areas raise generation in proportion to
their capacities
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Multiple Generators and Areas
Coherent generators The oscillation in
frequency/angle represent synchronizing
swings As generators exchange Kinetic energy
trying to synchronize or find a common
frequency The swing is large and slow when
systems are separated by long lines Within an
area generators synchronize quickly and swing as
one large unit against other areas. An area can
be modeled as one large unit.
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Multiple Generators and Areas
Regulation R cannot be made too small -- System
becomes oscillatory and/or unstable
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Multiple Generators and Areas
Isochronous governors cannot be used with
multiple generators -- Difficult to supply
identical reference frequency to each generators
Pm1
Pm2
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Governor-Turbine Generator Summary

• Load-generation imbalance produces frequency
  changes as well as power flow (interchange)
  changes
• Turbine generator dynamics is described by the
  swing equation
• Governors achieve load-generation balance by
  changing generation based on frequency deviation
• Regulation/droop permits proper load sharing
• All generators(areas) assist in the load
  balancing process
• Governors do not restore frequency error to zero

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Load Frequency Control- Restoring Frequency and
Interchange