lecture 44 agc 1
TRANSCRIPT
Automatic Generation Control
Dr M S R Murty
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Automatic Generation Control (AGC)
• Automatic frequency regulation bygoverning systems of individual turbine‐generators andAutomatic Generation Control (AGC) or Load frequency control ( LFC) system of the power system.
• In Energy Management system (EMS) at the Energy Control Center (ECC)
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AGC
• AGC componentsLoad frequency control (LFC)Economic Dispatch(ED)Interchange Scheduling (IS) • AGC is also referred as System Control Load Dispatch
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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
•A single generator and load is analyzed and then generalized to large system
Generator Turbine Governor Behavior
Pm
Pl
Pm‐Pl = M [dω/dt]
For small changes in parametersΔ Pm‐ Δ Pl = M[d (Δ ω) / dt]
Generator Turbine Governor Behavior
1/(Ms)
A sustained load – generation imbalanced would lead to a continuous change in frequency!!
Δ Pm(s)
Δ Pl(s)
Δ ω(s)
Load response to frequency change
• For Rotating components of load the real power increases with frequencyΔ Pl(s) = Δ Pl(s)+ DΔ ω(s)
Δ Pl(s) now is an ‘incipient’ load change ( a motor starts)DΔ ω(s) represents the response that the additional load causes frequency to drop, all motors slow down, and so load drops as DΔ ω(s)
Generator Turbine Governor Behavior
Δ Pm(s)‐ Δ Pl(s)‐D Δ ω(s) = sM Δ ω(s)
Δ ω(s)= [Δ Pm(s)‐ Δ Pl(s)]/ (Ms+D)
1/(Ms+D)
Δ Pm(s) +
Δ Pl(s) ‐Δ ω(s)
Generator Turbine Governor Behavior
Measures speed(frequency) and adjusts valves to change generation
Frequency drops => Raise generation
The Governor
PmPl
Pe
Speed
Governor Desired Generation
Generator Turbine Governor Behavior
Pm
Pl
Pe
Speed
Governor
Desired Generation
Generator Turbine Governor BehaviorSteady State Response
Steady state error
Using energy balance
Δ Pl ‐ D Δω ‐ (1/R) Δω = 0
Load Load GenerationChange Response Change from
Governor
Δω = ‐ Δ Pl /( D+1/R)
Typical R = 0.05 pu ( 5% factory set)
For ΔP = 1 , D = 1, R=0.05 Δω = 1/21 = ‐ 0.0476 pu
Single Turbine Generator with load
• For a change in load, speed/ frequency changes (with generation remaining unchanged):
• [Pm – Pl] = M [dω/dt ] Rotor Inertia Equation
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Pm
PlTurbine Gen
Speed Change due to load imbalance
• The governing system senses change in speed and adjusts steam control valve ( gate) so that mechanical power (Pm) matches with the changed load (Pl).
• The change in frequency (Δω) at steady state can be described using the DROOP equation in terms of change in load (Δ Pl) and a factor R called ‘speed regulation or ‘droop’.Δω = ‐ [Δ Pl ]( R) Droop equation
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Single Turbine Generator with load
• [Pm – Pl] = M [dω/dt ] Rotor Inertia Equation• Δω = ‐ [Δ Pl ]( R) Droop regulation equation
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Pm
PlTurbine Gen
LOAD DROP RESPONSE
Load
100%
Time(sec)
t
100%80%
Speed(%)
Speed does not returnTo 100 %
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Proportional Control : Droop
Rotor Inertia_
+
Load
Generation
Speed Reference
Speed‐
+
Control action stops when the power error has zero value
Speed error
Speed error present at steady state
Steady state: Generation = Load , butMachine speed differentfrom Speed set point
Proportional
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1.0
1.0
Power (p.u)
Speed( p.u)
Droop Characteristic
0.5
1.02
0.0
1.04
1.0 p.u or 100 % change
0.04 p.uor 4 %change
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NEED FOR SUPPLEMENTARY CONTROL
• Speed variation stops but at a different steady value.
• The speed however has to be brought back to the original value for which speed/ load reference has to be adjusted either by the operator or by a supplementary control system called Load Frequency Control (LFC) system
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+
Pref
-
Combined Mechanical
Power
+Composite Governor Composite Turbine Power System
Inertia
BLOCK DIAGRAM SHOWING POWER SYSTEM FREQUENCY VARIATION
Total Elec. load
Frequency
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Set point○
Generator Power
Frequency
Total Generation
TotalLoad
Primary regulation
Other m/c
To OtherMachines
Set point AreaFreq-uency
Secondary regulation
-- +
++
+○
○
○
AUTOMATICLOAD REQUENCY
CONTROLLER
Governor Turbine GRIDINERTIA
Fig 7 AUTOMATIC LOAD RFEQUENCY CONTROL SYSTEM
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Energy Management System(EMS)-Automatic Generation Control (AGC)
Electro Hydraulic Governor (EHG)
Electro Hydraulic Governor (EHG)
Electro Hydraulic Governor (EHG)
Electro Hydraulic Governor (EHG)
Turbine-Generator (TG)
Turbine-Generator (TG)
Turbine-Generator (TG)
Turbine-Generator (TG)
Set Point
Set Point
Set Point
Set Point
Frequency (f)
f
f
f
SYSTEM CONTROL CENTER (SCC)
HYDRO POWER PLANTS
Telemetry
------…..
Generation Signals (MW)
System Frequency
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Load Frequency ControlLFC Implementation
FrequencyMeasured At a centralLocation Tie line flows(MW)
Desired Frequency
Net Interchange
ACE
Filters K ∫ AllocationTo Plants
Other Considerations
∆PrefTo Units
Economic Dispatch SeverityActual Unit Movement Unit Energy BalanceMinimum Movement Response Rate Time error
~ every 4 sec
~ every 4 sec
Multiple Generators and Areas
Pm1
P1l
Pe1
Pm2
Pl2
Pe2
jX
Area 1 or Gen 1 Tie Line Area 2 or Gen 2
Ptie
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
Multiple Generators and Areas
Pm1
P1l
Pe1
Pm2
Pl2
Pe2
jX
Area 1 or Gen 1 Tie Line Area 2 or Gen 2
Ptie
Qualitative Response
Load increase in area 1Area 1 frequency dropsArea 1 voltage phase angle falls behind are 2Ptie decreases (stabilizes Area 1 frequency, drags down area 2)Area 2 frequency dropsBoth governors raise generationSteady state achieved at a lower frequency and PtieArea 1 assists Area 2 in meeting the load increase; frequency drop is lower
Area Control Error (ACE)
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• TIE‐LINE BIAS CONTROL. In this control strategy each area of an interconnected system tries to regulate its area control error (ACE) to zero, where:
Difference between the actual (a) and the scheduled (s) net interchange on the tie lines. Frequency error
System natural response coefficient
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ACE Generation
ACE > 0, DECREASE Generation
ACE< 0 , INCREASE GENERATION
Load Frequency Control
• Governors ensure that frequency is restored to near‐nominal
• This happens irrespective of location of load/generation change
• The purpose of LFC is to reallocate generation so – System wide frequency is restored – Each area meets its obligation Load+Interchange
Load Frequency Control• Definition
– Area Control Error (ACE)
ACE = Δ Net Interchange + β Δ f
Δ Net Interchange = Interchange error = Scheduled – Actual
Δ f = Δ ω = frequency deviationβ = frequency bias ( pu MW/ pu frequency)
Definition is sometimes written with negative sign on both terms
Load Frequency Control• Basic Idea
– ACE> 0 decrease generation– ACE<0 increase generation
• Assume load increases in one area only– Frequency drops everywhere Δf<0– Interchange from affected area decreases Δ Net Interchange <0– Interchange from other areas increases Δ Net Interchange >0
– Affected area has negative ace– In other are ACE is small or zero
– Affected area increases generation– Others stay put
Load Frequency ControlProperties of ACEAs long as one frequency bias β≠0
If all areas have ACE=0
then Δω = 0and all Δ Net Interchange =0
Driving ACE to zero restores frequency and interchange
Load Frequency Control
Properties of ACE 1Two areas ( loss ignored)
ACE 1 = ΔNet Interchange + β1 ΔωACE 2 = ‐ΔNet Interchange + β2 ΔωΔω= (ACE1+ACE2)/(β1 + β2 )
= 0 if ACE1=ACE2=0 and β1 + β2 ≠0Then Interchange error is also zero
Load Frequency Control
Properties of ACE 1Two areas ( loss ignored)ACE 1 = ΔNet Interchange + β1 ΔωACE 2 = ‐ΔNet Interchange + β2 ΔωΔω= (ACE1+ACE2)/(β1 + β2 )
= 0 if ACE1=ACE2=0 and β1 + β2 ≠0Then Interchange error is also zeroReasonable values of β1 , β2 will work
Load Frequency ControlProperties of ACE 2
Choose βi = Di + 1/RiIdeally
ΔNet Interchange = ‐(Di+1/Ri) Δω –ΔPliΔNet Interchange +Di+1/Ri) Δω = –ΔPliACEi = ‐PLi !!!!!!!!
Since 1/Ri >> Di we know Di+1/Ri pretty well
ACE measures area load change– should give us good control
From LFC to Economic Allocation
Time
MW
ΔωGovernor~seconds
LFC~minutes
Units pick up load α capacity
LFC distributes based on response
Economic dispatch
AGC Scheme
From Grainger and Stevenson Jr)
From LFC to Economic Allocation
Time
MWEconomic dispatch
Coal
Gas
Pref changes
The Economic dispatch problem• Given
– N units on‐line– System Load + Loss
– Equals Area Net Generation‐ Net Interchange when ACE~0
• Determine – MW allocation(Schedule) for each Unit– Minimize Fuel and Variable O&M cost
• Constraints A constrained– Unit capacity, Reserves optimization problem
The Economic dispatch problem
Minimize Pi
CT= C1(P1)+C2(P2)+…+CN(PN)
P1+P2+…+PN= PT
Pimin≤ Pi≤ Pimax i=1,2,…,N
Ci(Pi) = Fuel + Variable O&M cost ($/H) unit iPi = Net MW output Unit IPimin, Pimax = Min and Maximum Capacity Unit i
Load Variation with frequency
• Motor load in particular is affected by frequency • When frequency drops, motors slow down, produce less work, and consume less energy
• Frequency drops by 1%, motor load will drop 3%.• Non‐motor resistive load generally remainsconstant.• The net for both of the above is a general rule ofthumb:+/‐ 1% change in freq. = +/‐ 2% change in load
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Deadband
• An additional feature displayed by generators. • Deadband is the amount of frequency change a governor must “see” before it starts to respond.
• Deadband was really a natural feature of the earliest governors caused by gear lash (looseness or slop in the gear mechanism)
• Deadband serves a useful purpose by preventing governors from continuously “hunting” as frequency varies ever so slightly
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Policy of the NERC
• Generators with nameplate ratings of 10• MW or more must have governors installed.• Governors should provide 5% droop.• Deadband on all governors must be set to
+/‐ 0.036 Hz (on 60 Hz system)
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FRC
• Frequency Response or Frequency Response Characteristic (FRC) is the change in frequency that occurs for a change in load‐resource balance in a control area or interconnection
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• Example:• If a generator of 1,000 MW is lost somewhere in a control area, frequency will decline.
• The actual amount of decline will depend on:• Characteristics of the load (how much motor load)
• The total governor response available• Number of generators on line• Their relative loading• Their governor settings
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Graph shows frequency excursionsvs. generation loss
Line represents the averagefrequency response of 1,500MW/0.1 Hz.
• Graph shows frequency excursions• vs. generation loss• • 350 events were tracked in WECC• from 1994 to 2002• • Line represents the average• frequency response of 1,500• MW/0.1 Hz.• Note: The total Frequency• Response in an Interconnection• is the sum of the responses• from all control areas within the• Interconnection
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• As mentioned, Frequency Response Characteristic (FRC) is the actual response provided by control areas for a particular set of events.
• Control areas use Automatic Generator Control (AGC) systems to meet their minute‐to‐minute obligations to serve their internal load.
• When an excursion happens external to a control area, there should be an immediate outflow from the control area to arrest frequency decline.
• The outflow itself is from “load rejection” and governor• response.• In order to prevent AGC from “fighting” this natural frequency
support, a “Bias” term is added to the ACE equation.
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