Automatic Generation Control Meth Bandara and Hassan Oukacha EE194 Advanced Controls Theory February 25, 2013
Outline Introduction System Modeling Single Generator AGC Going Forward Conclusion
Introduction
Background In a power grid, the load is always changing. Generators need to adjust the power output to match the load requirements Manual control is costly
Introduction to AGC System that automatically adjusts the output in response to changes in the input Important in electric power systems power output of multiple generators should change in response to load changes
Purpose Maintain a system s power balance Maintain and control operating limits Maintain a constant system frequency
System Frequency Kept constant If increasing: power generated > power used If decreasing: power generated < power used
The generator s speed and frequency are used as control signals.
Effects of Large Frequency Deviation Damages equipment Degrades load performance Overloads transmission lines Interferes with system protection schemes Destabilizes power system
Automatic Generation Control returns the system to a normal state.
Value of AGC True centralized function operates in real time in a closed loop Maintains frequency and time in the system and saves cost
Control Components of AGC Primary control Secondary control Economic dispatch
Primary Control Immediate and automatic response to the sudden change in load. Simple example: reaction to a frequency change.
Primary control is the focus of today s presentation.
Primary Control
A Simple Example Driving a car Speed changed by driver according to speed limits
Case1: Constant Load
Case 2: Increased Load
Case 3: Decreased Load
System Modeling
Turbine-Governor Model Small signal analysis model Relates mechanical power to the control power and generator speed Maintains frequency and time in the system and saves cost
Turbine-Governor Model Steam/Water (Input) Valve Turbine Generator Governer (Feedback) Load Power (Output)
Turbine-Governor Model Behavior: P M P L = M dω dt P L (s) - + P M (s) + 1 ω(s) sm
Turbine-Governor Model Behavior: P M P L = M dω dt For a small change in parameters: P M P L = M d( ω) dt
Load Response to Frequency Change For rotating components of the load, the real power increases with increasing frequency P L is now P L + D ω P L = Incipient load change (e.g. a starting motor). D ω = Response caused by additional load (frequency drops, motor slows down). It is the frequency sensitive load change. D = % change in load % change in frequency
Modified Model Original behavior: Modified behavior: P M P L = M d( ω) dt P M ( P L +D ω) = M d( ω) dt
Switching Domains Time domain: P M ( P L +D ω) = M d( ω) dt Frequency domain: P M s P L s D ω s = sm ω s ω s = P M s P L s sm + D
Switching Domains P M s P L s D ω s = sm ω s P L (s) - + P M (s) + 1 ω(s) sm + D
Let: P M s P L s D ω s = sm ω s Be: P M s P L s D ω s = ω s R
Steady State Response P M P L D ω = ω R At the steady state, because of energy balance: P M = 0 Then P L D ω = ω R
Steady State Response P L + D ω + Load Change Load Response ω R Generation Change = 0 ω = P L D + 1 R
Example Typical factory setting is 5 percent: R = 0.05 pu For P L = 1, D = 1, R = 0.05 ω = 1 1 + 1 0.05 = 0.0476 pu
Speed-Power Relationship Small signal analysis model P M = G M s P L 1 R ω At steady state: s 0 and G M (s) 1 P M = P L 1 R ω
Single Generator AGC
Rotor Inertia If the power generation remains unchanged: P M P L D ω = M d( ω) dt Becomes: P M P L = M d( ω) dt
Speed Change from Load Imbalance Governing system senses the speed change. System adjusted to match mechanical power (P M ) to the changed load (P L ). At steady state, the change in frequency can be described by the speed regulation factor (R): ω = R P L
Speed-Power Curve At the steady state (static): P M = P C 1 R ω Immediate output change that corresponds to the sudden load change Note the load is signaled by the frequency
Secondary Control Speed/frequency stops varying, but at a new steady state. System can be brought back to original state by a secondary control system.
Generator Model Relates mechanical power to the power angle Ignores voltage changes Note the power angle is not the voltage angle
Generator Model P M (s) 1 s 2 M + sd + T δ The combined transfer function becomes: P M = P C 1 R ω 1 1 + st G 1 + st T
AGC for Single Generator Since ω = θ = ω 0 + δ ω = δ
Governor with Steady State FB
Block Diagram of Turbine-Governor System
Complete Block Diagram
Block Diagram of Generator
Dynamic Response of the system
Going Forward
Multiple Generators Consider the effect to the mechanical power of each generator by: - Power flows in transmission lines - Loads at each bus New method of analysis required
Area Control Error (ACE) The difference between the scheduled and actual electrical generation. Attempted minimization. ACE is calculated by: ACE = NI A NI S 10b F A F S T ob + I ME
Area Control Error (ACE) ACE is calculated by: ACE = NI A NI S 10b F A F S T ob + I ME NI A NI S b F A F S T ob I ME = Actual net interchange (MW) = Scheduled net interchange (MW) = Frequency bias setting (MW/0.1 Hz) = Actual system frequency (Hz) = Scheduled system frequency (Hz) = Scheduled interchange energy for correction (MW) = Manually entered compensation for known equipment error (MW)
Area Control Error (ACE) Each area assumes to: - Minimize ACE - Hold common system frequency ACE 1 = NI A1 NI S1 10b 1 F F 1 ACE 2 = NI A2 NI S2 10b 2 F F 2 ACE n = NI An NI Sn 10b n F F n 0 = NI S1 + NI S2 + + NI Sn
Matrix Form 1 10 b 1 1 10 b n 10 b n 1 1 1 0 0 NI S1 NI S2 NI Sn F = NI A1 + 10b 1 F 1 ACE 1 NI A2 + 10b 2 F 2 ACE 2 NI An + 10b n F n ACE n 0 The matrix is regular
Multi-Generator AGC
Multiple Generators Consider the effect to the mechanical power of each generator by: - Power flows in transmission lines - Loads at each bus New method of analysis required
Multi-area Power System Comprises areas that are interconnected by high-voltage transmission line or tie-lines The frequency measured in each control area is an indicator of the trend of the mismatch power in the interconnection and not in the control area alone The LFC of each control area should control the interchange power with the other control areas as well as its local frequency
N-Control-area Power System
Tie-line Power The power flow on the tie-line from area 1 to area 2: Tie-line power change between area 1 and 2
N- Control-areas In general, the total tie-line power change between area 1 and other areas:
Block Diagram Representation of Control Area i
Is the previous system complete?
Supplementary Control Loop Regulates area frequency Maintains the net interchange of with neighboring areas at scheduled values Linear combination of both Area Control Error (ACE) With the bias factor
Complete Control Area i system
Dynamic Response Following Disturbance in areas 1 and 3
Conclusion Consider the effect to the mechanical power of each generator by: - Power flows in transmission lines - Loads at each bus New method of analysis required
References Robust Power System Frequency Control by Hassan Bevrani Power System Stability and Control by P. Kundur http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnum ber=5269681&tag=1 http://www.sarienergy.org/pagefiles/what_we_do/activities/ceb_pow er_systems_simulation_training,_colombo,_sri_lanka/c ourse_ppts/lecture_44_45_agc_1_and_2.pdf