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Coordinated Voltage Control in Electrical Power Systems
Mohammad Moradzadeh, René Boel
SYSTeMS Research Group, EESA Department
{Mohammad.Moradzadeh, René.Boel}@ ugent.be
Research goal
Hybrid system design for coordination of discretely
acting devises for avoiding voltage instability in power
system
Automatic voltage regulators (AVRs)
12-bus IEEE standard power system
Controls the field current ifd to keep the terminal voltage close to the
desired setpoint.
What is voltage instability?
Power system are operated under much more stress than in the past due
to these issues:
• Competitive markets
• Continuous growth of consumption
• Transmission expansion doesn’t keep pace with generations and loads
• Renewable energy sources, micro generations (wind turbines, solar cells)
Voltage instability results from the attempt of loads to draw more power
than can be delivered by the transmission and generation system.
Load restoration dynamics
Loads are the driving force of voltage instability and for this reason this
phenomenon has also been called load instability.
OvereXcitation Limiter (OXL)
OXL Protects the field winding from an overheating due to excessive
current and we do need to include it in the model of AVR for voltage
instability studies. Among all other AVR limiting circuits, only OXL is
primarily related to voltage instability phenomena.
Fault: outage and reconnection of two parallel lines in the location F of
the model at t=30sec and t=60sec resp.
Loads: exponential dynamic load
• OXL action:
Ifd2
6
Load restoration is a process during which the dynamic of various load
components ( induction motors, thermostatic loads) and control
mechanisms ( load-tap changing transformers) tend to restore load
power at least to a certain extent.
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2
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Xt2
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• Exponential load:
0
V 
P  zP0 ( )
V0
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V
Q  zQ0 ( ) 
V0
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Xoxl2
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P active power consumed by the load
Q reactive power consumed by the load
z independent dimensionless demand variable
α, β depend on the type of load (motor, heating, lighting, etc.)
P0, Q0 nominal load powers
V0 reference voltage
0.04
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• Voltage behavior and LTCs action:
Tap3
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classical model
-4
0  g ( x, y, zc , zd )
x  g ( x, y, zc , zd )
zc  g ( x, y, zc , zd )
y: bus voltages vector
x: sate vector
zc: continuous long-term state vector
zd: discrete long-term state vector
-5
Load tap-changing transformer (LTC)
0
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20
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V Bus A
2
The LTCs are slowly acting, discrete devices changing the transformer
ratio r by one step at a time if the voltage error remains outside a
deadband longer than a specified time delay so that controls the
voltage of the distribution, medium voltage.
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Tap2
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• Instantaneous response of network:
load flow is represented by algebraic equations.
0
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V Bus B
2
1
• Short-term dynamics:
The state variable Zc represents fast dynamics like AVRs and
governors, excitation systems, turbines, induction motors, HVDCs
components and SVCs.
The corresponding dynamics last typically for several seconds.
0
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• LTCs interaction:
fault in t=50sec
Tap3
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• Long-term dynamics:
The state variable X represents slow dynamics like LTCs, generator
-4.5
limiters, boilers as well as secondary controllers.
The corresponding dynamics typically last for several minutes.
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V Bus A
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Hybrid system model
Components such as generators and loads drive the continuous
dynamic behavior. On the other hand, Limiters ( such as OXL), LTC,
HVDC and SVC actions lead to discrete events.
So, the behavior of power systems is characterized by complex
interactions between continuous dynamics and discrete events, i.e.,
power systems exhibit hybrid behavior.
Given an initial state and input trajectory, we have to solve the whole
set of differential-algebraic, discrete-continuous time equations
covering some of the short and long-term scales in simulation.
We only look at dynamics of time scale of seconds up to a few
minutes.
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• LTC tap adjustments:
Blocking: deactivating of control mechanism on its current position
Locking: moving to a specific tap position and then to be locked
Reversing: changing the control logic to control the transmission side
voltage instead of the distribution side
Voltage setpoint reduction: lowering the reference voltage
1
0.98
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25
30
Tap2
-2
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proposed solution
Distributed voltage control by coordinating LTC and
OXL operation via message exchange between them
and avoiding unnecessary reduction of the control
action.
-8
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LTC3 resp. LTC2 control the voltage of bus A resp. B.
The tap movement of LTC3 at t=50sec is a result of the tap movement
of LTC2 and its effect on the LTC3 controlled voltage at t=40sec.
often voltage collapse incidents are caused by uncoordinated
interactions of LTCs and the bigger power system, the more interactions
between them.