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Coordinated Voltage Control in Electrical Power SystemsMohammad Moradzadeh, René Boel

SYSTeMS Research Group, EESA Department {Mohammad.Moradzadeh, René.Boel}@ ugent.be

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.

classical model

Automatic voltage regulators (AVRs)

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

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.

y: bus voltages vector x: sate vector zc: continuous long-term state vector zd: discrete long-term state vector

• Instantaneous response of network:load flow is represented by algebraic equations.

• 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.

• Long-term dynamics:The state variable X represents slow dynamics like LTCs, generator limiters, boilers as well as secondary controllers.The corresponding dynamics typically last for several minutes.

Load restoration dynamics

Loads are the driving force of voltage instability and for this reason this phenomenon has also been called load instability.

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.

• Exponential load:

)(0

0 V

VzPP

)(0

0 V

VzQQ

P active power consumed by the loadQ reactive power consumed by the loadz independent dimensionless demand variableα, β depend on the type of load (motor, heating, lighting, etc.)P0, Q0 nominal load powersV0 reference voltage

),,,(0 dc zzyxg

Controls the field current ifd to keep the terminal voltage close to the desired setpoint.

Load tap-changing transformer (LTC)

• OXL action:

• Voltage behavior and LTCs action:

• LTCs interaction:fault in t=50sec

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.

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.

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.

Distributed voltage control by coordinating LTC and OXL operation via message exchange between them and avoiding unnecessary reduction of the control action.

),,,( dc zzyxgx

),,,( dcc zzyxgz

Research goal

Hybrid system design for coordination of discretely acting devises for avoiding voltage instability in power system

proposed solution

• LTC tap adjustments:Blocking: deactivating of control mechanism on its current positionLocking: moving to a specific tap position and then to be lockedReversing: changing the control logic to control the transmission side voltage instead of the distribution sideVoltage setpoint reduction: lowering the reference voltage

12-bus IEEE standard power system

0 10 20 30 40 50 60 70 80-5

-4

-3Tap3

0 10 20 30 40 50 60 70 800

1

2V Bus A

0 10 20 30 40 50 60 70 80-10

-5

0Tap2

0 10 20 30 40 50 60 70 800

1

2V Bus B

10 15 20 25 30 35 40 45 50 55-5

-4.5

-4Tap3

10 15 20 25 30 35 40 45 50 55

0.98

1

1.02V Bus A

10 15 20 25 30 35 40 45 50 55-8

-6

-4

-2Tap2

0 10 20 30 40 50 60 70 800

2

4

6Ifd2

0 10 20 30 40 50 60 70 80-10

-5

0

5Xt2

0 10 20 30 40 50 60 70 800

0.02

0.04

0.06Xoxl2