1. Flexible AC Transmission Systems: Placement, Control, and Interaction
Mariesa L. Crow
University of Missouri-Rolla
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2. Flexible AC Transmission Systems
Alternating current transmission systems incorporating power electronics-based and other static controllers to enhance controllability and increase power transfer capability
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3. “Without fundamental research in this area, very little use will be made with full confidence of the real opportunities offered by FACTS devices. For the time being we only have limited examples, entirely based on simulation, which demonstrate that fast regulation of reactive compensation on a transmission grid could be very useful in the future. Because of this, there may exist an immediate danger of uncoordinated system-wide fast regulation via FACTS devices which could become detrimental to system integrity under certain operating conditions.”
Marija Ilic, “Fundamental engineering problems and opportunities in operating power transmission grids of the future” Int'l Journal of Electrical Power & Energy Systems, vol. 17, no. 3, pp , June 1995.
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4. Constraints on Useable Transmission Capacity
Dynamic:
Transient and dynamic stability
Subsynchronous oscillations
Dynamic overvoltages and undervoltages
Voltage collapse
Frequency collapse
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5. Static: Uneven power flow Excess reactive power flows
Voltage capability
Thermal capability
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6. FACTS Controllers Static VAR Compensator - SVC
Thyristor Controlled Series Compensator - TCSC
Thyristor Controlled Phase Angle Regulator - TCPAR
Static Synchronous Compensator - StatCom
Solid State Series Compensator - SSSC
Unified Power Flow Controller - UPFC
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7. SVC
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8. Thyristor Controlled Series Compensator (TCSC)
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9. StatCom
shunt device
lower rated components since only carry a fraction of the line current
impacts bus voltage and reactive power support
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10. SSSC series device must have higher rated transformer and devices
impacts active power flow
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11. UPFC combination of StatCom and SSSC
may control voltage, impedance, and angle
impacts active and reactive power flow in line
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12. UPFC Topology
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13. Placement and Coordination of FACTS Devices
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14. Is there a one-size-fits-all controller? time Long-term Control
Power Flow control
FACTS scheduling
Economics
Dynamic Control
System oscillation damping
Voltage stability
FACTS “ringing”
Local Control
Control target acquisition
Power electronics topology
Modulation strategies
time
Is there a
one-size-fits-all
controller?
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15. Steady-State Power Flow Control
UPFC
SSSC
TCSC
TCPAR
These devices can affect active power flow
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16. Approaches Sensitivity analysis
Where  is the change in power transfer capacity in response to an addition of t compensation in line i-j with admittance bij+j gij and b and g are sensitivity parameters
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17. Optimization (optimal power flow) with genetic algorithms to minimize some cost function
Generation costs
Congestion
Problem is nonlinear, non-smooth, and non-convex
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18. Max-flow (graph theory) uses forward and backward labeling from source to sink to dynamically determine line flows
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19. Issues and Challenges Dynamic Coordination of FACTS settings
Security
Economics
Droop
Hierarchical or local control of FACTS?
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20. Dynamic Control transient stability improvement
inter-area oscillation damping
voltage collapse avoidance
subsynchronous resonance mitigation
Each control objective will (possibly) require a different FACTS placement
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21. Issues
Most dynamic control development has concentrated on SMIB or very small two-area systems
How is control implemented in a large nonlinear interconnected dynamic network?
FACTS-FACTS interaction
FACTS-generator interaction
Hardware/field verification limited
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22. Challenges to FACTS Implementation
Unbalanced operation
Harmonics
Integration of Energy Storage (BESS, SMES, flywheels)
Power electronic topologies
Power electronics devices
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23. StatCom/BESS voltages active power
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24. SSSC/BESS voltages active power
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25. Summary Responses voltages active power
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26. Issues Most work considers only: Simplified topologies
UPFC = variable impedance
StatCom = PV bus
Three-phase balanced operation
No harmonics
Simulation based
Isolated performance (no interactions)
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27. Cascaded Converter Advantages Disadvantages
Use fewer components to achieve the same number of levels
Has modularized circuitry which makes packaging possible
Does not have balancing problem when with batteries
Disadvantages
Needs separate DC sources for active power conversion
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28. Diode-Clamped Advantages
The harmonic content decreases as the number of levels increases, thus reducing the size of filters
Efficiency is high since devices are switched at the fundamental frequency
It is easy to realize bi-directional active power flow with a BESS or other energy storage system
Disadvantages:
Requires a large number of high power clamping diodes if the number of levels is high
A high voltage rating is required for the blocking diodes
There is potentially a voltage balancing problem
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29. Food for thought
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30. time Long-term Control Power Flow control FACTS scheduling Economics
Dynamic Control
System oscillation damping
Voltage stability
FACTS “ringing”
Local Control
Control target acquisition
Power electronics topology
Modulation strategies
time
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31. Conventional eigenvalue analysis cannot predict the high frequency self-modes of the several FACTS devices embedded in a large power system network.
High frequency control interactions among the several FACTS devices must be checked using an EMTP-type program
A promising technique is based on the use of high frequency eigenvalue calculation using Generalized Switching Function Models for the different FACTS devices under consideration.
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32. Series controllers
low loop impedance - the series controllers may experience a very strong interaction, and therefore these controllers must be designed in a coordinated way - the main linking variable among the series controllers is the ac current
high loop impedance - no control interactions may be expected among series controllers
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33. HVDC
HVDC converters embedded in a large network will not experience control interactions if the transference impedances between their commutation busbars are high. This means that, in this case, the dc control design of each station can be based exclusively on the Short-Circuit Ratio (SCR) at its ac connection point.
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34. SCADA systems
Dedicated SCADA systems will have to be developed if global control of multiple FACTS controllers is desired.
Currently available SCADA systems have a refresh rate of 1 second (maximum). This is sufficient for steady-state control dispatch of FACTS controllers.
However, this is completely inadequate for dynamic control, especially if we consider that high frequency modes ( Hz) may occur on FACTS assisted power systems
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35. Discussion
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