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Flexible AC Transmission Systems: Placement, Control, and Interaction

Flexible AC Transmission Systems: Placement, Control, and Interaction. Mariesa L. Crow University of Missouri-Rolla. Flexible AC Transmission Systems.

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Flexible AC Transmission Systems: Placement, Control, and Interaction

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  1. Flexible AC Transmission Systems: Placement, Control, and Interaction Mariesa L. Crow University of Missouri-Rolla

  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

  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. 207-214, June 1995.

  4. Constraints on Useable Transmission Capacity • Dynamic: • Transient and dynamic stability • Subsynchronous oscillations • Dynamic overvoltages and undervoltages • Voltage collapse • Frequency collapse

  5. Static: • Uneven power flow • Excess reactive power flows • Voltage capability • Thermal capability

  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

  7. SVC

  8. Thyristor Controlled Series Compensator (TCSC) TCSC

  9. StatCom • shunt device • lower rated components since only carry a fraction of the line current • impacts bus voltage and reactive power support

  10. SSSC • series device • must have higher rated transformer and devices • impacts active power flow

  11. UPFC • combination of StatCom and SSSC • may control voltage, impedance, and angle • impacts active and reactive power flow in line

  12. UPFC Topology

  13. Placement and Coordination of FACTS Devices

  14. 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?

  15. Steady-State Power Flow Control • UPFC • SSSC • TCSC • TCPAR These devices can affect active power flow

  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 gijand b and g are sensitivity parameters

  17. Optimization (optimal power flow) with genetic algorithms to minimize some cost function • Generation costs • Congestion • Problem is nonlinear, non-smooth, and non-convex

  18. Max-flow (graph theory) uses forward and backward labeling from source to sink to dynamically determine line flows

  19. Issues and Challenges • Dynamic Coordination of FACTS settings • Security • Economics • Droop • Hierarchical or local control of FACTS?

  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

  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

  22. Challenges to FACTS Implementation • Unbalanced operation • Harmonics • Integration of Energy Storage (BESS, SMES, flywheels) • Power electronic topologies • Power electronics devices

  23. StatCom/BESS voltages active power

  24. SSSC/BESS voltages active power

  25. 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)

  26. Cascaded Converter Advantages • 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

  27. 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

  28. Food for thought

  29. 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

  30. 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.

  31. 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

  32. 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.

  33. 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 (10-100 Hz) may occur on FACTS assisted power systems

  34. Discussion

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