Flexible AC Transmission Systems: Placement, Control, and Interaction Mariesa L. Crow University of Missouri-Rolla EPRI/NSF Workshop on Global Dynamic Optimization

Flexible AC Transmission Systems Alternating current transmission systems incorporating power electronics-based and other static controllers to enhance controllability and increase power transfer capability EPRI/NSF Workshop on Global Dynamic Optimization

“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. EPRI/NSF Workshop on Global Dynamic Optimization

Constraints on Useable Transmission Capacity Dynamic: Transient and dynamic stability Subsynchronous oscillations Dynamic overvoltages and undervoltages Voltage collapse Frequency collapse EPRI/NSF Workshop on Global Dynamic Optimization

Static: Uneven power flow Excess reactive power flows Voltage capability Thermal capability EPRI/NSF Workshop on Global Dynamic Optimization

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 EPRI/NSF Workshop on Global Dynamic Optimization

SVC EPRI/NSF Workshop on Global Dynamic Optimization

Thyristor Controlled Series Compensator (TCSC) EPRI/NSF Workshop on Global Dynamic Optimization

StatCom shunt device lower rated components since only carry a fraction of the line current impacts bus voltage and reactive power support EPRI/NSF Workshop on Global Dynamic Optimization

SSSC series device must have higher rated transformer and devices impacts active power flow EPRI/NSF Workshop on Global Dynamic Optimization

UPFC combination of StatCom and SSSC may control voltage, impedance, and angle impacts active and reactive power flow in line EPRI/NSF Workshop on Global Dynamic Optimization

UPFC Topology EPRI/NSF Workshop on Global Dynamic Optimization

Placement and Coordination of FACTS Devices EPRI/NSF Workshop on Global Dynamic Optimization

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? EPRI/NSF Workshop on Global Dynamic Optimization

Steady-State Power Flow Control UPFC SSSC TCSC TCPAR These devices can affect active power flow EPRI/NSF Workshop on Global Dynamic Optimization

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 EPRI/NSF Workshop on Global Dynamic Optimization

Optimization (optimal power flow) with genetic algorithms to minimize some cost function Generation costs Congestion Problem is nonlinear, non-smooth, and non-convex EPRI/NSF Workshop on Global Dynamic Optimization

Max-flow (graph theory) uses forward and backward labeling from source to sink to dynamically determine line flows EPRI/NSF Workshop on Global Dynamic Optimization

Issues and Challenges Dynamic Coordination of FACTS settings Security Economics Droop Hierarchical or local control of FACTS? EPRI/NSF Workshop on Global Dynamic Optimization

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 EPRI/NSF Workshop on Global Dynamic Optimization

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 EPRI/NSF Workshop on Global Dynamic Optimization

Challenges to FACTS Implementation Unbalanced operation Harmonics Integration of Energy Storage (BESS, SMES, flywheels) Power electronic topologies Power electronics devices EPRI/NSF Workshop on Global Dynamic Optimization

StatCom/BESS voltages active power EPRI/NSF Workshop on Global Dynamic Optimization

SSSC/BESS voltages active power EPRI/NSF Workshop on Global Dynamic Optimization

Summary Responses voltages active power EPRI/NSF Workshop on Global Dynamic Optimization

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) EPRI/NSF Workshop on Global Dynamic Optimization

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 EPRI/NSF Workshop on Global Dynamic Optimization

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 EPRI/NSF Workshop on Global Dynamic Optimization

Food for thought EPRI/NSF Workshop on Global Dynamic Optimization

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 EPRI/NSF Workshop on Global Dynamic Optimization

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. EPRI/NSF Workshop on Global Dynamic Optimization

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 EPRI/NSF Workshop on Global Dynamic Optimization

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. EPRI/NSF Workshop on Global Dynamic Optimization

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 EPRI/NSF Workshop on Global Dynamic Optimization

Discussion EPRI/NSF Workshop on Global Dynamic Optimization