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A power flow controller responsive to power circulation demand for optimizing power transfer is disclosed. When a power flow controller operates at its rated capacity, it can no longer regulate from-bus voltage set-points, line power flow set-points, or both. In such cases, the power flow controller

A power flow controller responsive to power circulation demand for optimizing power transfer is disclosed. When a power flow controller operates at its rated capacity, it can no longer regulate from-bus voltage set-points, line power flow set-points, or both. In such cases, the power flow controller switches to power circulation set-point control without exceeding the rated capacities of the voltage-sourced converters in the power flow controller. Power-voltage (PV) curves associated with voltage stability analysis for maximizing power transfer can be generated and stored for use with a power flow controller operating in automatic power flow control mode.

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What is claimed is: 1. A power flow controller, comprising: a controller responsive to a circulating power demand; a first voltage-sourced converter coupled to the controller and adapted to be coupled to a transmission line, the first voltage-sourced converter configured by the controller to contro

What is claimed is: 1. A power flow controller, comprising: a controller responsive to a circulating power demand; a first voltage-sourced converter coupled to the controller and adapted to be coupled to a transmission line, the first voltage-sourced converter configured by the controller to control power flow in the transmission line; and a second voltage-sourced converter coupled to the first voltage-sourced converter via a direct current link and adapted to be coupled to the transmission line, the second voltage-sourced converter configured by the power controller to control power flow in the transmission line, wherein the controller is configured to switch to power circulation set point control between the coupled first and second voltage-sourced converters when at least one of the first voltage-sourced converter and the second voltage-sourced converter is at an operating limit. 2. The power flow controller of claim 1, wherein the circulating power demand is determined in real-time. 3. The power flow controller of claim 1, where the circulating power demand is determined from a look-up table. 4. The power flow controller of claim 1, wherein the circulating power demand is determined automatically and continuously using a closed-loop feedback system. 5. The power flow controller of claim 1, wherein the controller implements a vector control scheme that regulates transmission line current using a synchronous reference frame. 6. The power flow controller of claim 1, wherein the first and second voltage-sourced converters are configured as a Unified Power Flow Controller (UPFC). 7. The power flow controller of claim 6, wherein the first voltage-sourced converter is operating at MVA rating and the circulating power demand Pcd, is based on a solution to a set of load flow equations given by description="In-line Formulae" end="lead"√ {square root over (Pse2+Q se2)}=se max,description="In-line Formulae" end="tail" wherein Qse is given by and a second set of load flow equations for the second voltage-sourced converter given by description="In-line Formulae" end="lead"V1 =V1d description="In-line Formulae" end="tail" description="In-line Formulae" end="lead"anddescription="In-line Formulae" end="tail" description="In-line Formulae" end="lead"Psh =-Pcd.description="In-line Formulae" end="tail" 8. The power flow controller of claim 6, wherein the first voltage source converter is operating at a voltage magnitude limit V m2max, and the circulating power demand Pcd is based on a solution to a first set of loadflow equations given by description="In-line Formulae" end="lead"Vm2 =Vm2max description="In-line Formulae" end="tail" description="In-line Formulae" end="lead"Pse =Pcd, anddescription="In-line Formulae" end="tail" a second set of load flow equations for the second voltage-sourced converter given by description="In-line Formulae" end="lead"V1 =V1d description="In-line Formulae" end="tail" description="In-line Formulae" end="lead"anddescription="In-line Formulae" end="tail" description="In-line Formulae" end="lead"Psh =-Pcd.description="In-line Formulae" end="tail" 9. The power flow controller of claim 6, wherein the second voltage-sourced converter is operating at MVA rating and the circulating power demand Pcd is based on a solution to a set of loadflow equations given by description="In-line Formulae" end="lead"√ {square root over (Psh2+Qsh 2)}=Ssh max,description="In-line Formulae" end="tail" wherein Qsh is given by and a second set of loadflow equations for the second voltage-sourced converter given by 10. The power flow controller of claim 6, wherein the second voltage-sourced converter is operating at a current magnitude limit I sh max, and the circulating power demand Pcd is based on a solution to a first set of loadflow equations given by description="In-line Formulae" end="lead"Psh =-Pcd, anddescription="In-line Formulae" end="tail" a second set of loadflow equations for the second voltage-sourced converter given by 11. The power flow controller of claim 6, wherein the first and second voltage-sourced converters are operating at MVA rating and the circulating power demand Pcd is based on a solution to a set of loadflow equations given by description="In-line Formulae" end="lead"√ {square root over (Pse2+Q se2)}=Sse max, description="In-line Formulae" end="tail" wherein Qse is given by wherein Qsh is given by description="In-line Formulae" end="lead"Psh =-Pcd, anddescription="In-line Formulae" end="tail" description="In-line Formulae" end="lead"Pse =Pcd.description="In-line Formulae" end="tail" 12. The power flow controller of claim 6, wherein the first voltage-sourced converter is operating at a voltage magnitude limit V m2max and the second voltage-sourced converter is operating at a current magnitude limit Ish max, and the circulating power demand Pcd is based on a solution to a set of loadflow equations given by 13. The power flow controller of claim 1, wherein the first and second voltage-sourced converters are configured as an Interline Power Flow Controller (IPFC). 14. The power flow controller of claim 1, wherein the power flow controller includes a third voltage-sourced converter and is configured as a Generalized Unified Power Flow Controller (GUPFC). 15. A method of controlling power flow in a transmission line using a power flow controller operated at rated capacity, comprising: determining if a power flow controller coupled to the transmission line is operating at rated capacity; determining a circulating power demand in response to the power flow controller operating at rated capacity; and configuring the power flow controller based on the circulating power demand to control power flow in the transmission line, wherein the configuring includes setting the power flow controller to include power circulation set point control between at least two coupled voltage-sourced converters if the transmission line is operating at rated capacity. 16. The method of claim 15, wherein the step of determining the circulating power demand further comprises: determining a desired voltage-stability margin for a bus on the transmission line; determining a power flow adjustment that will maintain the desired voltage-stability margin on the bus; and determining a circulating power demand that will provide the maximum power flow adjustment while maintaining the desired voltage-stability margin on the bus. 17. The method claim 15, wherein the circulating power demand is determined from predetermined voltage collapse curves. 18. The method claim 15, wherein circulating power demand is determined automatically from predetermined voltage collapse curves. 19. A computer-readable medium having stored thereon instructions which, when executed by a processor in a power flow control system coupled to a transmission line, causes the processor to perform the operations of: determining a desired voltage-stability margin for a bus on the transmission line; determining a power flow adjustment that will maintain the desired voltage-stability margin on the bus; and determining a circulating power demand that will provide the maximum power flow adjustment while maintaining the desired voltage-stability margin on the bus, wherein the determining the circulating power demand includes power circulation set point control between at least two coupled voltage-sourced converters. 20. The computer-readable medium of claim 19, wherein the circulating power demand is determined from predetermined voltage collapse curves. 21. The computer-readable medium of claim 19, wherein circulating power demand is determined automatically from a look-up table. 22. A system for controlling power flow in a transmission line using a power flow controller operated at rated capacity, comprising: means for determining if a power flow controller coupled to the transmission line is operating at rated capacity; means for determining a circulating power demand in response to the power flow controller operating at rated capacity; and means for configuring the power flow controller based on the circulating power demand to control power flow in the transmission line, wherein the power flow controller is configured to include power circulation set point control between at least two coupled voltage-sourced converters if the transmission line is operating at rated capacity.