1. Available Transfer Capability Determination Chen-Ching Liu and Guang Li University of Washington Third NSF Workshop on US-Africa Research and Education Collaboration Abuja, Nigeria, December 13-15, 2004

2. Third US-Africa Research and Education Collaboration Workshop Abuja, Nigeria, December 13-15, 2004 2 Overview Background of Available Transfer Capability (ATC) Definitions of ATC Determination of ATC Examples of ATC in Nigerian NEPA 330kV Grid Optimization Technique to Calculate ATC Stability-Constrained ATC Calculation Method Conclusions

3. Third US-Africa Research and Education Collaboration Workshop Abuja, Nigeria, December 13-15, 2004 3 Background ATC is the transmission limit for reserving and scheduling energy transactions in competitive electricity markets. Accurate evaluation of ATC is essential to maximize utilization of existing transmission grids while maintaining system security.

4. Third US-Africa Research and Education Collaboration Workshop Abuja, Nigeria, December 13-15, 2004 4 Transmission Service Types Recallable transmission service: Transmission service that a transmission provider can interrupt in whole or in part. Non-recallable transmission service: Transmission service that cannot be interrupted by a provider for economic reasons, but that can be curtailed for reliability.

5. Third US-Africa Research and Education Collaboration Workshop Abuja, Nigeria, December 13-15, 2004 5 ATC Under Operating Constraints Transfer capability must be evaluated based on the most limiting factor. Time Voltage Limit Stability Limit Power Flow A to B (MW) Thermal Limit Total Transfer Capability

6. Third US-Africa Research and Education Collaboration Workshop Abuja, Nigeria, December 13-15, 2004 6 Available Transfer Capability (ATC) (North American Electric Reliability Council) MW A->B Time Operating HorizonPlanning Horizon Nonrecallable Scheduled Recallable Scheduled Nonrecallable Reserved Recallable Reserved Total Transfer Capability (TTC) Transmission Reliability Margin TRM Nonrecallable Reserved Nonrecallable Available Transfer Capability Nonrecallable ATC Recallable ATC

7. Third US-Africa Research and Education Collaboration Workshop Abuja, Nigeria, December 13-15, 2004 7 Definition of ATC ATC = TTC – TRM – Existing Transmission Commitments (including CBM) Transmission Transfer Capability Margins –Transmission Reliability Margin (TRM) –Capacity Benefit Margin (CBM)

8. Third US-Africa Research and Education Collaboration Workshop Abuja, Nigeria, December 13-15, 2004 8 Transmission Reliability Margin (TRM) Uncertainty exists in future system topology, load demand and power transactions TRM is kind of a safety margin to ensure reliable system operation as system conditions change. TRM could be 8% or 10% of the TTC

9. Third US-Africa Research and Education Collaboration Workshop Abuja, Nigeria, December 13-15, 2004 9 Capacity Benefit Margin (CBM) CBM is reserved by load serving entities to ensure access to generation from interconnected systems to meet generation reliability requirements. Intended only for the time of emergency generation deficiencies

10. Third US-Africa Research and Education Collaboration Workshop Abuja, Nigeria, December 13-15, 2004 10 State of the Art: ATC Methods ATC Methods Description Linear Approximation Method DC Power Flow Model, Thermal Limit Only Optimal Power Flow Method AC Power Flow Model, Thermal Limit + Voltage Limit Continuation Power Flow Method AC Power Flow Model, Thermal Limit + Voltage Limit (Voltage Collapse) Stability-Constrained ATC Method Time Domain Simulations with Dynamic Model

11. Third US-Africa Research and Education Collaboration Workshop Abuja, Nigeria, December 13-15, 2004 11 First Contingency Incremental Transfer Capability (FCITC) & First Contingency Total Transfer Capability (FCTTC) FCTTC FCITC BASE POWER TRANSFERS

12. Third US-Africa Research and Education Collaboration Workshop Abuja, Nigeria, December 13-15, 2004 12 Total Transfer Capability (TTC) System Conditions Critical Contingencies Parallel Path Flows Non-Simultaneous and Simultaneous Transfers System Limits

13. Third US-Africa Research and Education Collaboration Workshop Abuja, Nigeria, December 13-15, 2004 13 Procedure to Calculate TTC Start with a base case power flow Increase generation in area A and increase demand in area B by the same amount Check the thermal, stability and voltage constraints. Evaluate the first contingency event and ensure that the emergency operating limits are met. When the emergency limit is reached for a first contingency, the corresponding (pre- contingency) transfer amount from area A to area B is the TTC.

14. Third US-Africa Research and Education Collaboration Workshop Abuja, Nigeria, December 13-15, 2004 14 Example 1: 2-Area NEPA 330kV Grid

15. Third US-Africa Research and Education Collaboration Workshop Abuja, Nigeria, December 13-15, 2004 15 2-Area Base-Case Tie Flow 2123 Area 1 4.64 MW Area 2 No thermal limit (assumed 120% base case flow) reached Single transmission line contingency First thermal limit reached Notation Tie Line Flow

16. Third US-Africa Research and Education Collaboration Workshop Abuja, Nigeria, December 13-15, 2004 16 Area 1 to Area 2 ATC Calculation 21 23 Area 1 > 4.64 MW Area 2 Increasing Generation  P MW Increasing Demand  P MW 2123 Area 1 4.96 MW Area 2 7-25 2-8 Increased Generation 0.32 MW Increased Demand 0.32 MW FCITC FCTTC

17. Third US-Africa Research and Education Collaboration Workshop Abuja, Nigeria, December 13-15, 2004 17 Area 2 to Area 1 ATC Calculation 2123 Area 1 < 4.64 MW Area 2 Increasing Demand  P MW Increasing Generation  P MW 2123 Area 1 4.54 MW Area 2 7-25 5-24 Increased Demand 0.1 MW Increased Generation 0.1 MW FCITC FCTTC

18. Third US-Africa Research and Education Collaboration Workshop Abuja, Nigeria, December 13-15, 2004 18 2-Area ATC Calculation DirectionArea 1 to Area 2Area 2 to Area 1 Critical Contingency Line 7-25 (Delta- Aladja) Thermal Limit Reached Line 2-8 (Jebba G.S.-Jebba T.S.) Line 5-24 (Alam- Aba) FCTTC4.96 MW  4.54MW FCITC0.32 MW0.1 MW

19. Third US-Africa Research and Education Collaboration Workshop Abuja, Nigeria, December 13-15, 2004 19 Example 2: 4-Area NEPA 300kV Grid AREA 1 AREA 2 AREA 3 AREA 4

20. Third US-Africa Research and Education Collaboration Workshop Abuja, Nigeria, December 13-15, 2004 20 4-Area Base-Case Tie Flows Area 1 4.64 MW Area 2 Area 4 16.6 MW Area 3 8.24 MW8.5 MW

21. Third US-Africa Research and Education Collaboration Workshop Abuja, Nigeria, December 13-15, 2004 21 Area 3 to Area 1 ATC calculation (Example of Parallel Path Flows) Area 1 4.64 MW Area 2 Area 4 17.2 MW Area 3 8.65 MW9.1 MW 7-25 1-7 Increased Generation 1.01 MW Increased Demand 1.01 MW FCTTC = 9.1+ 8.65 = 17.75 MW FCITC = 17.75  (8.5 + 8.24) = 1.01 MW

22. Third US-Africa Research and Education Collaboration Workshop Abuja, Nigeria, December 13-15, 2004 22 Area 4 to Area 2 Simultaneous ATC with a Pre- existing Area 3 to Area 1 17.75 MW Transfer Area 1 4.85 MW Area 2 Area 4 16.99 MW Area 3 8.65 MW9.1 MW 7-25 4-10 Increased Generation 0.21 MW Increased Demand 0.21 MW FCTTC =  16.99  (  17.2) = 0.21 MW FCITC = 4.85  4.64 = 0.21 MW

23. Third US-Africa Research and Education Collaboration Workshop Abuja, Nigeria, December 13-15, 2004 23 Optimization Technique to Calculate ATC Objective: Subject to sum of generation in sending area A - system dynamic behavior - power flow equations - active power output - thermal limit - voltage profile - energy margin

24. Third US-Africa Research and Education Collaboration Workshop Abuja, Nigeria, December 13-15, 2004 24 Stability-Constrained ATC Second-Kick based Energy Margin Computation Time Domain Simulation (ETMSP) System trajectory No Energy Margin Sensitivity Analysis with BFGS Method Generation Adjustment (EM = 0) ? Yes ATC

25. Third US-Africa Research and Education Collaboration Workshop Abuja, Nigeria, December 13-15, 2004 25 Second-kick-based energy margin computation Perform time-domain simulation Obtain system trajectory following a pre-specified disturbance sequence Compute potential energy of first- and second-kick trajectories Potential energy difference at the respective peaks of the first- and second-kick disturbances - Simulation - Trajectory - Potential energy - Energy margin

26. Third US-Africa Research and Education Collaboration Workshop Abuja, Nigeria, December 13-15, 2004 26 Energy margin sensitivity computation Determine the search direction with the Broyden-Fletcher-Goldfarb-Shanno (BFGS) method D is an approximation to the inverse of Hessian matrix

27. Third US-Africa Research and Education Collaboration Workshop Abuja, Nigeria, December 13-15, 2004 27 Generation adjustment - Adjustment - Update

28. Third US-Africa Research and Education Collaboration Workshop Abuja, Nigeria, December 13-15, 2004 28 2-Area Test System Net power transferred from area A to area B in the base case = 453 MW 453 MW Area A Area B

29. Third US-Africa Research and Education Collaboration Workshop Abuja, Nigeria, December 13-15, 2004 29 Stability-Constrained ATC Results

30. Third US-Africa Research and Education Collaboration Workshop Abuja, Nigeria, December 13-15, 2004 30 Conclusions ATC provides a reasonable and dependable indication of available transfer capabilities in electric power markets. ATC considers reasonable uncertainties in system conditions and provides operating flexibility for the secure operation of the interconnected network. The effects of simultaneous transfers and parallel path flows are studied. Need for ATC calculation method to incorporate voltage, angle stability limits as well as thermal limits.

31. Third US-Africa Research and Education Collaboration Workshop Abuja, Nigeria, December 13-15, 2004 31 References [1] North American Electric Reliability Council, “Available Transfer Capability Definitions and Determination”, June 1996. [2] North American Electric Reliability Council,“Transmission Transfer Capability”, May 1995. [3] S. K. Joo, C. C. Liu, Y. Shen, Z. Zabinsky and J. Lawarree, “Optimization Techniques for Available Transfer Capability (ATC) and Market Calculations,” IMA Journal of Management Mathematics (2004) 15, 321-337.