ECE620 – CURENT Course: Decision Support for Power System Restoration

1. ECE620 – CURENT Course:Decision Support for Power System Restoration Kai Sun November 6, 2013

2. Content • Historical power system blackouts • Industry practices in system restoration • Why do we need decision support tools? • Introduction of a Generic Restoration Milestone based approach • Case studies • Demonstration using OTS software (PowerSimulator by POWERDATA and IncSys)

3. Historical Blackout Events

4. Sequence of Events in Blackouts • Initial event • Vulnerable conditions • System islanding • Load/generation imbalance in islands • Blackout of islands

5. Tripped by Zone 3 relay 7 4 Faulty zone 3 relay 1GW generation tripped by SPS 9 3 2 1 5 Tree contact and relay mis-opt. 6 8 Loss of key hydro units 10 7 4 6 1 9 5 8 2 3 Example of Voltage Collapse - July 2nd, 1996 Western Cascading Event

6. Blackout Event on August 10, 1996 970 MW loss 2,100 MW loss 11,600 MW loss 15,820MW loss Malin-Round Mountain #1 MW 15:48:51 1500 15:42:03 15:47:36 1400 0.276 Hz oscillations Damping>7% 1300 0.252 Hz oscillations Damping 1% 0.264 Hz oscillations 1200 3.46% Damping 1100 200 300 400 500 600 700 800 Time in Seconds 1. Initial event (15:42:03): Short circuit due to tree contact  Outages of 6 transformers and lines 2. Vulnerable conditions (minutes) Low-damped inter-area oscillations  Outages of generators and tie-lines 3. Blackouts (seconds) Unintentional separation  Loss of 24% load System islanding and blackouts

7. Losses Due to Blackouts [1][2] The faster we bring the system back, the less we would lose

8. Common Factors: The 3 “T”s • Tools • Inability of system operators or coordinators to visualize events on the entire system • Failure to ensure that system operation was within safe limits • Training • Inadequate training of operating personnel • Ineffective communication, failure to communicate status to neighboring systems • Trees • Conductor contact with trees, inadequate vegetation management

9. Status of Power System Restoration • Restoration is basically manual work performed by operators in control rooms • Restoration plans or guidelines are offline designed by planning engineer and evaluated once/twice a year • Regional system restoration trainings/drills based on OTS (Operator Training Simulators) are conducted every year • Typical Restoration stages (assume 6-10 hours) [1]-[3]: • Preparation (1-2 hours) • System restoration (1-3 hours) • Load restoration (4-6 hours)

10. Generating Units with Black-Start (BS) Capabilities [1]-[4] • Hydro • may be started in 5-10 min. • Diesel • small but has fast response • may provide the start-up requirement of larger units • cannot be used to pick up sizable loads or energize transmission lines. • Gas turbine • units with local battery power • larger units with an on-site diesel unit

11. North American Electric Reliability Corporation (NERC) Standards for System Restoration

12. Sample Restoration Procedure • Initial assessment • Assessment of the extent of a blackout • Communications (essential) • Verify communication with ISO/RTO, control centers, energy providers, hydro, and other affected systems • Verify backup communications • Effective communication with all stakeholders • Determine generator status • online/offline, location, type, • damaged equipment, stability, reserve, connectivity to the system, and • blackstart capability. • Call for extra manpower

13. Sample Restoration Procedure (cont’d) • Start generation units • Restoration of offline units • Hydro: quickly started without an outside source • Combustion turbine: quickly (10min) started, may be voltage-dependent to allow starting • Thermal steam: 1-20 hours (24-48 hours for nuclear); hot units may be returned quicker • Prioritization of units to start • NERC requirements • Individual restoration plan • Start-up time of a unit • Availability of on-site auxiliary power • Distance to blackout resources • Generating plant operators • Safe plant shutdown (prepared for restoration) • Governors and AVR should be on • Plant operators control frequency around 60Hz

14. Sample Restoration Procedure (cont’d) • Restore the system • Multiple islands (bottom-up) • Stabilize remaining available generation • Determine restoration transmission paths • Expand islands by restoring transmission and load • Synchronize islands when appropriate • Large islands (Top-down) • Restore the EHV transmission (maybe from outside sources if available) • Restore critical generating plants and substations along the restored transmission • Bring on more generation • Restore underlying transmission

15. Sample Restoration Procedure (cont’d) • Restore load • Prioritize loads for restoration • Auxiliary power for generating plants • Auxiliary power for substations • Natural gas or oil supply facilities • Customers: • Critical (hospitals, airports, etc.) • Dispatchable (others) • Frequency control • Maintain frequency around 60Hz (e.g. 59.75-60.05Hz) • Increase frequency to >60Hz (e.g. 60-60.05Hz) prior to restoring a block of load

16. Restoration Strategies Build-Upward (Bottom-Up) (e.g. PJM [4]): Build-Downward (Top-Down) (e.g. Hydro Quebec [5]): Re-energizing the transmission network to pool blackstartpower first Actions include: Start up BS units, Energize the transmission network Crank non-BS units Pick up loads • Based on offline define electrical islands with blackstart capabilities • Actions include • Start up BS units • Crank non-BS units • Restore multiple islands to pick up loads • Synchronize islands

17. Decision Support Tools • Why important? • Supporting planning engineers in developing and evaluating restoration strategies • Supporting system operators in developing, rehearsing, coordinating and implementing restoration strategies Today’s Restoration plan Restoration decision support Offline, non-interactive Online, interactive

18. Optimize the path (minimizing the restoration time) Able to re-calculate when necessary (operators make mistakes or meet unexpected events) Online Interactive Decision Support Tool TVA Control Center (source: TVA.com Duke Energy Control Center (source: Patrick Schneider Photo.Com)

19. Restoration Milestone-based Decision Support Path (Strategy) Milestones • Stop 1 (Milestone 1) • Stop 2 (Milestone 2) • Turn Left (Action 1) • Turn Right (Action 2) • Turn Right (Action 3) • Turn Left(Action 4) • Turn Right (Action 5) • … Decision Support Tool Restoration Path Optimization (Minimizing Duration Time) Actions Simulation Tools (Security Constraints)

20. Generic Restoration Milestones (GRMs) Generic Restoration Actions (GRAs) A Restoration Milestones based Approach for Developing and Evaluating Restoration Strategies [6][7] • A specific restoration strategy is a combination of specific milestones • Under each milestone, an optimization problem can be formulated to solve restoration actions achieving that milestone with the shortest time • Constraints about, e.g., voltages, overloading and stability, can be checked for each restoration action by a power system simulation tool • GRM1: Form BS_NBS_Building Blocks • GRM2: Establish Transmission Grid • GRM3: Form Electrical Island • GRM4: Synchronize Electrical Islands • GRM5: Serve Load in Area • GRM6: Connect with Neighboring System • GRA1: start_black_start_unit • GRA2: find_path • GRA3: energize_line • GRA4: pick_up_load • GRA5: synchronize • GRA6: connect_tie_line • GRA7: crank_unit • GRA8: energize_busbar

21. Achieving GRMs by GRAs

22. GRM1: Form BS-NBS Building Block • Objectives • crank all generators (from a BS unit to NBS units) • pick up all critical loads as quickly as possible. • Dispatchable loads are picked up when necessary to balance restored generation and maintain voltage. • GRAs: • Start the BS unit (GRA1) • Find transmission path from the BS unit to a NBS unit (GRA2) • Build a transmission path (GRA3) • Pick up load (GRA4) • Crank a NBS unit (GRA7)

23. GRM1: Form BS-NBS Building Block (cont’d) • At stage S, solve the shortest time fS to restore all generators and critical loads by Dynamic Programing: S: the set of restored generators xi: the state (restored generators and loads) at stage S • Constraints: • Power flow equations are solved • No violation on generation limits, transmission limits or voltage limits

24. Algorithms • Split the complex multistage optimization problem into two sub-problems Alg-1: Finding a neighboring region (within a given depth) around an energized block Alg-2: Finding a transmission path to crank a generator Alg-3: Solve OPF to find an operating point without violation to minimize the duration time Alg-4: Finding dispatchable loads by OPF

25. Modeling of Generating Units

26. Demonstration Using a WECC Model • 200-bus system • 31 generating units • 3 critical loads • 5 black start units • Time for energizing a line is 5minutes

27. Generator and Load Characteristics Generators Critical Loads Dispatchable Loads

28. Develop Restoration Strategies by GRMs • The system is restored as 5 islands first and then synchronized • GRMs: • GRM 1: Form BS_NBS_BuildingBlocks • GRM2: Establish Transmission Grid • GRM3: Form Electrical Island • GRM4: Synchronize Electrical Islands • GRM5: Serve Load in Area

30. Restoration Strategy for GRM1 in Island 1

31. Island 1

32. Island 2

33. Island 3

34. Island 4

35. Island 5

37. Voltage Profiles GRM1 for Island 1 GRM3 for synchronizing Islands 1&2

38. Total Generation Output During Restoration

39. Comparison of Different Ramping Rates of the BS Unit (Island 5)

40. “Detour” Function • If line 137-143 in Island 5 is unavailable Original Detour

41. EPRI’s System Restoration Navigator [8] • Establish GRM-based algorithms to develop or evaluate a restoration strategy • Interactive GUI to provide automatic or interactive strategy development • Milestones and priorities assigned by users • Restoration report on on-line diagram or in text format • Accept PSS/E raw data

42. Integration with OTS • Operator Training Simulator (OTS) • Simulation engine: • power-flow based pseudo-dynamic • transient simulation • Products: • EPRI OTS • PowerSimulator by POWERDATA and IncSys (Source: powersimulator.net)

43. Integration with OTS (cont’d) On-line diagram GIS visualization System Restoration Navigator System messages

44. References • M. M. Adibi and L. H. Fink, "Overcoming restoration challenges associated with major power system disturbances - Restoration from cascading failures," Power and Energy Magazine, IEEE, vol. 4, pp. 68-77, 2006. • M. M. Adibi and N. Martins, "Power system restoration dynamics issues," IEEE Power and Energy Society General Meeting 2008. • L. H. Fink, K.-L. Liou, and C.-C. Liu, "From generic restoration actions to specific restoration strategies," IEEE Trans. Power Syst., vol. 10, pp. 745-752, 1995 • J. W. Feltes and C. Grande-Moran, "Black start studies for system restoration," presented at Power and Energy Society General Meeting 2008 • F. Levesque, S. T. Phan, A. Dumas, and M. Boisvert, "Restoration plan — The Hydro-Québec experience," presented at Power and Energy Society General Meeting - Conversion and Delivery of Electrical Energy in the 21st Century, 2008 IEEE, 2008. • Y. Hou, C. C. Liu, K. Sun, et al, “Computation of Milestones for Decision Support during System Restoration”, IEEE Trans. Power Systems, vol. 26 , No. 3, pp. 1399 1409, Aug. 2011 • Y. Hou; C.-C. Liu; P. Zhang; K. Sun, “Constructing power system restoration strategies”, IEEE International Conference ELECO 2009. Page(s): I-8 - I-13, 2009 • System Restoration Navigator (SRN) Version 2.0, EPRI Product ID: 1021715, 2011