3Western Electricity Coordinating Council (WECC) system - Aug 10th, 1996 Blackout Initiating eventsSystem becomes unstableBlackout15:48 p.m. Keeler-Allston 500-kV line contacts a tree due to inadequate right-of-way maintenance. Additionally the Pearl-Keeler line is forced out of service due to the Keeler 500/230-Kv transformer being OOS.Mild .224 Hz oscillations were seen throughout the system and began to appear on of the PDCI.The WECC broke into 4 asynchronous islands with heavy loss of load.7.5 million people lost power.Shunt capacitor banks were switched in to raise the voltage but the oscillations were not being damped.With the loss of these 2 lines, 5 lines are now out of service, removing hundreds of MVAR.15:48:51 p.m. Oscillations on the POI reached 1000MW and 60-kV peak-to-peak.WAMTNDLines throughout the system begin to experience overloads as well as low voltage conditions. Additional lines trip due to sagging.ORPDCI Remedial Action Schemes (RAS) began to actuate. Shunt and series capacitors were inserted.IDSDWYNVNECAUTCO15:47:40-15:48:57 p.m. Generators at the McNary power house supplying 494 MVAR trip. The system begins to experience “mild oscillations”.AZNM
4Eastern Interconnection – August 14th, 2003 Blackout Initiating eventsSystem becomes unstableBlackout1:07 p.m. FE turns off their state estimator for troubleshooting.4:05:57 p.m. The loss of 138-kV lines overloads the Sammis-Star line.265 power plants tripped off line and 50 million people are without power.1:31 p.m. Eastlake 5 generation unit trips and shuts down.2:02 p.m. Stuart-Atlanta 345-kV line tips off due to contact with a tree.4:08:59 p.m. Galion-Ohio and Central-Muskinghun 345-kV lines trip on Zone 3 causing major power swings through New York and Ontario and into Michigan.2:14 p.m. FE’s control room lost alarm functions followed by a number of the EMS consoles.Low voltage/ high load conditions and system disturbances propagate through the system tripping transmission lines and generators.NDMEMNVTMINHSDWINYMA2:54 p.m. The primary and secondary alarms servers failed.CTRINEIAPAILOHNJINMDDEKSWVMOVA3:05:41-3:57:35 p.m kV lines trip due to contact with trees. This overloads the underlying 138-kV system and depressed voltages.4:13 p.m. most of the North East and parts of Canada blacked out. There are only a few islands which remain operating.KYNCOKTNARSCMSALGALAFL3:39:17-4:08:59 p.m kV lines trip due to overloading.
6Blackout Propagation (without defense systems) Complete System CollapseTriggering Event
7Occurrences and Extent of Blackouts in North America 1000Number of Blackouts10010100102104106108Number of customers affected
8Strategic Power Infrastructure Defense (SPID) Design self-healing strategies and adaptive reconfiguration schemesTo achieve autonomous, adaptive, and preventive remedial control actionsTo provide adaptive/intelligent protectionTo minimize the impact of power system vulnerability
9SPID System Fast and on-line power & comm. system assessment Hidden failure monitoringAdaptive: load shedding, generation rejection, islanding, protection
10Multi-Agent System for SPID Knowledge/Decision exchangeProtection AgentsGenerationAgentsFaultIsolation AgentsFrequencyStabilityModelUpdateCommandInterpretationPlanningAgentRestorationHiddenFailureMonitoring AgentsReconfigurationVulnerabilityAssessmentPower SystemControlsInhibition SignalPlans/DecisionsEventIdentificationTriggering EventsEvent/AlarmFilteringEvents/AlarmsInputsUpdate ModelCheckConsistencyComm.DELIBERATIVE LAYERCOORDINATION LAYERREACTIVE LAYER
11Multi-Agent System for SPID Subsumption Architecture (Brooks) for CoordinationAgents in the higher layer can block the control actions of agents in lower layersLoad SheddingAgentGlobal View/Goal(s)RUnder Frequency RelayLocal View/Goal(s)Tripping SignalInhibition Signal
12combined into sequences Cascaded EventsSome Basic PatternsLine tripping due to overloadingGenerator tripping due to over-excitationLine tripping due to loss of synchronismGenerator tripping due to abnormal voltageand frequency system conditionUnder-frequency/voltage load sheddingIdentifying the basic patterns of cascaded events and explore how these patterns can becombined into sequences
13Cascaded Zone 3 Operations Zone 3 Relay Operations Contributed to Causes of Blackouts.Heavy Loaded LineLow VoltageHigh CurrentZone 3RelayOperation(s)Lower Impedance Seen by RelayLoss of Transmission LinesOther Heavy Loaded LinesCatastrophic Outage
14Prediction of Zone 3 Relay Tripping Based on On-Line Steady State Security Assessment Contingency Evaluation Performed On Line Every Several MinutesContingency Evaluation…CaseRelayStatusContingency Description1N/ASecure3 phase fault at bus 12Zone3Insecure3 phase fault at bus 2..…N3 phase fault at bus NCaseRelayStatusContingency Description1N/ASecure3 phase fault at bus 12Zone3Insecure3 phase fault at bus 2..…N3 phase fault at bus NCaseRelayStatusContingency Description1N/ASecure3 phase fault at bus 12Zone3Insecure3 phase fault at bus 2..…N3 phase fault at bus NCaseRelayStatusContingency Description1N/ASecure3 phase fault at bus 12Zone 3Insecure3 phase fault at bus 2..…N3 phase fault at bus NPost-ContingencyPower FlowFISPost-ContingencyApparent ImpedanceFuzzy Inference System (FIS) DevelopedUsing Off-Line Time-Domain SimulationsCorrected Post-ContingencyApparent Impedance
15Impedances Obtained by Power Flow and Time Domain Simulation Post-Contingency Impedance Obtained by Power Flow Does Not Coincide with Impedance Obtained by Time-Domain Dynamic Simulations
16Automatic Development of Fuzzy Rule Base Wang & Mendel’s algorithm is a “learning” algorithm:1) One can combine measured information and human linguistic information into a common framework2) Simple and straightforward one-pass build up procedure3) There is flexibility in choosing the membership functionPre-determine number ofmembership functions NGive input and outputdata setsFIScreatedautomatically(Inp1, Inp2, Out) = (10, 1, -2)(Inp1, Inp2, Out) = (8, 3, -1)(Inp1, Inp2, Out) = (5, 6, -4)(Inp1, Inp2, Out) = (2, 8, -5)…In this example, N is 7
18Impedance on R-X (Case A) Z obtained by power flow solution is outside Zone 3 circle.Case A
19Load SheddingStudies have shown that the August 10th 1996 blackout could have been prevented if just 0.4% of the total system load had been dropped for 30 minutes.According to the Final NERC Report on August 14, 2003, Blackout, at least 1,500 to 2,500 MW of load in Cleveland-Akron area had to be shed, prior to the loss of the 345-kV Sammis-Star line, to prevent the blackout.
20Automatic Load Shedding Under VoltageUnder FrequencyRate of Frequency DecreaseRemedial Action Scheme
21Adaptive Self-healing: Load Shedding Agent A control action might failUnsupervised adaptive learning method should be deployedReinforcement LearningAutonomous learning method based on interactions with the agent’s environmentIf an action is followed by a satisfactory state, the tendency to produce the action is strengthened
22Load Shedding OptionsfrequencyTime (multiples of 0.02 sec)
23Adaptive Self-healing: Load Shedding Agent 179 bus system resembling WSCC systemETMSP simulationRemote load shedding scheme based on frequency decline + frequency decline rateTemporal Difference (TD) method is used for adaptation: Need to find the learning factor for convergence
24Adaptive Load Shedding Agent State 1State 2State 3Freq := 59.5Dec.rate > threshold valueFreq := 58.8Freq := 58.6179 bus system
25Adaptive Load Shedding Agent Expected normalized system frequency that makes the system stableNormalized frequency“The load shedding agent is able to find the proper control action in an adaptive manner based on responses from the power system”Number of trials
26Flexible Grid Configuration to Enhance Robustness Flexible Grid Configuration can play a significant role in defending against catastrophic events.Power infrastructure must be more intelligent and flexible.To allow coordinated operation and control measures to absorb the shock and minimize the potential damages caused by radical events .
27Cascading EventsA Cascading Event Refers to a Series of Tripping Initiated by One or Several Component Failures in a Power SystemHere the initial component(s) failure is designated as “shock” to the power infrastructure
28Simulated Cascading Events (179 Bus System) Compute Power Flows after TrippingSix lines are found on limit violationTrip these linesIdentify New Network Configuration and Solve Power Flows AgainFifteen lines are found with limit violationsContinue This Simulation ProcedureFinally system collapses: most transmission lines are tripped and most loads are lost
292-Area Partitioning Algorithm (from VLSI) Spectral 2-way Ratio-Cut PartitioningTheorem Given an edge-weighted graph G = (V, E), the second smallest eigenvalue λ2 of the graph’s Laplacian matrix Q yields a lower bound on the cost c of the optimal ratio cut partition, with c = e(U,W)/(|U|·|W|) ≥ (λ2/n)Cut-Size: e(U,W) ≥ (λ2/n) (|U|·|W|)
32Flexible Grid Configuration to Absorb the Shock Solve Power Flows of Area OneAll MW loads are supplied, no line flow constraints violationsSolve Power Flows of Area TwoSeven lines on limit violation:(Bus158-Bus164), (163-8), (64-163), double lines (16-19), and double lines ( )
33Flexible Grid Configuration to Absorb the Shock Use “Power Redispatching & Load Shedding” in Area TwoTotally, = MW load are shedBus #Original Load (MW)Load Shed (MW)Load Supplied (MW)82391885116793.464.47291541066601006
35Flexible Grid Configuration to Absorb the Shock Shed Load vs. System Total LoadK=1K=2K=3
36Flexible Grid Configuration to Avoid Cascaded Failures Step 1 : Compute power flows after initial tripping event(s).Step 2 : Convert power network to an edge-weighted graph G, weight of each edge is absolute value of real power flow.Step 3: Multilevel graph partitioning with minimum edge-cut.Network is separated into k areas to minimize generation / load imbalance.Graph COARSENED to a smaller number of vertices,Bisection PARTITIONING of much smaller graph,UNCOARSENING back toward original graph.Coarsest graph small, Coarsening can be parallelized, Partitioning efficiency high.
37Emergency Control with Multilevel Graph Partitioning Partitioning a 22,000 bus and 32,749 branch system into 2, 3, 4 islands with 0.07s, 0.081s and 0.09s on 2 GHz Pentium CPU and 1GB RAM.Fast computational speed makes it possible to determine partitioning strategy and identify new network configuration in on-line environment.An adaptive relaying architecture for controlled islandingCCUs acquires system data, generates system separation strategy.SCUs receive system separation commands from CCU and send breaker opening commands to specific auxiliary relays.
38Flexible Grid Configuration to Avoid Cascaded Failures Controlled islanding on a 200-bus system:199 buses, 31 generators, 248 branches.Sequence of cascading events:At t=0 s, three transmission lines out-of-service.At t=60 s, line tripped due to line fault.At t=120 s, line tripped due to line fault. Generator G70 at bus 70 overloaded.At t=240 s, generator G70 tripped by over-excitation protection. At t= 260 second, system collapses.Cut SetLoad-Generation (MW)Bus , , , , , (1,2)North: Gen=37862 , Load=37104South: Gen=24517, Load=23794
39Flexible Grid Configuration to Avoid Cascaded Failures Load bus voltages without/with islanding strategySystem islanding initiated at 241s.Islanding strategy results in balanced generation / load in both islands.All loads in the system are served. Load shedding scheme not applied.Islanding strategy successfully prevents the collapse of the system.
40Conclusion Cascading failures remain a grand challenge New communication, information and computer technologies enable wide area protection and controlConnectivity also brings cyber vulnerability“Smart” grid?“Self-healing” grid?
41Further InformationJ. Li, C. C. Liu, “Power System Reconfiguration Based on Multilevel Graph Partitioning,” IEEE PES Power Tech, Bucharest, Romania, 2009.K. Yamashita, J. Li, C. C. Liu, P. Zhang, and M. Hafmann, “Learning to Recognize Vulnerable Patterns Due to Undesirable Zone-3 Relay Operations,” IEEJ Trans. Electrical and Electronic Engineering, May 2009, ppJ. Li, K. Yamashita, C. C. Liu, P. Zhang, M. Hoffmann, “Identification of Cascaded Generator Over-Excitation Tripping Events,” PSCC, Glasgow, U.K., 2008.H. Li, G. Rosenwald, J. Jung, and C. C. Liu, “Strategic Power Infrastructure Defense,” Proceedings of the IEEE, May 2005, ppJ. Jung, C. C. Liu, S. Tanimoto, and V. Vittal, “Adaptation in Load Shedding under Vulnerable Operating Conditions,” IEEE Trans. Power Systems, Nov. 2002, ppH. You, V. Vittal, J. Jung, C. C. Liu, M. Amin, and R. Adapa, “An Intelligent Adaptive Load Shedding Scheme,” Proc PSCC, Seville, Spain, June 2002.C. C. Liu, J. Jung, G. Heydt, V. Vittal, and A. Phadke, “Strategic Power Infrastructure Defense (SPID) System: A Conceptual Design,” IEEE Control Systems Magazine, Aug. 2000, pp