OPF Studies: Strategies and Tips - PowerWorld

OPF Strategies and Tips2006 PowerWorld Client ConferenceJune 14-15Chattanooga, Tennessee2001 South First StreetChampaign, Illinois 61820+1 (217) 384.6330Scott R. Dahman, P.E.scott@powerworld.comhttp://www.powerworld.comPotential OPF Challenges• Unenforceable Transmission Constraints• Unenforceable Area Constraints due toinsufficient generation reserves• Too much power transfer in LP causespower flow solution to divergeJune 14-15, 20062006 PowerWorld Client Conference: OPF Strategies and Tips 21

Analysis of UnenforceableConstraints• Example: PJM Case• Initial solution has 16 unenforceable constraints• Of these many seem to be caused by radial– Change Limit Monitoring Settings to “Ignore RadialLines and Buses”• Radial Bus is connected to the system by only onetransmission line• Radial Line is a line connected to a radial bus.– Choosing this eliminates the unenforceableconstraintsJune 14-15, 20062006 PowerWorld Client Conference: OPF Strategies and Tips 3Unenforceable Constraints• Check unenforceable lines for VERY largeMVar flows– Add Columns for Max MW and Max MVar onLP OPF, Lines and Transformers• LTC tap ratios• Phase-shifters in series with an overloadedbranchJune 14-15, 20062006 PowerWorld Client Conference: OPF Strategies and Tips 42

Reset the Tap Settings• Set all transformers on tap control to have a ratio of1.00• Set all phase-shifters to be controlled by the OPFsolution– Phase Shifters have three control options• None – leave at a fixed angle• Power Flow – Allow the power flow solution to dispatchaccording to the setpoints of the controller• OPF – Allow the OPF’s linear program to “dispatch” thetransformer for a more global optimization– Use caution with Phase Shifter MW limits when usingOPF control – large ranges may cause OPF to dispatch toan unstable pointJune 14-15, 20062006 PowerWorld Client Conference: OPF Strategies and Tips 5Unenforceable Constraints Left• This results in a reduced list of 17 unenforceableconstraintsJune 14-15, 20062006 PowerWorld Client Conference: OPF Strategies and Tips 63

AMVAAMVAAMVAAMVAAMVAAMVAAMVAAMVAAMVAAMVAAMVAAMVAAMVAAMVAExample: Internal MERCK59.29 $/MWh1.0131 pu4212NWALES82.2 MW25.2 Mvar82.2 MW14.1 Mvar23.2 MW 99%9.0 Mvar90.5 Mvar4217N WALES459.64 $/MWh1.0185 pu4216NWALES59.63 $/MWh1.0186 pu82.2 MW25.2 Mvar20.9 Mvar4214NWALES59.63 $/MWh1.0186 pu113%4215NWALES59.63 $/MWh1.0186 pu4217-4216 line has a largeimpedance of 0.15 compared tothe lines 4214-4217, 4214-4215,4216-4215 which haveimpedances of 0.0002415359.63 $/MWh L 107001.0184 pu59.64 $/MWh1.0181 pu3.9 MW1.6 Mvar59.63 $/MWh1.0184 pu4195MERCK4154L 16000June 14-15, 2006This means that 4216-4217 willNEVER have any flow on it.Thus the line 4214-4217 isessentially radial.41964544MERCKMERCK 359.64 $/MWh59.62 $/MWh1.0283 pu 1.0149 pu0.0 MW1.4 MW2006 PowerWorld Client Conference: 0.0 Mvar OPF -1.5 Strategies Mvar and Tips 9After these changes we removeall unenforceable Constraints• Still some very high cost constraints remain• BIRDBORO – Pine LNE = 772.8 $/MVAhrJune 14-15, 20062006 PowerWorld Client Conference: OPF Strategies and Tips 105

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$/MWh1.01 pu1589MUHLENBG36.13 $/MWh1.02 pu1603SIMON TP54.04 $/MWh1.02 puAM V A1602SIMON54.04 $/MWh1.02 puAM V AAM V A41.58 $/MWh1.02 pu1605ST PETRS1589MUHLENBG36.13 $/MWh1.02 puAM V A41.58 $/MWh1.02 pu54.04 $/MWh1.02 puAM V A1605ST PETRS43.11 $/MWh1.02 pu1558BERK 83550.00 $/MWh1.02 puAM V A43.11 $/MWh1.02 puAM V A1558BERK 83550.00 $/MWh1.02 puAM V A1597ROSEDALEAM V AAM V AAM V A1597ROSEDALEAM V AAM V A1595RIVRVIEWAM V A49.44 $/MWh1.02 pu1559BERKLEY50.00 $/MWh1.06 pu63.23 $/MWh1.01 puAM V A1595RIVRVIEWAM V A49.44 $/MWh1.02 pu1559BERKLEY50.00 $/MWh1.06 puAM V A63.23 $/MWh1.01 pu1591NTEMPLE1552ALTN CMTAM V A1591NTEMPLE1552ALTN CMTA AM V A M V A57.67 $/MWh1.01 puAAM V A M V A57.67 $/MWh1.01 puAM V A1159NTEMPLEAM V A1159NTEMPLEBirdboro – Pine LNE• Yellow Region forms a“load pocket” for twolarge loads– 85.3 MW– 193.7 MW• The 69 kVlines feedingthis regionhave high loadingsJune 14-15, 2006100%100%85.3 MW-25.6 Mvar94%193.7 MW-16.4 Mvar16.4 MW-3.3 Mvar12.6 MW5.0 Mvar268.5 MW-35.7 Mvar2006 PowerWorld Client Conference: OPF Strategies and Tips 1195%83%97%Contour of Prices aroundBirdboro – Pine Lne• Load Pocket• These pricescould bereasonable.16.4 MW-3.3 Mvar12.6 MW5.0 Mvar268.5 MW-35.7 Mvar85.3 MW-25.6 Mvar193.7 MW-16.4 MvarJune 14-15, 20062006 PowerWorld Client Conference: OPF Strategies and Tips 126

Unenforceable ConstraintsSummary• Look for radial systems and “load pockets”• Look for generators or phase-shifters which canrelieve problems– Give the OPF more controls to FIX the problems• Look for constraints which don’t make sense– Radial lines serving load– Radial transformers/lines leaving generators• Use your judgment to setup a reasonable case• Realize that some unenforceable constraints areinevitable at firstJune 14-15, 20062006 PowerWorld Client Conference: OPF Strategies and Tips 13Insufficient Reserves• In this example, Area 28 (JCP&L) does not haveenough AGCable generation• Message Log: “Insufficient controls to enforcearea … constraint”June 14-15, 20062006 PowerWorld Client Conference: OPF Strategies and Tips 147

Insufficient Reserves: Tips• Examine Generator records or Area field“Gen MW AGC Range Up”• To resolve– Commit more generation– Make more generation AGCable– Increase imports, or make Area part of a SuperArea– Decrease load, or make load dispatchableJune 14-15, 20062006 PowerWorld Client Conference: OPF Strategies and Tips 15Too Much Power Transfer• The linear program (LP) iterates with thenon-linear power flow to achieveconvergence of the entire solution• If an AC OPF is performed over a verylarge area, the LP may dispatch generatorsin a manner that exceeds voltage stabilitymargins• May occur more frequently in WECC casesJune 14-15, 20062006 PowerWorld Client Conference: OPF Strategies and Tips 168

Too Much Power Transfer• Example: A Northern and a Southern section ofthe eastern interconnection in one Super Area– North: includes eastern PJM, AEP, First Energy– South: includes TVA, Southern Company, Entergy• Assume generation much less expensive in thesouth, so OPF will try to increase the transferfrom south to north• OPF may exceed stability margin of power flowJune 14-15, 20062006 PowerWorld Client Conference: OPF Strategies and Tips 17Too Much Power Transfer• Message LogJune 14-15, 20062006 PowerWorld Client Conference: OPF Strategies and Tips 189

Too Much Power Transfer: Tips• Tighten MW Limits on generators withunrealistic limits (e.g. Max MW = 9999)• Place less of the system on OPF control• Use interface limits• Break a large area (or super area) into two ormore smaller areas; use OPF dispatchabletransactions between the smaller areas• Manually move generation in the direction of theLP transfer, resolve power flow, restart OPF• Use DC Power FlowJune 14-15, 20062006 PowerWorld Client Conference: OPF Strategies and Tips 19OPF Dispatchable Transactions• Example: Break MMWG super area intoNorthern and Southern super areas• Add a new transaction between a Northern area(e.g. AEP) and a Southern area (e.g. TVA)• Set MW limits on the new transaction• If OPF and power flow solve, try increasing thelimits of the transaction – stop when the powerflow will not convergeJune 14-15, 20062006 PowerWorld Client Conference: OPF Strategies and Tips 2010

OPF Dispatchable TransactionsSouth-North transaction limited to 500 MW beyond base casetransferOPF determinesoptimal transaction.If transaction isnon-binding at thesolution, then areasare acting as asingle super area.June 14-15, 20062006 PowerWorld Client Conference: OPF Strategies and Tips 21Manually Move Generation• Solve a “Single Outer Loop” of the OPF• Look at OPF controls following the failure• Move generation in direction of transfer (e.g.10% of the transfer)• Attempt to resolve the OPF– Often additional transmission constraints will becomebinding before the full transfer is made– OPF will know to move in a different directionJune 14-15, 20062006 PowerWorld Client Conference: OPF Strategies and Tips 2211

Manually Move Generation:OPF ControlsJune 14-15, 20062006 PowerWorld Client Conference: OPF Strategies and Tips 23Manually Move Generation:Spreadsheet• Use a spreadsheet to step the transfer• Set GenMW = Orig. Value + (Value – Orig. Value)* PercentMove1. Send OPFcontrols to thespreadsheet2. Paste GenRecords back intoSimulator, solvepower flow, andrestart LP OPFJune 14-15, 20062006 PowerWorld Client Conference: OPF Strategies and Tips 2412