Integration of Geomagnetic Disturbance (GMD) Modeling into the Power Flow Thomas J. OverbyeFox Family Professor of Electrical and Computer EngineeringUniversity of Illinois at Urbana-Champaignoverbyeillinois.eduMarch 13, 20121.OverviewGeomagnetic disturbances (GMDs) have the potential to severely disrupt operations of the electric grid, yet power engineers currently have few tools to help them assess the impact of GMDs on their systemsGoal of the this work is to perform the research necessary to help move GMD assessment into the realm of power system planning and operations engineersGuiding motto: “All models are wrong but some are useful,” George Box, 1979GIC impact is certainly still an area of research, but power engineers need tools to consider its impact on their systems2NERC Interim GMD ReportOn February 29, 2012 NERC issued an Interim GMD Report available at In section I.10 of the Executive Summary there are four high level recommended actionsImproved tools for industry planners to develop GMD mitigation strategiesImproved tools for system operators to manage GMD impactsDevelop education and information exchanges between researchers and industryReview the need for enhanced NERC Reliability Standards I hope this project helps with at least the first 3 of these3My Background in GMDMy interest comes from PSERCs DOE funded “The Future Grid to Enable Sustainable Energy Systems” project In June 2011 I attended the JASON GMD briefingRealized GMD calculations could added to the power flowImplemented these calculations in PowerWorld Simulator while working there full-time in summer 2011Presented results in November 2011 at NERCs Geo-Magnetic Disturbance Task Force; based on feedback changed GMD modeling approachIn 2012 started EPRI large-scale system study and also working with several other utilities for studies4Geomagnetic Disturbances (GMDs)Solar events can cause changes in the earths magnetic field (i.e., dB/dt). These changes in turn produces an electric field at the surfaceChanges in the magnetic flux are usually expressed in nT/minute; from a 60 Hz perspective they produce an almost dc electric field1989 North America storm produced a change of 500 nT/minute, while a stronger storm, such as the one in1921, could produce more than 5000 nT/minute variationStorm “footprint” can be continental in scale, for example covering much of the U.S.For reference, Earths magnetic field is normally between 25,000 and 65,000 nT, with higher values near the polesImage source: J. Kappenman, “A Perfect Storm of Planetary Proportions,” IEEE Spectrum, Feb 2012, page 295March 8-9, 2012 “Event”Forecasted solar storm last week was not nearly as large as some thought (graph is dB/dt at Ottawa, 45.4N)Image source: Less than100 nT/minute,compared to 500 nT/minute forthe 1989 event and5000 in 19216Electric Fields and Geomagnetically Induced Currents (GICs)As described by Faradays law, changes in the magnetic flux intensity produce a (non-uniform) electric field on the surface; values are impacted by ground conductivityElectric fields are vectors with a magnitude and direction; values are usually expressed in units of volts/mile (or volts/km);A 2400 nT/minute storm could produce 5 to 10 volts/mile. The electric fields cause geomagnetically induced currents (GICs) to flow in electrical conductors such has the high voltage transmission gridFrom a modeling perspective the induced voltages that drive the GICs can be modeled as dc voltages in the transmission lines. The magnitude of the dc voltage is determined by integrating the electric field variation over the line length7Geomagnetically Induced Currents (GICs)8Power System Impacts of GICsThe dc GICs are superimposed upon the ac currents. In transformers this can push the flux into saturation for part of the ac cycleThis can cause largeharmonics; in the positive sequence(e.g., power flow and transient stability) theseharmonics can be represented by increased reactive power losses on the transformer. 9Mapping Transformer GICs to Transformer Reactive Power LossesTransformer specific, and can vary widely depending upon the core typeSingle phase, shell, 3-legged, 5-leggedIdeally this information would need to be supplied by the transformer ownerCurrently support default values or a user specified linear mappingFor large system studies default data is used when nothing else is available. Scaling value changes with core typeStill debate in the industry with respect to the magnitude of damage GICs would cause in transformers (from slightly age to permanently destroy)10The Impact of a Large GMD From an Operations PerspectiveThere would be a day or so warning but without specifics on the actual magnitudeIt could strike quickly (they move at millions of miles per hour) with rises times of less than a minute, rapidly covering a good chunk of the continentReactive power loadings on hundreds of transformers could sky rocket, causing heating issues and potential large-scale voltage collapsesPower system software like state estimation could failControl room personnel would be overwhelmedThe storm could last for days with varying intensityWaiting until it occurs to prepare might not be a good idea Image source: J. Kappenman, “A Perfect Storm of Planetary Proportions,” IEEE Spectrum, Feb 2012, page 2911GMD Enhanced Power Analysis SoftwareBy integrating GIC calculations directly within power analysis software (like power flow) power engineers can readily see the impact of GICs on their systems, and consider mitigation optionsGIC calculations use many of the existing model parameters such as line resistance. But some non-standard values are also needed; power engineers would be in the best position to provide these values, but all can be estimated when actual values are not availableSubstation grounding resistance, transformer grounding configuration, transformer coil resistance, whether auto-transformer, whether three-winding transformer, generator step-up transformer parameters12GIC G-MatrixWith knowledge of the pertinent transmission system parameters and the GMD-induced line voltages, the dc bus voltages and GIC flows can be calculated by solving a linear equation I = G VThe G matrix is similar to the Ybus except 1) it is augmented to include substation neutrals, and 2) it is just conductancesThe current vector contains the Norton injections associated with the GMD-induced line voltagesFactoring the sparse G matrix and doing the forward/backward substitution takes about 1 second for the 62,600 bus Eastern Interconnect Model 13Four Bus ExampleThe line and transformer resistance and current values are per phase so the total. Substation grounding values are total resistance. Brown arrows show GIC flow. 14Determining GMD Storm ScenariosThe starting point for the GIC analysis in the power flow is an assumed storm scenario; this is used to determine the transmission line dc voltagesMatching an actual storm can be complicated, and requires detailed knowledge of the associated geology Feb 2012 NERC report recommended for planning purposes a similar approach could be usedUniform electric field: All locations experience the same electric field; induced voltages in lines depend on assumed directionMaximum value in 1989 was 1.7 V/km (2.7 V/mile) We also consider a more detailed non-uniform modelNon-uniform electric field: Magnitude of electric field varies according a some function; induced voltages in lines depend on magnitude and assumed direction15East-West (90 Degrees) Non-uniform GMD Scenario, Centered at 42N, 88.5W; 5 volt/mile16NE-SW (45 Degrees) Non-uniform GMD Scenario, Centered at 42N, 88.5W; 5 volt/mile17Integrated Geographic InformationThe potentially time-varying GMD induced dc voltages are determined by knowing the latitude and longitude of the transmission linesJust knowing the geo-coordinates of the terminal buses should be sufficientHence buses need to be mapped to substations, and substations to their geo-coordinatesFor this project substation/geographic data was supplied by PowerWorldBuses mapped to substations; note bus numbers do vary even within cases in a particular “series” like the 2010 MMWG cases Latitude and longitude provided for substationsData can easily be imported/exported18Impact of Electric Field Angle on GICsAs mentioned earlier, the resultant GICs depend upon the electric field angle; different angles have different system impactsThe results shown to the right give the transformer Mvar GIC-induced losses by area in the Eastern Interconnect (using pseudo-IDs for confidentiality) assuming a uniform 2 V/mile field. 19Getting Access to the ResultsGIC studies involve the traditional power system results (voltages, flows, etc.), along with a new set of variables such as Estimated parameters, such as substation grounding and transformer coil resistanceSubstation neutral dc voltagesBus dc voltagesGIC flows in branchesGIC induced reactive power losses in transformersProviding users with easy access to the results/data is key, as is good wide-area visualization20Per Unit Voltage Drops Caused by East-West 5V/mile GMD Storm ScenarioThese results are just illustrative. Actualmodel parameters willhelp give better results. 21GIC-enhanced Power Flow List Display22Power Flow Convergence IssuesIntegrated GIC modeling can certainly impact power flowconvergence since the GIC induced reactive power losses simultaneously add lots of reactive power.Several techniques can help prevent divergenceJust calculating the GICs without solving the power flowNot calculating GMD induced voltages for equivalent linesGradually increasing the assumed electric fields to avoid simultaneously adding too much reactive power at one timeOnly calculating the GIC transformer reactive power losses for specified areas; reactive power doesnt tend to travel farFreezing the transformer taps and switched shunts in certain problematic areas Solving in transient stability for a gradually increasing GMD23Integrating GIC Calculations into Power System PlanningA large GMD could cause substantially different power system flows and voltagesStudies allow for testing various mitigation strategiesOperational (short-term) changes include redispatching generation to avoid long distance power transfers and reducing transformer loading values, and strategically opening devices to limit GIC flowsLonger-term mitigation actions include the installation of GIC blocking devices on the transformer neutrals (such as capacitors) and/or increased series capacitor compensation on long transmission linesDetermining relay settings when to trip the transformer24Research Example: Should the Lower Voltage Network be IncludedNERC report stated with respect to GIC calculations, “Transmission lines below 230 kV are typically not modeled”Our research indicates ignoring these low voltage lines might substantially under report the total GIC flows and the associated increase in reactive power loadingFor Eastern Interconnect case with a uniform 2.0 volt/mile east-west field the total GIC related reactive power losses are 64,525 Mvar considering all lines, 44,341 Mvar if lines below 150 kV are neglected and just 40,984 if lines below 200 kV are neglected.No reason not to include these lines since they are in the standard power flow modelsComputationally neglecting lines is faster, but the calculations are quite computationally efficient25Research Example: Determining Mitigation StrategiesGIC flows can be reduced both through operational strategies such as strategically opening lines, and through longer term approaches such as installing blocking devicesAlgorithms are needed to provide power engineers with techniques that go beyond trial-and-errorSuch approaches require a coupling between the GIC calculations and the power flow solutionsFor example, determining lines that would 1) substantially reduce the GIC flows and 2) are not crucial from an operational perspective 26Small System Operational Mitigation ExampleThe system on the right shows how the GIC impacts can be reduced by doing generation dispatch to allow opening a 345 kV lineResearch is needed to determine optimal solutions 27ValidationValidating the GIC models and results is an important area of researchReal-time and historical magnetic field measurements are available at Readings are available for about 60 locations worldwideWe are working with EPRI using their Sunburst dataWe would like to analyze SCADA data to relate transformer neutral flows to changes in transformer lossesValidation is challenging because there are not a lot of measurements, and the GIC flows depend upon system topology, which is often difficult to reconstruct after the fact28Thank You!29