1 GMD Impacts on Power System Voltage Stability Komal S. Shetye Research Engineer University of Illinois at Urbana-Champaign shetye1@illinois.edu
2 Overview of GMDs and GICs GMD: Geomagnetic Disturbances Cause variations in Earth s magnetic field inducing electric fields Result in GICs in the power grid, which are quasi-dc Image Source: http://www.davidreneke.com/wp-content/uploads/2012/07/chart1.jpg
3 Overview of GMDs and GICs GMD: Geomagnetic Disturbances Cause variations in Earth s magnetic field inducing electric fields Result in GICs in the power grid, which are quasi-dc GIC: Geomagnetically Induced Currents Can cause half-cycle saturation in transformers Harmonics can cause protection-device mis-operation Transformer heating and potential damage Increased reactive power absorption in transformers Compromise system reliability Equipment damage Voltage stability issues caused by increased reactive power absorption
4 Efforts to Address GMD Impacts NERC is developing planning standards for GMDs Includes required GMD vulnerability assessments Benchmark GMD event to perform assessments Regional peak geoelectric field amplitude E peak = 8 * α * β V/km Replaced with E max later in this talk 1 in a 100 year event Details can be found at http://www.nerc.com/pa/stand/project201303geomagneticdisturbance Mitigation/Benchmark_GMD_Event_June12_clean.pdf
5 Geomagnetic Latitude Scalar (α) Image Source: http://www.ngdc.noaa.gov/stp/cdrom/ionocd.html
6 Earth Resistivity Scalar (β) Image Source: http://www.naturalhistorymag.com/sites/default/files/imagecache/large/media/200 9/05/0309partner2_jpg_18912.jpg
7 Combined Regional Scalar (α*β) Image Source: http://www.ngdc.noaa.gov/stp/cdrom/ionocd.html Image Source: http://www.naturalhistorymag.com/sites/default/files/imagecache/large/media/200 9/05/0309partner2_jpg_18912.jpg
1 256 511 766 1021 1276 1531 1786 2041 2296 2551 2806 3061 3316 3571 3826 4081 4336 4591 4846 5101 5356 5611 5866 6121 6376 6631 6886 7141 7396 7651 7906 8161 8416 8671 8926 9181 9436 9691 9946 10201 10456 10711 GIC (A) GIC (A) 8 Quantifying Scaling Effect on GICs 550 500 450 400 350 300 250 200 150 100 50 0 Comparison of Transformer Effective GICs using an Eastward, 8 V/km uniform* vs scaled electric field on a large-system case 350 300 250 200 150 100 50 0-50 -100-150 -200 Transformer Number Scaled Uniform * A uniform electric field across the whole EI is not a realistic assumption (scenario is purely illustrative)
9 Key Topics of Discussion GMD analyses of a large scale system Focus on steady-state voltage stability What happens if E max exceeds 8 V/km? Comprising of two parametric studies 1. Effects of including/excluding neighboring regions At which value of E max does the power flow lose convergence, due to increased reactive power losses? 2. Uncertainty of substation grounding resistance values Scaling resistance values by a factor γ
10 GIC Model GIC calculation: V = G -1 I G : Conductance matrix with line, bus and substation data I : Norton equivalent injections of GMD-induced dc voltages V : Substation neutral and bus dc voltages GICs in the system calculated from V Transformer reactive power losses: Q loss = K *V pu *I GIC V pu : Terminal voltage (p.u.) I GIC : Effective per-phase GIC (p.u.) K : Loss factor - depends on core-type, number of phases Values assumed*~ based on highest nominal kv level * X. Dong, Y. Liu, J.G. Kappenman, Comparative Analysis of Exciting Current Harmonics and Reactive Power Consumption from GIC Saturated Transformers, Proc. IEEE 2001 Winter Meeting, Columbus, OH, Jan. 2001, pp. 318-322. ~ Study of the Impact of Geomagnetically Induced Currents on the North American Eastern and Western Interconnects. EPRI, Palo Alto, CA: 2013. 3002000818.
11 Large System Example 2010 Series, 2012 Summer Case from MMWG/ERAG of the North American Eastern Interconnect (EI) system Bus and substation coordinates added GIC model parameters estimated/assumed Transformer K values Transformer winding resistances from series resistances Substation grounding resistances (SubR) based on number of lines and highest nominal kv (0.1 2.0 Ω) Estimation method heuristic, not accurate Actual data is generally not easily/readily available Prior work $ has shown that accurate SubR values are important! $ Uyen Bui; Overbye, T.J.; Shetye, K.; Hao Zhu; Weber, J., "Geomagnetically induced current sensitivity to assumed substation grounding resistance," North American Power Symposium (NAPS), 2013, vol., no., pp.1,6, 22-24 Sept. 2013
12 Large System Study Next slide shows a video of EI system with An Eastward electric field applied to whole EI case E max increased in steps of 0.5 V/km (Left-half of screen) Regional scaling factors modeled Voltages at each step with Q loss -included power flow (Right-half) Q loss considered for transformers/areas in US only Video stops at point of power flow non-convergence Caused by increased reactive power demand Leads to voltage collapse in part of the system For an actual system study, actual data is key! Defaults and estimates used here for illustration only
13 Large system study video Electric Field in V/km Voltages in p.u.
14 Main Results and Further Analysis Non-convergence at E max = 14.5 V/km (E max, c ) Collapse occurred in Area A on the East Coast Some other Areas also have low voltage profiles e.g. Northwest portion of EI, and a region to the North of Area A Next, studies focusing on Area A What portion of the system apart from Area A needs to be modeled for voltage stability studies? Considered 1) Only Area A, 2) Tie-line connected Areas, and 3) Whole EI How to account for uncertainties in SubR values? Scaled SubR values by γ = 1/5, 1/4, 1/3, 1/2, 2, 3, 4, and 5 Regional scaling factors used for these studies
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E_max (V/km) 16 E max and γ Parametric Studies Emax,c for different system sizes and grounding resistances (step-size 1 V/km) 32 28 24 20 16 12 8 0.20 0.25 0.33 0.50 1.00 2.00 3.00 4.00 5.00 Substation Resistance Scaling Factor γ* *γ applied to all substations of EI Area A plus first neighbors Series4
E_max (V/km) 17 E max and γ Parametric Studies Emax,c for different system sizes and grounding resistances (step-size 1 V/km) 32 28 24 20 16 12 8 0.20 0.25 0.33 0.50 1.00 2.00 3.00 4.00 5.00 Substation Resistance Scaling Factor γ* *γ applied to all substations of EI Area A plus first neighbors Series4
E_max (V/km) 18 E max and γ Parametric Studies Emax,c for different system sizes and grounding resistances (step-size 1 V/km) 32 28 24 20 16 12 8 0.20 0.25 0.33 0.50 1.00 2.00 3.00 4.00 5.00 Substation Resistance Scaling Factor Area A only Area A plus first neighbors Whole EI Series4 *γ applied to all substations of EI γ*
19 SubR Uncertainty in One Footprint Previous results showed effects of varying SubR values throughout the EI, to study their uncertainty What if only a certain region had uncertain values? What would the impacts be on system voltages and E max, c? Next slide shows snapshots taken at E max, c when only the SubR values in Area A were scaled by γ
Emax (Volts/km) 20 Area A SubR Variations 17 16 15 14 13 12 0.20 0.25 0.33 0.50 1.00 2.00 3.00 4.00 5.00 Substation Resistance Scaling Factor γ Series4 SubR scaled for Area A only
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22 Key Takeaways Impacts of size of study system: Study with only Area A losses overestimates the level of E max, c Including losses of first neighbors of Area A has an effect similar to considering the whole EI Considering individual Areas by themselves may not be sufficient as a worst case scenario, for accurate voltage stability studies Extent of neighboring region that needs to be modeled will be system dependent Next Steps: To formalize how much of the system should be modeled for voltage stability studies
23 Key Takeaways Substation grounding resistance uncertainty: Varying SubR values within a factor of 5 E max, c varies ±5V/km for the Area A study Uncertainty in one Area can influence E max, c of the larger system In simulations, under (over) estimating SubR values in a subsystem can pull (push) more GICs from (to) neighboring regions, than what is expected in the real world Uncertainty in SubR data Range of values for E max, c Desired certainty of E max, c Tolerable uncertainty of SubR data
24 Questions? The GMD research group at University of Illinois at Urbana-Champaign welcomes discussions on performing individual system studies Email: shetye1@illinois.edu