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GMD Impacts on Power System Voltage Stability
Komal S. ShetyeResearch Engineer
University of Illinois at [email protected]
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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
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Image Source: http://www.davidreneke.com/wp-content/uploads/2012/07/chart1.jpg
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
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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
Epeak = 8 * α * β V/km
– “1 in a 100 year” event
– Details can be found at
• http://www.nerc.com/pa/Stand/Project201303GeomagneticDisturbanceMitigation/Benchmark_GMD_Event_June12_clean.pdf
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Replaced with “Emax” later in this talk
Geomagnetic Latitude Scalar (α)
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Image Source: http://www.ngdc.noaa.gov/stp/cdrom/ionocd.html
Earth Resistivity Scalar (β)
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Image Source:
http://www.naturalhistorymag.com/sites/default/files/imagecache/large/media/200
9/05/0309partner2_jpg_18912.jpg
Combined Regional Scalar (α*β)
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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
Quantifying Scaling Effect on GICs
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* A uniform electric field across the whole EI is not a realistic assumption
(scenario is purely illustrative)
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GIC
(A
)
GIC
(A
)
Transformer Number
Comparison of Transformer Effective GICs using an Eastward, 8 V/km uniform* vs scaled electric field on a large-system case
Scaled Uniform
Key Topics of Discussion
• GMD analyses of a large scale system– Focus on steady-state voltage stability
– What happens if Emax exceeds 8 V/km?
– Comprising of two parametric studies
1. Effects of including/excluding neighboring regions– At which value of Emax 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”
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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: Qloss = K *Vpu *IGIC
– Vpu : Terminal voltage (p.u.)
– IGIC : Effective per-phase GIC (p.u.)
– K : Loss factor - depends on core-type, number of phases
• Values assumed*~ based on highest nominal kV level
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* 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.
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!
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$ 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
Large System Study
• Next slide shows a video of EI system with– An Eastward electric field applied to whole EI case
• Emax increased in steps of 0.5 V/km (Left-half of screen)
• Regional scaling factors modeled
– Voltages at each step with Qloss -included power flow (Right-half)
• Qloss 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
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Large system study video
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Electric Field in V/km Voltages in p.u.
Main Results and Further Analysis
• Non-convergence at Emax = 14.5 V/km (Emax, 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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Emax and γ Parametric Studies
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0.20 0.25 0.33 0.50 1.00 2.00 3.00 4.00 5.008
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E_m
ax (
V/k
m)
Substation Resistance Scaling Factor
Emax,c for different system sizes and grounding resistances (step-size 1 V/km)
Area A plus first neighbors Series4
γ*
*γ applied to all substations of EI
Emax and γ Parametric Studies
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0.20 0.25 0.33 0.50 1.00 2.00 3.00 4.00 5.008
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E_m
ax (
V/k
m)
Substation Resistance Scaling Factor
Emax,c for different system sizes and grounding resistances (step-size 1 V/km)
Area A plus first neighbors Series4
γ*
*γ applied to all substations of EI
Emax and γ Parametric Studies
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0.20 0.25 0.33 0.50 1.00 2.00 3.00 4.00 5.008
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E_m
ax (
V/k
m)
Substation Resistance Scaling Factor
Emax,c for different system sizes and grounding resistances (step-size 1 V/km)
Area A only Area A plus first neighbors Whole EI Series4
γ*
*γ applied to all substations of EI
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 Emax, c?
• Next slide shows snapshots taken at Emax, c when only the SubR values in Area A were scaled by γ
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Area A SubR Variations
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0.20 0.25 0.33 0.50 1.00 2.00 3.00 4.00 5.0012
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Emax
(V
olt
s/km
)
Substation Resistance Scaling Factor
Series4 SubR scaled for Area A only
γ
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Key Takeaways
• Impacts of size of study system:
– Study with only Area A losses overestimates the level of Emax, 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
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Key Takeaways
• Substation grounding resistance uncertainty:
– Varying SubR values within a factor of 5 Emax, c varies ≈ ±5V/km for the Area A study
– Uncertainty in one Area can influence Emax, 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 Emax, c
– Desired certainty of Emax, c Tolerable uncertainty of SubR data
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Questions?
The GMD research group
at University of Illinois at Urbana-Champaign welcomes discussions on performing individual system studies
Email: [email protected]
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