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Improving GMD‐Induced Power System Voltage Stability with Transformer Neutral Blocking and Operational SchemesScott Dahman, P.E., IEEE member
PowerWorld Corporation
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• Modern methods model GIC as DC voltage sources in transmission lines
• With pertinent parameters, GIC computation is a straightforward linear calculation
GIC Modeling
• The dc GICs are superimposed upon the ac currents. In transformers this can push the flux into saturation for part of the ac cycle
• This can cause large harmonics; in the positive sequence(e.g., power flow and transient stability) these harmonics can be represented by increased reactive power losses on the transformer.
Transformer Saturation and Losses
Image Source: Craig Stiegemeier, JASON Presentation, June 2011
NERC Planning Requirements• Vulnerability Assessment of the system for its ability to withstand a Benchmark GMD Event without causing a wide area blackout, voltage collapse, or damage to transformers
• Transformer thermal impact assessment to ensure that all high‐side, wye grounded transformers connected at 200kV or higher will not overheat based on the Benchmark GMD Event
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Source: North American Electric Reliability Corporation, TPL-007-1 Technical Conference, July 2014
• A large GMD could substantially affect power system flows and voltages
• Studies allow for testing various mitigation strategies– Operational mitigation could include redispatching generation to avoid long distance power transfers and reducing transformer loading values, and opening devices to limit GIC flows
– Longer‐term strategies could include installation of GIC blocking devices (such as capacitors) on transformer neutrals and/or series capacitor compensation on long transmission lines
Power System Planning for GMD
GMD‐Induced Voltage Collapse
• We can examine the behavior of the system by stressing the GMD‐induced surface electric field
• As the E‐field increases, GIC transformer reactive power losses increase and system voltages decrease
• We can plot voltage vs. E‐field until reliability criteria are violated or until voltage collapses
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• Eastern Interconnect power flow model, peak load conditions
• NOTE: The examples that follow contain no specific parameters for transformers or substation grounding resistance; results are hypothetical and for illustration only
Uniform Electric Field Example
• Examine GIC transformer reactive power losses at constant E‐field magnitude at various orientations
• Worst‐case for this system is 75 degrees
Worst‐Case E‐field Orientation
GIC Q‐V Curve• Plot bus voltage vs. E‐field to examine the effect of increasing GIC transformer reactive power losses
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0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
Volta
ge at Low
est pu
Voltage Bus, 230kV
+ Nom
inal
Field Strength (V/km)
Increasing E‐Field Magnitude:Wide‐Area AC Bus Voltage Drop Contour
2 V/km 3 V/km
4 V/km 5 V/km
Mitigation for Voltage Collapse
• Transformer neutral blocking– Placing capacitive or resistive device in series with the transformer neutral to effectively create an open circuit for quasi‐DC GIC currents
• Opening transmission lines or transformers
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Transformer GIC Sensitivities• The sensitivity of a transformer’s effective GIC current with respect to the GIC E‐field on transmission lines may be expressed as [1]
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Effective,r,r T r
T
dId
-1= C G B SE• where
– Cr is a column vector of conductances for transformer r,– G is the real GIC DC bus admittance matrix,– and B is the transmission line conductance‐distance matrix
• This is a fast computation due to the sparsity of the vector and matrices in the sensitivity calculation
• A small number of nearby transmission facilities are often responsible for a large percentage of GIC in transformers
[1] T. J. Overbye, K. S. Shetye, T. R. Hutchins, Q. Qiu, J. D. Weber, “Power Grid Sensitivity Analysis of Geomagnetically Induced Currents,” IEEE Trans. on Power Systems, vol. 28, no.4, pp. 4821-4828, November 2013.
• Applying this analysis to the Eastern Interconnect example to the set of transformers in the substation with the greatest GIC Mvar losses
Transformer GIC Sensitivities
Four lines are responsible for 75% of the transformer GIC; opening some or all of these lines can be an effective mitigation strategy for off-peak load conditions
• Blocking the neutrals of transformers with the highest GIC neutral currents can reduce GIC Mvar losses and improve performance
• Neutral blocking in one transformer can often lead to increases in GIC flow in nearby transformers
• It is assumed that neutral blocking would be applied to all transformers in a substation in which any transformer is blocked
Transformer Neutral Blocking
Greedy Neutral Blocking Heuristic• Do until n substations protected
– Place neutral blocking on all transformers in the msubstations with the greatest sum of transformer neutral GIC
– Recalculate GIC flows• Repeat
• Can also use other criteria for blocking selection:– transformer effective GIC current– transformer GIC Mvar loss– changes in bus voltage or dV/dQ sensitivity– reactive power reserve margin (for generator substations)
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Eastern Interconnect Example• Perform with m=5; first 2 iterations:
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Iteration Substation Neutral Amps Mvar Losses
1 Conemaugh 500 160 62
1 Keystone 500 140 66
1 Cuthbert 138 108 8
1 Sunbury 500 98 17
1 Dickerson 230 95 8
2 A29 500 88 35
2 Three Mile Island 500 88 34
2 New Freedom 500 87 44
2 Steel City 500 85 24
2 Juniata 500 85 36
GIC Q‐V Curves with Blocking17
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0.0 2.0 4.0 6.0 8.0 10.0
Volta
ge at Low
est pu
Voltage Bus, 230kV
+ Nom
inal
Field Strength (V/km)
Base
5 Substations
10 Substations
15 Substations
20 Substations
25 Substations
Voltage collapse margin improves from 6.0 V/km to 9.1 V/km
Opportunities for Future Work
• Examine sensitivity of transformer GIC effective current to transformer neutral blocking– Analogous to line outage distribution factors (LODF) used in AC power flow and contingency analysis
• Combine these with bus voltage sensitivities to develop better blocking algorithms
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Questions?19