1 The View from the Other Side: How Transformers affect GIC D.H. Boteler Geomagnetic Laboratory Ottawa Geomagnetic Disturbance Workshop, Idaho National Laboratory, April 7-8, 2015

2 Factors affecting GIC in Power Systems Geomagnetic Induction  Earth Conductivity  Rate of change of magnetic field System Characteristics  Network Configuration  Impedances The higher the sampling rate the larger the dB/dt value GIC modelling uses resistances only. What about inductances? How does system L/R time constant affect GIC?

3 Hydro-Quebec HVDC Tests Bolduc et al, 1989

4 Hydro-Quebec HVDC Tests

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6 Later Reports Dong, 2002 Walling and Khan, 1991

7 Calculations of growth of GIC Bolduc et al (1989b)

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10 Bolduc’s formula for time to saturation Fraction of steady-state current reached after time, t As a function of parameter Gives time

11 Using Bolduc’s formulas a)“Backwards” calculation determine T (secs) for specified i/I b) “Forwards” calculation determine i/I for specified T (secs)

12 Effective Inductance seen by GIC Walling, 2013

13 Effective Inductance seen by GIC t sat T L unsaturated L saturated Inductance time Fraction of time in saturation, Fraction of time not in saturation,

14 Effective Inductance seen by GIC T L unsaturated L saturated Inductance

15 Relation between θ and GIC where

16 Incremental Growth of GIC 1. Initially, i = 0, L eff = L unsaturated 2. Calculate current increase, Δi for time interval, Δt 3. For new value of i, calculate fraction of time in saturation, θ 4. Calculate new value of effective inductance, L eff 5. Return to Step 2, etc

17 Incremental Growth of GIC

18 3-leg 3-phase transformer Slope L2 Magnetising curve for AC Magnetising curve for DC Inductance is constant

19 Transformer with delta tertiary DC current Flux offset Current circulating in delta tertiary “Delta” phase “Normal” phase Walling, 2013

20 Slope L2 Magnetising curve for “normal” phase Magnetising curve for “Delta” phase Transformer with delta tertiary “Delta” phase “Normal” phase

21 Questions?Answers?

22 References  Bolduc, L., Pelletier, P. and Boisclair, J-G., DC current in AC transmission system near the electrode of the HVDC converter at DES Cantons substation, Proceedings of the EPRI Conference on Geomagnetically Induced Currents, Burlingame, California, November 8-10,  Bolduc, L. and Kieffer, P., A recipe for fast simulation of the effect of the DC component of magnetizing steady-state currents in transformers using EMTP, Proceedings of the EPRI Conference on Geomagnetically Induced Currents, Burlingame, California, November 8-10,  Bolduc, L., Formula to approximate the setting time for the DC component of magnetizing currents in transformers, Proceedings of the EPRI Conference on Geomagnetically Induced Currents, Burlingame, California, November 8-10,  Bolduc, L., A. Gardreau, and A. Dutil, “Saturation time of transformers under dc excitation,” Elect. Power Syst. Res., vol. 56, pp. 95–102,  Boteler, D.H., Characteristics of time-varying inductance, IEEE Trans. Magnetics, 30, ,  Dong, X., Study of power transformer abnormalities and IT applications in power systems, PhD Thesis, Virginia Polytechnic Institute,  Walling, R.A. and Khan, A.H., Characteristics of Transformer Exciting Current During Geomagnetic Disturbances, IEEE Trans. Power Delivery, Vol. 6, No. 4, October 1991.