b'Electronics | Engineer Innovationis often wrongly approximated in thesystems including transformer literature leading to erroneous results.time response," PhD(Eng) Thesis University of Cape Town, South Conclusions Africa, http://hdl.handle.The combination of practical testingnet/11427/16708, 2015.guided by preliminary FEM simulations yielded some important results[6] O. Br, G. Kockza, G. Leber, K.regarding the flux distributions underPreis and B. Wagner, "Finite simultaneous AC-DC excitation. ThisElement Analysis of Three-Phase resulted in the submission of a paper atThree Limb Power Transformers the Advanced Research Workshop onunder DC Bias," IEEE Transaction Transformers (ARWtr2016, Spain) whichon Magnetics, vol. 50, no. 2, pp. led to a journal publication [8]. The565-568, February 2014. main finding of the study was the importance of details in the transformer[7] N. Hihat, E. Napieralska-Juszczak,core joints for better representation ofJ. Lecointe, J. Sykulski and K. practical transformers both in the FEMKomeza, "Equivalent Permeability and in topologically derived models. Theof Step-Lap Joints of Transformer approach allows for better modeling ofCores: Computational and stray flux when designing for GIC. This isExperimental Considerations," IEEE different from the conventionalTransactions on Magnetics, vol. 47, modeling in industry that assumes solidno. 1, pp. 244-251, 2011.Figure 6. Flux distribution at normal AC joints for electromagnetic devices underexcitation for calibrating the FEM model of no normal operating conditions. The[8] H. K. Chisepo, L. D. Borrill and C. T.load current with measurements. Left: coreassumption of solid joints in the FEMGaunt, "Measurements show needmodeled as solid with air gaps at the joints. model resulted in the underestimationfor transformer core joint details inRight: core modeled with explicit 0.3 TKES H111-30 grain oriented electrical core steel of the no-load currents in the transientfinite element modeling of GIClaminations stacked close to the core surface. calculations performed in theand dc effects," The International differential core structure bench-scaleJournal for Computation and FEM analysis.Mathematics in Electrical and Electronic Engineering, vol. 37, no. References 3, https://doi.org/10.1108/[1] J. Raith and S. Ausserhofer, "GICCOMPEL-11-2016-0511, pp. 1011- Strength verification of power1028, 2017b. ntransformers in a high voltage laboratory," in GIC Workshop, University of Cape Town, South Africa, 2014. [2] H. Chisepo, "The response oftransformers to geomagneticaly induced-like currents," MSc(Eng) Dissertation, University of Cape Town, South Africa, http://hdl.handle.net/11427/8685, 2014.[3] P. R. Price, "Geomagneticallyinduced current effect on transformers," IEEE Transactions on Power Delivery, vol. 8, no. 3, pp. 1002-1008, October 2002. [4] R. Girgis and K. Vedante, "Effectsof Geomagnetically Induced Currents on Power Transformers and Power Systems," in PES T&D, Orlando, 2012. [5]D. T. O. Oyedokun, "Geomagnetically Induced Currents (GIC) in large powerFigure 7. Core stacking lamination detail close to the surface of a mostly solid core. 39'