Methodology for Fast Calculation of Impedance Matrix of Power Transformers for High Frequency Transient Studies

Power transformers are exposed to a variety of disturbances that may jeopardize their lifespan and the safe operation of the power system. The transient behavior of the transformer can be assessed by means of white-box models. The main prerequisite for the development of the white-box model is the frequency-dependent impedance matrix of the transformer. The main obstacle that restricts the use of white-box models at industrial level lies in the computation of the impedance matrix, since this calculation requires magnetic-field numerical simulations, which demand disproportionate calculation times when performed with traditional techniques such as the finite element method (FEM). That is why this paper proposes a new methodology for the fast calculation of the frequency-dependent impedance matrix of power transformers, which produces matrices suitable for the development of accurate white-box models. The proposed methodology was validated on a real 50 MVA transformer. Two white-box models were elaborated, one from the impedance matrix calculated with the proposed method and other using the matrix obtained with FEM. Excellent agreement was found between the transient response measurements and the response obtained with both models, however the proposed method reported a drastic reduction in computation time compared to FEM.