High frequency transformer model derived from limited information about the transformer geometry

To represent transformer behaviour during a transient state which includes high frequencies, it is necessary to consider the resonances which occur inside the transformer. One strategy is to deduce the transformer model from the measurements of the transformer’s frequency response, another one is to construct the model based on a careful representation of the inside of the apparatus. In the paper a model is presented which is compatible with EMTP-type software programs based on a finite element method (FEM) calculations and the complex permeability approximation. The model can be classified as a Grey Box transformer model, according to the terminology of the CIGRE. The model’s frequency dependent parameters are derived from limited information about the transformer geometry. State space equations are used to input the model into an electromagnetic transient calculation software program. This approach requires specific mathematical treatments to avoid stability issues during simulations. The model is validated for lightning impulse studies using the field test measurements of over-voltages that had occurred at the external transformer’s terminals.