Low cost surge voltage linearization methods for small rating power transformers

Non-linear initial voltage distribution often emerges along transformer windings under surge type incident voltages. Excessive concentration of voltage stresses at line end of the transformer often arises due to the presence of earth capacitance and the resulting free oscillations lead to high voltage peaks in the initial part of the winding with magnitudes exceeding the peak value of the incident surge voltage. If no special method is adopted to reduce these voltage stresses, heavy insulation has to be provided in the initial portion of winding, which is practically and economically infeasible. Thus, special winding techniques have evolved to protect the transformer windings against surge voltages, such as interleaved and inter shield windings, both of which are expensive and not economically viable for small rating transformers. This paper proposes external application of shunt resistors and shunt capacitors across transformer windings to achieve linear voltage distribution. Both time and frequency domain performance of the proposed voltage linearization methods were tested on developed model of a 33/11 kV, 1 MVA transformer using EMTP (ATP) with encouraging results.