Mitigation of lightning surge stresses in the high voltage windings of distribution transformers by introducing an electrostatic shield

The Medium Voltage (MV) Electricity Distribution Networks are frequently exposed to lightning and it is the main suspected reason for distribution transformer failures. Appreciable number of transformer failures occur due to insulation failures notably at points near line ends. The short rise time of a surge prompted by a lightning impulse can cause deterioration in the insulation and ultimately lead to a dielectric breakdown. The voltage distribution along transformer High Voltage (HV) winding becomes non-linear under surge conditions due to the capacitive-inductive currents. This research focuses on the possibility of neutralizing this effect with the aid of an electrostatic shield inserted in to the HV winding. Comparison of the behaviour of the winding under surge conditions with and without electrostatic shield requires appropriate transient models suitable for simulations. The necessary calculations required for this purpose were carried out and the proposed models are presented. The simulations were carried out with PSCAD software and the results are included in this paper. The oscillatory nature due to the combined effect of capacitors and inductors is observed in the wave tail region and some of the peaks created have potentials exceeding the peak potential of the surge waveform as well. The proposed transformer model have a definite positive effect compared with the available model as the results obtained by simulations clearly reveal the reduction of the stresses on the transformer HV windings when a shield is used. ENGINEER, Vol. 48, No.03, pp. 1-11 2015