The effect of the secondary loads on the voltage surges transferred through distribution transformers

Over voltages associated with nearby or direct lightning discharges on distribution networks have a great effect on the power quality. The surges transferred from the primary to the secondary via transformers should be correctly evaluated having in mind the protection of the installations and equipment connected to the low-voltage network. Thus a transformer model adequate for these calculations and that can be used in the presence of loads, is required. In this paper such a model, able to represent typical three-phase distribution transformers, is presented. Some measured and calculated waveforms of voltages transferred through typical distribution transformers, under different load conditions, are also shown. Based on comparisons of the results, the proposed model was validated. Simulations results are also presented for the case of direct lightning hits on the primary line of a typical distribution network, making it possible to evaluate the effect, on the over voltages, of installing surge protective devices at different points of the secondary network, the simulations were performed through the use of the EMTDC.