Derating line voltage as an option for reliability enhancement of lightly loaded networks

Networks with light loadings can suffer from steady state overvoltages and this can lead to damaged insulation of the equipments, accelerated electrical aging of equipment due to frequent operation under exceeded voltage limits. Common solutions for this problems entail the use of shunt reactors, series reactors, using tap changing of transformers, and ultimately, switching off of lines. In this paper, the authors evaluate the potential of derating the voltage of lines in the load centre, using step down transformers, as a solution for overvoltages. Load flow studies are conducted for a healthy network, with and without step down transformers, and voltages in these two cases are compared. The results showed that it is possible to operate the network with acceptable voltages in the case with step down transformers. There is no need to disconnect lines, putting load at risk, thereby worsening reliability, to maintain acceptable voltages however, it is necessary in the case without step down transformers. The use of step down transformer, thus enable voltages to be kept within limits and improves the reliability of the network.