Overcurrent Protection Philosophy Using Microprocessor Based Relays For a South African Power Distribution Network

This paper investigates the application of the overcurrent protection function on a South African high voltage consumer network and addresses the shortcomings that arise with the application of electromechanical relays. These shortcomings are overcome when microprocessor based relays are applied due to their agility. Simulations were conducted, of what is likely to happen in a real network - single line to ground (SLG) fault that get misjudged by an overcurrent relay, leading to an incorrect relaying decision. An incorrect relaying decision leads to improper discrimination and loss of service to sections of the network that are otherwise not affected. The proposed microprocessor based relaying solution is devised, as presented in this paper, whereby the microprocessor relay is marshalled for reliable operation under SLG fault conditions and for sensitivity to the direction of power flow. The solution prevents possible mal-operation of protection and ensures accurate selectivity.