Performance evaluation of traction and utility network interface: Fault location and protection coordination

Abstract

Single phase AC traction systems pose unique challenges to power utilities at interface points of connection on the power delivery network. Traction systems are usually supplied from dedicated utility networks in order to minimise the negative effects that traction loads have on conventional three phase loads, particularly in instances where customer equipment may be sensitive to the quality of supply. The electric utility’s power network is exposed to faults, short duration thermal overloads, temporary over-voltages, high magnitude transient recovery voltages, load unbalance and harmonics which emanate from the traction system. This research investigation evaluates and analyses Eskom’s network and Transnet Freight Rail’s network and presents a methodology for determining fault location and effective discrimination between electric utility and traction network faults. This is essential where there is a lack of protection coordination or difficulty in achieving coordination between the protective devices in each system. The method is further developed into a protection coordination philosophy to ensure that the utility’s protection relays operate for utility network faults only, while the traction system relays operate for faults within the traction networks. The balance of this paper assesses the withstand capability of network equipment against temporary overvoltage caused by single phasing on the utility system; a performance evaluation of high magnitude transient recovery voltage following network faults, and determination of the minimum technical specification of equipment on the system. Results obtained and proposed solutions to minimise the levels of overvoltage for both the temporary and transient cases are presented and discussed.