Mitigating Faults Effects on Equipment and Personnel on Substations

Abstract

Faults occur in substations and the effects of these faults range from damage of electrical equipment connected to the substation to electrocution of personnel operating the substations. When protective devices fail in their operations after fault occur, mitigating the fault effects can be the only option of saving the lives of the personnel and equipment around the fault location in the substation. A balanced three phase fault, line-to-ground (LG) fault, line-to-line (LL) fault and double line to ground (LLG) fault at the primary and secondary of the respective transformers in the 80 MVA 132/33 kV Ohia Transmission substation were simulated in Electrical Transient Analyzer Program (ETAP).   Ranges of fault current flow into the bus of the primary of the respective transformer causing corresponding arc flash energy in calories/cm², arc flash boundary in centimeters and arc flash distance in centimeters, appropriate recommendations of respective levels of radiating personnel protective equipment (PPE) amongst levels A, B, and D based on the results were made in accordance with National Fire Protection Association (NPFA) 70E2009/2012. The results recorded show the highest fault current flow of 451.01 kA in the primary of the second transformer as a result a three-phase fault and LG fault causing 8.962 cal/cm² energy to radiate within an arc flash boundary of 76.50 cm (2.51 ft.) given an arc flash distance of 9.75 cm (3.840 inches) and a corresponding choice level D PPE. Further mitigation of the faults effects can be done with a well-designed earth grid with touch potential of 797.7 volts, step potential of 938.5 volts and earth resistance of 2.9 ohms all being within the IEEE tolerable range. Mitigating faults effects can also be achieved using a well-designed fault current limiter (FCL) with normal operating condition of negligible impedance and maximum impedance during fault as 70 ohms and 1200 ohms respectively for the secondary sides of superconducting fault current limiter (SFCL).