Analysis of frequency distribution of ground fault-current magnitude in transmission networks for electrical safety evaluation

Ground Potential Rise (GPR) evaluation under fault conditions is important for the evaluation of electrical safety risk to human beings who are present in and around electric installations. Current national and international standards recommend such evaluation is based on a set of 'worst-case' conditions that include a simplified fault current calculation procedure based on assuming a maximum value. Accordingly, this approach may lead to over-design of grounding systems in certain cases by overestimating individual risk. To address this and move towards a comprehensive probabilistic assessment of risk, a more detailed model of the electric network is required for fault current magnitude evaluation. This paper describes the application of the multi-conductor method to determine fault current distribution on transmission networks. First, a model of a portion of the UK transmission network is built and the fault current and its distribution among the phases, ground wire and ground is evaluated for three example fault positions. It is found that the position of the fault along the line, substation bus section status and proximity to generation affect greatly the current distribution. Then, a transmission model based on the national network is built, and the effects of system loading and generation on fault current magnitude were also considered. A complete frequency distribution of fault current magnitude is obtained and the results demonstrate the value of such description for the probabilistic assessment of human safety in grounding system design.