Proposal for Probabilistic Risk Assessment in Grounding Systems and Its Application to Transmission Substations

An approach to probabilistic risk assessment of electrical system grounding is proposed. The method uses all significant factors that affect the risk of electrocution at substations and takes into account their probabilistic nature. The approach implements an accurate statistical description of IEC479-1 fibrillation and body impedance data, and it uses detailed computer simulations of the modeled grounding system to provide safety voltage distributions that take into account the individual's presence at a site as a random variable. Variation in the power system fault level is accounted for, and extensive data of actual system fault clearance time are included. It is proposed that the probabilistic risk assessment is utilized as a second stage of the grounding system assessment when the first-stage deterministic analysis requires expensive or impractical mitigation. Implementation of the second stage probabilistic risk assessment yields a measure of individual risk. This is then benchmarked against industry-accepted “as low as reasonably practicable” values to determine whether investment in mitigation is required. To illustrate the applicability of the proposed approach, the probabilistic risk assessment is applied to a practical case study of a transmission substation.