Research on the Method of Risk Probability Evaluation of Grounding Grid

Individual and equipment accidents of substations are mainly caused by the ground potential rise and the uneven voltage. The consequences of an accident are affected by lots of random elements, such as the type of the grounding faults, the magnitude of the fault current, the duration of the fault clearance time and the withstand voltage of equipment. While the traditional threshold means by judging the resistance of the grounding system, makes it difficult to accurately evaluate the risk of grounding system. Therefore, this paper presents a comprehensive method to calculate and assess the risk of grounding grid, especially for the equipment safety which has not been discussed before. Firstly three kinds of characteristic parameters was proposed which relate closely to individual and equipment safety, namely, the step voltage (Us), the contact voltage (Ut), and the ground potential difference (GPD), to calculate the risk of individual and equipment safety. Afterwards, the security level of the ground system was obtained according to the as low as reasonably practicable (ALARP criteria).