On Human Life Risk-Assessment and Sensitive Ground Fault Protection in MV Distribution Networks

The minimum phase-to-ground fault current required to be sensed by protection systems in medium-voltage (MV) networks can be as low as 0.7 A in a few countries, leading to a lot of undesired relay trips and poor service quality to costumers. However, these settings raise the protection threshold above the minimum fault current that concerns network operators regarding human safety, although they cannot be practiced when grounding is distributed. The purpose of this paper is to present a risk assessment foundation to determine the required protection sensitivity to ensure human safety in MV distribution networks. The proposed approach is based on a biophysical model included in IEC standards, the consideration of current paths models for typical faults and Monte Carlo methods to deal with nonlinearity, and the many involved random variables. Downed conductors and line-to-concrete pole faults are investigated and sensitivity analysis performed to highlight some important determinants of the results.