Return paths of earth faults current in medium voltage grids with underground shielded cables

Abstract

The design of earthing schemes in Belgian MV networks was established in the past based on the electrical knowledge of the time and has led to the implementation of a certain earthing strategy, around the idea of global earthing. New simulation tools lately have allowed for a more detailed modelling and analysis of the fault current return paths and it appears that the impacts of the conductors’ metallic screens have been underestimated.It appears that in a configuration thanks to which the fault current can return via a continuous path of interconnected screens all the way back to the earthing transformer, most if not all of the fault current will return via this path rather than via the multiple earth connections. This is due to a suction effect induced by the fault current in the conductors, following Lenz law. Several use cases have been investigated and tend to confirm this conclusion.