MODELLING AND LIVE MEASUREMENTS OF STEP AND TOUCH VOLTAGES AT LV CUSTOMERS IN DEVELOPING GRIDS, CAUSED BY MV FAULTS

Earth faults in the medium voltage (MV) grid cause touch and step voltages in the low voltage (LV) grid. If a MV and LV grid share their earthing systems at the MV-LV transformer, one may question the safety with respect to LV customers in case of a phase-to-earth fault in the MV grid. In 2005 models and actual measurements in an operating grid were preformed to determine the current distribution and touch voltages in a global earthing system. The results showed that a phase-to-earth fault leads to low touch voltages in urban global earthed areas. Furthermore it has been demonstrated that the highest touch voltage appeared during a short circuit at the end of a cable. New developed grids start with a single: MV cable, MV/LV transformer and LV cable from the substation. Therefore this assembly does not benefit from a global earthing system, leading to higher touch and step voltages. To identify the magnitude of these voltages calculations and measurements have been preformed. The result of this study is that Continuon (grid operator of Nuon) sees no more objections to apply TN grounding in their network.