Impacts of Grounding Concepts in IT DC-Grids on SPD Safety

Modern DC-grids fed by DC/DC-converters can be built easily as isolated (IT) grids. To have a non-floating grid potential different grounding system are discussed such as capacitive or high ohmic grounding. In both cases residual currents can flow to ground. This current can be either a DC-current caused by asymmetric ohmic grounding or AC-current driven by high switching frequency of the DC/DC-converters. Both currents can cause a permanent arc once a spark gap based surge protective device (SPD) has been triggered. This paper deals with the possible range of residual currents due to different grounding systems. Derived from this the turn-off limits of spark gaps in DC-grids are discussed and possible protective measures will be shown.