Studies on asymmetrical short circuit currents in shipboard medium voltage direct current distribution systems fed by AC generators

DC distribution is a promising technology for addressing incoming issues in AC conventional distribution. In marine power systems, given the high power requirements of electric propelled vessels, Medium Voltage DC (MVDC) distribution is on study. Among all the relevant issues of DC distribution, short circuit currents definition and interruption is the most problematic now. Rail-to-rail faults are commonly studied, and models and methods to address their calculation are present in literature. Conversely, rail-to-ground faults are more complex to assess. Indeed, their fault loop include also AC generation systems, so their grounding configuration must be considered. Moreover, rail-to-ground faults produce currents that can be non-symmetrical and non-sinusoidal on AC side, depending on DC side grounding configuration, rendering unsuitable common system models and methods to assess fault currents. In this paper, a study on fault currents in a shipboard power system with different DC side grounding configurations has been done, highlighting when the resulting currents are symmetrical or not. Considerations about practical applicability of proposed configurations has been also done, pointing out pros and cons and defining the most recommendable practices.