Achieving protection selectivity in DC shipboard power systems employing additional bus capacitance

With the implementation of energy efficiency regulations for all ships, DC shipboard power systems (SPS) have attracted much attention from the shipbuilding industry due to their advantages in fuel savings with variable speed engines and the closed bus-tie operation for dynamic positioning vessels. However, DC protection coordination is one of the main obstacles to employ DC power systems into ship power networks. Because, in the DC SPSs, fast fault clearing, e.g., several milliseconds, is necessary to avoid the failure of power converters which have much lower short-circuit withstand capabilities than conventional AC electrical equipment, e.g., generators, transformers and cables. This paper presents a comprehensive analysis of voltage drops and fault clearing time to achieve protection selectivity for centralized and distributed DC SPSs. Furthermore, impacts of additional bus capacitance, which is combined with the existing DC SPSs, are analysed in terms of the protection selectivity. The results show that employing the additional bus capacitance has great advantages in a bus protection by mitigating the voltage drop at the unfaulted bus and a feeder protection by providing the selectivity between the faulty and the adjacent feeders.