DC shipboard power systems have been connsidered as a promising solution for stricter environmental regulations on ships due to their main benefits in fuel savings with variable speed engines and easy integration of energy storage systems. In order to employ the DC solution in the shipboard power systems, the DC power systems have to be protected from a system fault with protection selectivity to minimise impacts of the fault or to avoid other undesirable situations in the system. For low-voltage DC shipboard power systems, a three-level protection has been proposed: fast action (1st) - bus separation by solid-state DC bus-tie switch, medium action (2nd) - feeder protection by high-speed fuse and slow action (3rd)- generator-rectifier fault controls. This paper proposes a new method by means of additional bus capacitance added in main DC buses to help the reliable operation of the three-level protection. The principle of the proposed method is introduced and the sizing of the additional bus capacitanceis addressed in this paper. With the modelling of the DC shipboard power systems, the analyses of voltage drops for the bus separation failure and fault clearing time for the feeder protection are carried out to verify the proposed method. The results show that the proposed method not only mitigates the voltage drop for the bus separation failure, but also achieves the selectivity and the sensitivity for the feeder protection.