The increasing penetration of renewable energy sources, energy storage systems (ESSs), and DC loads has opened the door to DC technology to pave its way into future grids. The shift has been more obvious in the marine domain supported by track records of installed onboard DC solutions since 2013. This new type of power system comes with technical challenges in protection coordination due to no natural current zero-crossing and low fault ride-through capability of power converters. This thesis has focused on the technical challenges tied to protection coordination in marine DC power distribution networks (PDNs).

The thesis starts with the state-of-the-art analysis of marine DC PDN protection. The analysis shows that three-level protection, which consists of three-different time frame protections: fast action - bus separation; medium action - feeder protection; and slow action - power supply protections, has become the dominant trend for marine low-voltage DC PDNs selected for the main research field in the thesis. Along with the analysis, each protection measure is investigated by simulation studies in terms of fault tolerance of equipment against fault energy limited by each measure. In these studies, additional bus capacitance (ABC) and artificial short-circuit methods have been proposed for extending selectivity of feeder protection and protecting voltage source converters (VSCs). The ABC method ensures the fault clearing by a reliable fuse operation that is assisted by the additional energy from the ABC only to the fuse on the faulty feeder. As an alternative to the fuse solution for the VSC protection, the artificial short-circuit method, which blocks the fault current to the rectifier by providing a low impedance path on the AC side, is verified by in-depth simulation studies.

Two-bus DC PDNs have been implemented for experimental studies. The DC PDNs consist of two DC motor-synchronous generator sets, two diode rectifier systems, a solid-state bus-tie switch, two supercapacitor banks, resistive loads, busbars, and a central controller. In the DC PDNs, the power supply protection method by generator deexcitation is characterized by comprehensive experimental tests for various system parameters. The analytical solution on the fault current during the deexcitation event is newly introduced for rectifier sizing and protection coordination. Apart from the protection topic, a wide range of DC PDN operations have been demonstrated in the implemented DC PDNs: DC voltage regulation, power sharing control, soft start, seamless transition, load leveling, transient mitigation, and zero-emission.

As the last work in the thesis, a protection scheme based on the three-level protection has been implemented in the DC PDNs. The influences of the system inductance and capacitance are investigated for the bus separation by the bus-tie switch to ensure its proper operation as well as the continuous operation of the adjacent healthy loads after the fault clearing by the fuse. Each protection measure and its settings are thoroughly coordinated from the investigation and the implemented protection scheme is verified by bus and feeder faults artificially generated in the networks. The results prove that the system protection can successfully isolate the faults from the networks with the correct operation of protection measures and enough time margins between the different protections.