**1. Introduction**

Recently, hydrogen has started to be regarded as an alternative ocean fuel source. As a result, studies on hydrogen-fueled fuel cells have been actively conducted in Europe and the United States [1–3].

Fuel cells are known as a technology in which the chemical energy associated with hydrogen molecules is converted to electricity and thermal energy through electrochemical reactions with air. Thanks to the minimization of emissions, the noise in operation and high adaptability of various fuel sources, fuel cells are considered a next-generation technology for clean production [4,5].

In particular, fuel cell-based power plants are expected to reduce greenhouse gas emissions by 30% compared to fossil fuel-based power generation. Considering such benefits, many developed countries have been carried out a number of projects to stimulate the fuel cell application to industries [4–10].

According to a report of the European Maritime Safety Agency (EMSA), since the first project 'US SSFC' was launched in 2000, 24 projects have been made available to facilitate the application of marine fuel cells. Eleven of them selected the Proton Exchange Membrane Fuel Cell (PEMFC) type composed of polymer resin with several advantages: Relatively low operating temperature, about 80 ◦C; shorter time to reach the operating temperature; unnecessary of peripheral devices [11].

On the other hand, to ensure the reliability of the stakes, it requires pure hydrogen which is operated in a very low temperature with the late response time. In order to reduce such drawbacks, expensive catalyst and electrode are applied [12–14].

To obtain pure hydrogen, a separate reforming system is additionally required. That limits the application of PEMFC for the propulsion of medium-large ships. The Zero Emissions Ships (ZEMShip) project has demonstrated the excellent performance of a fuel cell by applying a 96 kW fuel cell to the Motor Vessel (MV) 'Alsterwasser' [15–19].

In 'FellowSHIP' and 'Molten Carbonate Fuel Cells for Waterborne Application (MC-WAP)' projects, molten carbonate fuel cell (MCFC) type was applied to supplement auxiliary power rather than the main propulsion power. In particular, 'FellowSHIP' from 2003 to 2017, applied MCFC fuel cells for MV 'Viking Lady', 6,000 Deadweight Tonnage (DWT) Offshore Support Vessel (OSV), The project results showed excellent performance of the fuel cell in reducing emission levels [8,20–22].

Because MCFCs can operate at high temperatures, low-cost catalysts are available, which simplifying system design and reducing costs. In addition, even with long voyages, this type of fuel cell can utilize natural gas or coal gas as a direct fuel instead of using an external reformer. These advantages may be suitable for application as a major source of power for the ship's fundamental loads. Despite many research and projects for fuel cell applications in the marine industry, attempts to use the MCFC type for medium and large vessels for propulsion are scarce. Given this background, this study was motivated to investigate the suitability of the MCFC for the large vessel propulsion [23–27].

This paper was focused on analyzing the power quality of each power source in synchronization and breakaway. A test bed with a capacity of 180 kW was constructed using 100 kW fuel cells, 30 kW batteries and 50 kW diesel generator.

An actual voyage data for 5,500 TEU container vessel was used to verify the power quality of the fuel cell. Three load scenarios were developed by examining the performance characteristics of the fuel cell, battery and diesel generator systems based on normal navigational conditions. Each developed scenario was applied to the hybrid power sources, and the quality of voltage and frequency was examined during synchronization and breakaway phases.
