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The hybridization of power systems offers the low-emission and energy-efficient marine vessels. However, it increases the complexity of the power system. Design, analysis, control, and optimization of such a sophisticated system require reliable and efficient modeling tools to cover a wide range of applications. In this work, a typical DC hybrid power system model is developed using a bond graph modeling approach. Necessary component models of varying degrees of fidelity are developed and integrated to build a system model with reasonable accuracy. The developed system model, along with the rule-based energy management system, is used to simulate the entire system and to investigate the load-sharing strategies as well as the system stability in various operating scenarios. Moreover, the simulation results are validated with experimental results conducted on a full-scale laboratory setup of DC hybrid power system and with a ship load profile. The results show that the system model is capable of capturing the fundamental dynamics of the real system. The hybrid power system model is further used to analyze the bus voltage deviation from its nominal value. The computational efficiency presented by the system is fairly good as it can simulate faster than real-time. The developed system model can be used to build up a comprehensive simulation platform for different system analysis and control designs. Index Terms-Marine hybrid power systems, onboard DC power systems, modeling, stability, power and energy management I. INTRODUCTION T HE International Maritime Organization (IMO) has proposed stringent regulations to reduce emissions and improve energy-efficiency from the shipping industry. The energy-efficiency, low-, zero-emission, and innovative technologies are being investigated to comply with the regulations [1]. The energy storage device (ESD) based zero-emission vessel is still challenging for all types of marine vessels due to the low energy density of ESDs [2], [3]. The low energy density of ESD is compensated by the conventional Manuscript

In this paper, a data-driven dynamic efficiency model is developed for efficiency evaluation and comparison of ship electric powertrain with various system configurations and load-sharing methods. Based on the proposed method, the entire powertrain efficiency is assessed from the fuel consumption to the propulsion unit and the rest of the onboard load. The efficiency model is repeated for the conventional diesel-electric and the hybrid power system with batteries. In the latter case, the efficiency of the battery system is also included in the model. Then, the analysis is extended for different power system architectures such as AC-and DC onboard power systems. As a case study, system efficiency in a cruise ship is investigated using a real operational profile. A comprehensive analysis is performed to demonstrate the loss distribution in each subsystem of a hybrid AC-and DC power system. For a fair comparison between AC and DC, the battery charge level is equalized based on fuel compensation. The case study shows that hybridizing the ship power system increases system efficiency and enhances operational flexibility for the studied use case vessel. Further, the DC hybrid power system can improve the efficiency of the whole ship powertrain thanks to the variable speed operation of engines.Index Terms-Electric propulsion, ship hybrid power systems, power system efficiency, AC & DC power system. I. INTRODUCTIONW ITH the ever-increasing onboard electrical power demand, the ship power systems evolve gradually to a complex system [1]. The complexity is even increased due to incorporating energy storage devices (ESDs), highly dynamic consumers, and various energy carriers, which do not operate in the same power system. Thus, the discussion of alternating current (AC) vs. direct current (DC) is once Manuscript

The electrification of a ship power-train is growing at a fast pace to improve efficiency and reduce emissions. The implementation of new technologies requires test and validation using various modeling approaches. However, many of the existing models of the ship hybrid power system are too complicated and demand high computational requirements, which make them inappropriate for the real-time applications. The realtime simulation model offers the benefits of testing different control algorithms along with hardware-in-the-loop testing. The bond graph-based dynamic modeling of a ship hybrid power system with a DC grid is presented as applicable to realtime simulation. The overall system model is established using different component models with varying fidelity, so-called mixedmodeling approach. In this approach, the components and control functions are modeled with different complexity such that it can capture the necessary system dynamics while minimizing the computational time. Results show that the modeled system is capable of simulating different operating strategies of the hybrid power system. Moreover, the mixed-modeling approach has enabled the system to simulate in nearly 2.5 times faster than the real-time.

With the share of high-power electronic converters, the emerging marine DC hybrid power systems have been increasingly attractive for ship designers because of their higher operational flexibility. The system efficiency analysis, one of the critical factors while embracing an emerging system, requires a detailed estimation and evaluation. Conventionally, the rated efficiency for each component is the basis for the system efficiency estimation. However, the efficiency may vary with the loading conditions in any component, directly affected by the actual load and the load-sharing strategies. In this work, dynamic efficiency models are developed for the DC hybrid power system components, which are used to estimate the overall system efficiency using a realistic load power profile and a rule-based power and energy management system (PEMS). The efficiency analysis shows that the overall power efficiency increases with optimal battery usage in a hybrid power system. Moreover, three different power-sharing control strategies are compared. The modified rule-based PEMS offers the highest efficiency, while conventional diesel generator operation offers the least efficiency for the given load profile and power system configuration.