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Advanced DC microgrids design requires considering interdependent aspects. In these flexible grids with large penetration of high-bandwidth controlled converters, both system stability and short circuit limitation are crucial aspects to be attained. The design of filtering stages directly affects both: on one side the stability is beneficially influenced by a large filter capacitor, while on the other the short circuit current increases with oversized capacitors. Thus, voltage stability and filter’s short circuit contribution are discordant goals, which need to be carefully balanced by the designer. To solve this issue, a design procedure for sizing the LC filtering stage and the main bus voltage in a cascade-connected DC power system is proposed. The aim is to ensure meeting both voltage/current ripple requirements, as well as accomplishing system stability and limiting the energy supplied into a fault. Final tests on a Hardware-In-the-Loop system and electro-magnetic transients’ simulations are used to verify the effectiveness of the filter’s designing procedure.

Nowadays, in the large ships the electric propulsion solution is a viable alternative to the mechanical one. In fact, at present the latter is limited only to ships with peculiar requirements, such as the need of a high cruise speed or use of specific fuels. The use of electric propulsion, paired with progressive electrification of onboard loads, led to the birth of the All Electric Ship (AES) concept. An AES is a ship where all onboard loads (propulsion included) are electrically powered by a single power system, called Integrated Power System (IPS). The IPS is a key system in an AES, thus requiring both accurate design and management. Indeed, in AES electricity powers almost everything, highlighting the issue of guaranteeing both the proper Power Quality and Continuity of Service. The design of such a complex system has been conventionally done considering all the single components separately, to simplify the process. However, such practice leads to poor performance, integration issues, and oversizing. Moreover, the separate design procedure affects heavily system's reliability, due to the difficulty in assessing the effect on the ship of a fault in a single subsystem. For these reasons, a new design process is needed, able to consider the effect of all components and subsystems on the system, thus improving the ship design's most important drivers: efficiency, effectiveness, reliability, and cost saving. Therefore, the aim of the research has been to obtain a new design methodology, applicable to the AES’ IPS, which is able to consider the systems as a whole, with all its internal interdependencies. The results of such research are depicted in this thesis work, as a sub-process to be integrated into IPS’s design process. In this thesis, a wide review of the state of the art is done, to allow understanding the context, why such innovative process is needed, and which innovative techniques can be used as an aid in design. Each point is discussed focusing on the aim of this thesis, thus presenting topics, bibliography, and personal evaluations tailored to direct the reader to comprehend the impact of the proposed design process. In particular, after a first chapter dedicated to the introduction of All Electric Ships, in which are described how such ships have evolved, and what are the most impacting applications, a reasoned discussion on the conventional ship-design process is given in the second chapter. In addition to that, an in-depth analysis of the IPS design is done, to explain the context in which the proposed innovative design process has to be integrated. Several examples of issues coming from the conventional design process are given, to motivate the proposal of a new design process. Not only the above mentioned design issues, but also the upcoming introduction of innovative distribution systems onboard ships and the recent emergence of new requirements, whose impact on IPS is significant, are motivations calling for a new design process. Due to that, an excursus of both these two topics is given in the third chapter, referring to recent literature and research activities. Chapter four is dedicated to the description of the tools that will be used to build the innovative design process. The first part is dedicated to dependability theory, which is able to give a systematic and coherent approach to the determination of faults effects on complex systems. Through dependability theory and its techniques, it is possible: to assess the effect of single components faults on the overall system; to assess all the possible causes of a given system failure; to evaluate mathematical figures related to the system in order to compare different design solutions; and to define where the designer must intervene to improve the system. The second part of the fourth chapter is dedicated to power system’s software simulators and hardware in the loop testing. In particular, the use of such systems as an aid in designing power systems is discussed, to allow comprehending why such tools have been integrated in the innovative design process developed. The fifth chapter is dedicated to the developed design process. Discussion is presented on how such process work, how it should be integrated in ship design process, and which is the impact it have on the design. In particular, the developed procedure implies both the application of dependability theory techniques (in particular Failure Tree Analysis), and the simulation of the dynamic behavior of the power system through a mathematical model of the system tailored on electromechanical transients. Finally, to demonstrate the applicability of the proposed procedure, in chapter six a case of study has been analyzed: the IPS of a Dynamic Positioned Offshore Oil & Gas drillship. This has been done due to the stringent requirements these ships have, whose impact on power system’s design is significant. The analysis of the IPS done through the Fault Tree Analysis technique is presented (though using a simplified detail level), followed by the calculation of several dependability indexes. Such results, together with applicable rules and regulations, have been used to define the input data for simulations, carried out using a mathematical model of the IPS built on purpose. Simulations outcomes have been used in turn to evaluate the dynamic processes bringing the system from relevant faults to failure, in order to improve the system’s response to the fault events.

The degree of complexity of the Integrated Power Systems (IPS) for All Electric Ships (AESs) promotes the need to review the design process from a dependability perspective. This paper suggests the application of dependability concepts and definitions to demonstrate adherence to requirements set forth by recent rules and regulations as regards to threat levels for shipboard electrical equipment. Guidelines on 1) how to properly decompose the IPS into conceptual subsystems; 2) how to define appropriate mathematical models suitable to investigate such sub-systems in a quantitative fashion using RBD (Reliability Block Diagram) and DRBD (Dynamic Reliability Block Diagram); 3) how to define the threat level the IPS (or the sub-system object of the study) should be tested against to prove its dependability are given. Such guidelines and investigation methods will be then applied on a part IPS of a recently built cruise vessel IPS, to illustrate their application to a real case study. A small sub-system will be decomposed according to guidelines, down to a level matching typical ship design practice, defining entities here called “atoms”. A model, showing interactions among atoms, will be prepared applying the RBD and DRBD techniques. Threats and associated performance levels will be derived from relevant international rules and regulations in force, and “translated” into operations with atoms and “propagation path lengths” within the model; acceptance criteria will, in this framework, be rendered as accept/reject conditions. Considerations regarding the adopted decomposition and applied techniques will be given in the conclusions.

Modern shipboard power systems are complex systems that rely on automation for their correct operation. The power and the control layers are strictly interrelated, and the data infrastructure is as critical as the power one. Future power systems exploiting resilient architectures (like the zonal medium voltage dc one) will rely more and more on controlled components (e.g., power electronics converters) to achieve their operational advantages, thus increasing the integration among data and power infrastructure. In such a context, the cyber security of the data infrastructure constitutes a critical point for the correct operation of the ship. Existing approaches mostly focus on enforcing the dependability on the cyber infrastructure, taking the power infrastructure as a given. However, ensuring the dependable operation of the power system means ensuring the supply of the onboard critical loads, which directly depends on the power system architecture, its design, and how it is operated. Therefore, it is critical to evaluate the effect on the power system of the malicious actions performed on the data infrastructure, and consider the possibility of acting on the power system itself to avoid or react to the threats (i.e., designing a dependable power system). In this paper such a holistic approach for the dependability enforcement of integrated cyber & power systems on ships is presented, discussing some of the solutions for the actual power systems and presenting an overview in regards to future medium voltage dc power systems.

The design of naval electric ships is a complex task, requiring the combined effort of several experts to provide a feasible, efficient, and effective design. In this regard, modifying an existing “parent” design still seems to be the easiest way to achieve cost reduction, acquisition process anticipation, and reduced technical risks. In last decades, many methods have been proposed to approach new design concepts. In this paper the authors support the use of set-based design (SBD) paradigm, especially at concept design stage, with the aim of identifying the best possible design enabling naval smart grid onboard. A synthesis is made of the primary properties of a number of baseline ships, to ease the metamodeling of ships” attributes. The latter are used to drive a random mathematical design model in a MADM framework. A large set of feasible designs is generated by an adaptive Monte Carlo sampling, followed by identification of the Pareto frontier and selection of the best designs through a rank ordering. In this paper it is shown how the best non-dominated designs can improve performance, while providing a significant advantage compared with the baseline ships in terms of lower onboard occupied volumes and weight. The latter can then be used for installing additional equipment onboard the vessel, like energy storage systems, in order to enable the integration of modern and future electric systems. A preliminary assessment of the achievable energy storage capacity enabled by the SBD approach is also given, to allow evaluating the order of magnitude of the advantages provided by the SBD approach.

Innovation in platform systems, as well as requirements on energy efficiency, are pushing towards the naval vessels' electrification. In fact, the power required by new platform systems, sensors, and new subsystems (for supporting the latter) is so big to make either full or hybrid electric-propelled ships the new target for all but the smallest vessels. This makes the power system a critical system, whose onboard integration requires a significant effort. Several different power system architectures (e.g., ac vs dc, or radial vs. zonal) can be used to power the loads, exploiting different pros and cons depending on their peculiarities. While most of the focus is usually on topics regarding electric and survivability performance, the integrability of such architectures in a ship is less addressed. In this paper, three power system configurations (ac radial, de radial, dc zonal) for a multirole frigate are designed. Their correct onboard integration is then assessed, by means of 3D parametric models, and comparisons in terms of volumes and weights of all the main electrical components are provided. Moreover, a discussion about the effect of the proposed configurations on the overall ship arrangement is given, pointing out some useful considerations about the opportunity of decoupling power system and hull's design processes, provided that some conditions are verified.

DC current is considered one of the most promising solutions for power systems distribution when the quota of power electronics converters to be installed is significant. Among DC distribution systems, zonal electrical distribution systems (ZEDSs) are the most dependable ones, thanks to the possibility to provide more than one path to supply vital loads. However, fault protection may be harder to achieve due to the DC ZEDS additional complexity in respects to conventional radial distribution system. Several IEEE Standards provide useful recommendations, but a comprehensive universally recognized framework for the design of protections in DC ZEDS is still lacking. Moreover, most of the approaches proposed in literature require specific converter architectures or complex control algorithms. Thus, in this paper an approach to the protection design for a MW-scale ZEDS based on Commercial-Off-The-Shelf (COTS) components is presented, aimed at setting a base level of performance complying with the Standards requirements. An example of its practical implementation is shown, discussing its behavior for some significant fault cases.

The increasing sensibility to the environment protection and emission reduction in the atmosphere is also influencing the maritime shipping. The more restrictive regulations on pollutants suggest the adoption of alternative onboard propulsion and energy generation systems. To this end, electrification is a promising solution for pollution reduction, giving some flexibility on the strategy to use for the onboard system. Moreover, a hybrid-electric system can be combined with the exploitation of green fuel, increasing the potential reduction of total emissions. However, the final efficiency, both in term of emission reduction and expected operative expenditure, is strongly influenced by the operative profile of the vessel. This is of utmost importance both in case of a newly designed vessel and in the retrofitting of an existing one. In the present work, the impact of electrification on a double-ended ferry is presented, taking into consideration the operative profile of the vessel derived from real navigation data.

The application of hybrid-electric solutions for the propulsion of pleasure crafts is increasing in recent years. The more restrictive regulations on pollutants and the growing sensibility of the people on greenness make this solution even more attractive. In case the craft is stationing in a marine equipped with renewable energy sources, the attractiveness of such a solution increases even more. Besides, a hybrid-electric configuration can reduce the consumptions and, consequently, the operative costs against an initial higher investment. Therefore, hybrid-electric propulsion could give a benefit to raise a stationary market, as the pleasure craft one. This study compares in terms of consumptions three different solutions, besides conventional diesel one, for the propulsion of a sailing yacht.

Over the past decades, the growth of international cruise tourism has been constant. The forecasts made by the Ships and Maritime Equipment Association indicates an annual growth rate of about 7%, leading to the triplication of global cruise passengers in the future (from 19 million passengers in 2010 to more than 54 million in 2035). To cover the future market needs, roughly six or eight new cruise ship buildings per year are planned until 2031, along with an increase in ship size (up to 8,000 persons accommodated onboard). These two facts have led to a growing interest in the passengers' safety, which has been addressed by the International Maritime Organization (IMO) Maritime Safety Committee (MSC) in 2000, with the launch of an initiative for adequate international safety regulation for large passenger ships. The first issue that emerged from such an initiative was the difficulty in safely evacuating passengers toward lifeboats during emergencies (in particular, during a fire or flooding). The solution proposed by the MSC requires that future passenger ships be designed to increase their intrinsic survivability, exploiting the concept that the ship is its own best lifeboat (thus avoiding abandoning the ship as much as possible). Such a solution was formalized in December 2006 through a package of amendments to regulations dedicated to large passenger ships and published in the 82nd session of the MSC, i.e., the Resolution MSC.216(82), commonly known as safe return to port regulations (SRtP).

Complex energy vessels such as large platforms or drillships require more efficient use of electrical power. As shipboard electrical systems become larger, problems and limits arise with the AC distribution architecture. Hybrid AC/DC onboard distribution systems are today available, which provide higher efficiency and redundancy. IEEE Std. 1662, 1709 and 1826 set technical rules and recommendations for the design of hybrid AC/DC shipboard electrical systems. Among these. Zonal Electrical Distribution Systems (ZEDS) are considered a next technological evolution, as they provide optimal power sharing (and energy storage) along with high reliability.The paper is organized in sections: 1. energy specifications of new electric ships/platforms endowed with ZEDS; recommended design of ZEDS, mostly focused on relevant integration issues such as 2. voltage/power controls and 3. protections for DC faults and electrical safety.

Direct current (DC) distribution is one of the most important enabling technologies for the future development of microgrids, due to the ease of interfacing DC components (e.g., batteries, photovoltaic systems, and native DC loads) to the grid. In these power systems, the large use of controlled power converters suggests the need of a careful analysis of system stability, as it can be impaired in particular conditions. Indeed, in DC power systems, a destabilizing effect can arise due to the presence of inductor/capacitor (LC) filtering stages (installed for power quality requirements) and high-bandwidth controlled converters, behaving as constant power loads (CPLs). This issue is even more critical when the CPL is potentially fed only by the battery, causing the DC bus to be floating. In this context, Lyapunov theory constitutes a valuable method for studying the system stability of DC microgrids feeding CPLs. Such a theory demonstrates how the region of asymptotic stability (RAS) shrinks as the state of charge of the battery diminishes (i.e., as the bus voltage decreases). Once the accuracy of the RAS is validated by comparing it to the real basin of attraction (BA), numerically derived using continuation methods, a smart power management of the CPL can be proposed to preserve the system stability even in the presence of a low bus voltage. Indeed, a suitably designed criterion for limiting the load power can guarantee the invariance of RAS and BA for each equilibrium point. An electric vehicle was used herein as a particular DC microgrid for evaluating the performance derating given by the power limitation.

The naval vessels electrification is one of the most important issues to be tackled, because not only it allows a more efficient operation, but also is considered a technological game changer. Indeed, electrification offers improved operational benefits and enables mission systems continuous upgrade. Several technologies can be used, each presenting different impact on ship performance. Thus, instruments to analyze and compare different designs must be used already during early-stage design, allowing an aware and reasoned selection. In this paper, a Computer System Integrator (CSI) software is presented and an application example is given, about the electrification of a naval logistic support ship.

The paramount voltage stability requirement may be difficult to accomplish in Medium-Voltage DC (MVDC) power systems supplying Constant Power Loads. Focusing on the importance in complying with such a requirement (because instability triggers ship blackout), the aim of the paper is to design the Active Damping voltage control of two MVDC power systems, where the interface role is covered either by Thyristor Converters (TCs) or by DC-DC converters (DCs). Although most scientific papers propose DCs as the best solution, a voltage dynamics comparison will demonstrate how it is possible to assure voltage stability also with a mature technology like TCs.

Nowadays, it is of paramount importance reducing the pollutant emission also of small crafts operating near the coast. This action is an essential issue to preserve the marine and coastal environment. In this paper, the design of a coastal-navigation multipurpose craft to be used in North Adriatic Sea area is presented. The particular service areas are characterized both by inland waterways with shallow and restricted waters and by open sea. Hull-form of the craft along with hybrid-electric propulsion have been properly designed in order to obtain an efficient solution for the different operational conditions. Specifically, the hybrid-electric power system has been conceived to allow the Zero Emission Mode navigation for a reasonable range.