State-of-the-Art Review on Shipboard Microgrids: Architecture, Control, Management, Protection, and Future Perspectives

: Shipboard microgrids (SBMGs) are becoming increasingly popular in the power industry due to their potential for reducing fossil-fuel usage and increasing power production. However, operating SBMGs poses signiﬁcant challenges due to operational and environmental constraints. To address these challenges, intelligent control, management, and protection strategies are necessary to ensure safe operation under complex and uncertain conditions. This paper provides a comprehensive review of SBMGs, including their classiﬁcations, control, management, and protection, as well as the most recent research statistics in these areas. The state-of-the-art SBMG types, propulsion systems, and power system architectures are discussed, along with a comparison of recent research contributions and issues related to control, uncertainties, management, and protection in SBMGs. In addition, a bibliometric analysis is performed to examine recent trends in SBMG research. This paper concludes with a discussion of research gaps and recommendations for further investigation in the ﬁeld of SBMGs, highlighting the need for more research on the optimization of SBMGs in terms of efﬁciency, reliability, and cost-effectiveness, as well as the development of advanced control and protection strategies to ensure safe and stable operation.


Background
Nowadays, the shipping industry has a significant impact on our environment. It is estimated that by 2050 the shipping industry will account for 12-18% of global anthropogenic carbon dioxide (CO 2 ) emissions [1]. Pollutants released by ships in coastal areas may lead to serious health problems for the populations nearby [2,3]. Therefore, the International Maritime Organization (IMO) has created effective regulations to limit ship emissions and protect the environment [1]. Such restrictions are designed to reduce emissions from ships and help the shipping industry significantly reduce its carbon footprint in the future [4]. However, 95% of vessels use diesel-fueled engines, making it difficult to meet the IMO's targets [5]. However, the evolution of the power electronics used in electrical power systems has led to developments in ship power systems [6]. Hence, the shipboard microgrid (SBMG) concept has been developed. Since alternative energy and electrical propulsion systems have evolved in the ship power system, warships have been equipped with electric-drive propulsion systems to maximize fuel efficiency and minimize noise pollution [7][8][9][10]. Such innovations have led to a growing variety of power and propulsion architectures that make it complex to balance efficiency with the ability to operate in a [26][27][28][29] 2016 ------

Contribution
In this paper, a comparison of SBMGs and land-based microgrids is first introduced. A review of the SBMG is presented based on classifying SBMGs in terms of their distribution system types, propulsion systems, and power system architectures, along with their developments. The review includes a detailed discussion of AC, DC, and hybrid AC/DC distribution systems. It also includes a detailed comparison of mechanical propulsion, electric propulsion, and hybrid propulsion systems and of radial, integrated, and zonal architectures. Moreover, a review of the most recent research in terms of the control, management, and protection of SBMGs is presented with a comparison of the contributions and issues of each study, and a co-occurrence analysis of these recent research works is performed to study the most recent trends concerning SBMGs. Finally, a review of uncertainties and issues relating to SBMGs is presented to compare the contributions of and issues considered in the most recent research. The main contributions of this research are as follows: • A comparison of SBMGs and land-based MGs; • An exploration of the classification of SBMGs based on distribution types, propulsion system types, and architecture types; • A comparison of the most recent studies on the control, management, and protection of SBMGs based on the contributions and shortcomings of each study; • A bibliometric analysis to study the most recent trends concerning SBMGs in terms of control, management, and protection; • An investigation of the uncertainties that may be encountered with SBMGs; • A presentation of the most recent new trends related to SBMGs.
The rest of the paper is organized as follows: Section 2 introduces the paper's motivation and a bibliometric study for the most recent research concerning SBMGs. Section 3 compares shipboard and land-based microgrids' IEEE/IEC standards. Section 4 discusses the structure and design of the SBMG and outlines a quest for a future integrated SBMG. Section 5 represents and differentiates between control methods applied for SBMGs. Section 6 discusses the uncertainties related to the SBMG, while Section 7 investigates the management techniques for the SBMG. Section 8 discusses the protection techniques for the SBMG, Section 9 represents examples of real ships worldwide, and Section 10 investigates the research gaps and recommendations concerning the SBMG. Finally, Section 11 concludes the paper.

Motivation
A systematic review of the Scopus database was performed to study the research progress concerning SBMGs to effectively employ the tightly coupled power components in SBMGs. SBMGs have to deal with many variable load demands caused by propulsion motors. In addition, a high pulsed power is required in naval ships due to the high-power weapons used. Therefore, it is essential to achieve an efficient and reliable power supply by properly coordinating different power sources with each other. Such coordinated control prevents voltages in SBMGs from falling below acceptable limits, and it distributes power between sources based on their characteristics. SBMGs allow for a 10% voltage tolerance, as defined in IEEE Standard 1709-2010 [55]. Thus, the main goals for SBMGs are not just to maintain the steady bus voltage but rather to ensure system reliability and survivability. Additionally, in modern ships, more than one source ensures power-supply reliability (e.g., diesel generators and batteries). Thus, coordinated control is required to prevent generators from overloading and to extend the ESS to avoid overloading. Ref. [56] aims to optimize and control ESSs to support critical mission loads and improve energy efficiency for multi-mission activities. It also involves developing real-time control algorithms for this purpose. Furthermore, maintaining the ship's degree of freedom is also essential to maintain its position during maneuvering under severe environmental conditions to maintain its destination. Moreover, it is essential to provide reliability and survivability for SBMGs under fault conditions to maintain the crews and keep people safe. So, a protection layer is very important for SBMGs, and ensuring one could be complex due to the existence of pulsed power loads (PPLs) that are characterized by power change rates and a large peak power which make it difficult to distinguish between fault currents and normal faults caused by PPLs. A shipboard power and cooling system model with a PPL (electromagnetic railgun) is presented in [57] which implemented traditional and exergy-based control schemes. Moreover, it is difficult to design a grounding system for an SBMG because it is considered an isolated power system. Furthermore, designing an EMS to achieve an optimization objective considering the system constraints is mandatory to coordinate different power components with different time scales. The objective of the optimization problem can be minimizing operation cost, minimizing fuel consumption, or minimizing the power losses. The challenges of multi-objective EMSs can be found in [58]. Therefore, the control, management, and protection of SBMGs are essential issues that should be reviewed. Figure 1 shows the statistics of the papers published in the last ten years on control, uncertainties, management, and protection in IEEE and Elsevier's magazines and journals. plex due to the existence of pulsed power loads (PPLs) that are characterized by power change rates and a large peak power which make it difficult to distinguish between fault currents and normal faults caused by PPLs. A shipboard power and cooling system model with a PPL (electromagnetic railgun) is presented in [57] which implemented traditional and exergy-based control schemes. Moreover, it is difficult to design a grounding system for an SBMG because it is considered an isolated power system. Furthermore, designing an EMS to achieve an optimization objective considering the system constraints is mandatory to coordinate different power components with different time scales. The objective of the optimization problem can be minimizing operation cost, minimizing fuel consumption, or minimizing the power losses. The challenges of multi-objective EMSs can be found in [58]. Therefore, the control, management, and protection of SBMGs are essential issues that should be reviewed. Figure 1 shows the statistics of the papers published in the last ten years on control, uncertainties, management, and protection in IEEE and Elsevier's magazines and journals. Therefore, one of the main objectives of this research was to classify and analyze the SBMG integration projects that are widely cited to fully understand their developments. To this end, we categorized and analyzed keywords used in a selection process consisting of the ship power system (SPS), control, protection, and energy management. Throughout the process, numerous publications were found but only those that passed the criteria were chosen by analyzing each publication's title, focus, and contributions.
The primary research identified 1815 articles in the Scopus database. The number of articles decreased to 1341 by selecting the year range of 2010 to 2021. Then, by selecting article papers only, the number of articles decreased to 473. These were then reduced to 390 by selecting English-language publications only. Thus, 327 papers were collected by selecting Engineering and Energy subject areas. The collected data were analyzed by determining the number of articles in each year from 2010 to 2021, as shown in Figure 2. Based on the provided data, we can see that the number of articles related to the research topic has been steadily increasing over the years. In 2010, there were only 5 articles published, but this number increased to 76 in 2021. The highest number of articles (57) was published in 2020, followed closely by 2021, with 76 articles. This suggests that research interest in the topic is growing and that more researchers are working on it. Article numbers for different journal publishers are shown in Figure 3. It was concluded that IEEE has the highest frequency of publication, with 177 articles, followed by Elsevier, with 53 articles. MDPI, IET, Springer, and Taylor and Francis Online have relatively lower publication frequencies, with 25, 13, 12, and 9 articles, respectively. The bibliometric analysis suggests Therefore, one of the main objectives of this research was to classify and analyze the SBMG integration projects that are widely cited to fully understand their developments. To this end, we categorized and analyzed keywords used in a selection process consisting of the ship power system (SPS), control, protection, and energy management. Throughout the process, numerous publications were found but only those that passed the criteria were chosen by analyzing each publication's title, focus, and contributions.
The primary research identified 1815 articles in the Scopus database. The number of articles decreased to 1341 by selecting the year range of 2010 to 2021. Then, by selecting article papers only, the number of articles decreased to 473. These were then reduced to 390 by selecting English-language publications only. Thus, 327 papers were collected by selecting Engineering and Energy subject areas. The collected data were analyzed by determining the number of articles in each year from 2010 to 2021, as shown in Figure 2. Based on the provided data, we can see that the number of articles related to the research topic has been steadily increasing over the years. In 2010, there were only 5 articles published, but this number increased to 76 in 2021. The highest number of articles (57) was published in 2020, followed closely by 2021, with 76 articles. This suggests that research interest in the topic is growing and that more researchers are working on it. Article numbers for different journal publishers are shown in Figure 3. It was concluded that IEEE has the highest frequency of publication, with 177 articles, followed by Elsevier, with 53 articles. MDPI, IET, Springer, and Taylor and Francis Online have relatively lower publication frequencies, with 25, 13, 12, and 9 articles, respectively. The bibliometric analysis suggests a growing interest in the field of study, as well as the importance of choosing reputable journals and publishers for disseminating research findings. The distribution of the top 15 keywords from 2021 to 2010 is shown in Figure 4. The keyword "Energy efficiency" was the most frequently used keyword in 2021 (57 articles), followed by "Ships" (51 articles) and "SPS" (24 articles). In 2020, "Ships" was the most frequently used keyword (47 articles), followed by "ESS" (22 articles) and "Ship propulsion" (13 articles). Similarly, "Ships" was the most frequently used keyword in 2019 (35 articles), followed by "Electric ship equipment" (15 articles) and "Ship propulsion" (11 articles). Overall, "Ships" was the most frequently used keyword from 2021 to 2010, indicating that the maritime industry is a significant area of research. Other popular keywords included "Energy efficiency", "Ship propulsion", and "ESS", suggesting a focus on sustainable energy solutions in the industry. Additionally, there was a noticeable increase in the number of articles related to "SPS" in 2021 compared to previous years, indicating growing interest in this area of research. The ten authors with the most publications concerning SBMGs are shown in Figure 5. It can be concluded that Guerrero, J.M. and Khooban, M.H. are the top two most productive authors, with 21 and 19 publications, respectively. This suggests that they are likely experts in their respective fields and have significantly contributed to the literature. Additionally, the fact that the remaining eight authors have published between 9 and 13 articles indicates that they too are prolific and influential researchers. Overall, this analysis suggests that these ten authors are important figures in the field and that their research has significantly impacted the academic community. journals and publishers for disseminating research findings. The distribution of the to keywords from 2021 to 2010 is shown in Figure 4. The keyword "Energy efficiency" the most frequently used keyword in 2021 (57 articles), followed by "Ships" (51 arti and "SPS" (24 articles). In 2020, "Ships" was the most frequently used keyword (47 cles), followed by "ESS" (22 articles) and "Ship propulsion" (13 articles). Simil "Ships" was the most frequently used keyword in 2019 (35 articles), followed by "Ele ship equipment" (15 articles) and "Ship propulsion" (11 articles). Overall, "Ships" wa most frequently used keyword from 2021 to 2010, indicating that the maritime indus a significant area of research. Other popular keywords included "Energy efficien "Ship propulsion", and "ESS", suggesting a focus on sustainable energy solutions i industry. Additionally, there was a noticeable increase in the number of articles relat "SPS" in 2021 compared to previous years, indicating growing interest in this area o search. The ten authors with the most publications concerning SBMGs are shown in Fi 5. It can be concluded that Guerrero, J.M. and Khooban, M.H. are the top two most ductive authors, with 21 and 19 publications, respectively. This suggests that they likely experts in their respective fields and have significantly contributed to the litera Additionally, the fact that the remaining eight authors have published between 9 an articles indicates that they too are prolific and influential researchers. Overall, this ana suggests that these ten authors are important figures in the field and that their rese has significantly impacted the academic community.  journals and publishers for disseminating research findings. The distribution of the top 15 keywords from 2021 to 2010 is shown in Figure 4. The keyword "Energy efficiency" was the most frequently used keyword in 2021 (57 articles), followed by "Ships" (51 articles) and "SPS" (24 articles). In 2020, "Ships" was the most frequently used keyword (47 articles), followed by "ESS" (22 articles) and "Ship propulsion" (13 articles). Similarly, "Ships" was the most frequently used keyword in 2019 (35 articles), followed by "Electric ship equipment" (15 articles) and "Ship propulsion" (11 articles). Overall, "Ships" was the most frequently used keyword from 2021 to 2010, indicating that the maritime industry is a significant area of research. Other popular keywords included "Energy efficiency", "Ship propulsion", and "ESS", suggesting a focus on sustainable energy solutions in the industry. Additionally, there was a noticeable increase in the number of articles related to "SPS" in 2021 compared to previous years, indicating growing interest in this area of research. The ten authors with the most publications concerning SBMGs are shown in Figure  5. It can be concluded that Guerrero, J.M. and Khooban, M.H. are the top two most productive authors, with 21 and 19 publications, respectively. This suggests that they are likely experts in their respective fields and have significantly contributed to the literature. Additionally, the fact that the remaining eight authors have published between 9 and 13 articles indicates that they too are prolific and influential researchers. Overall, this analysis suggests that these ten authors are important figures in the field and that their research has significantly impacted the academic community.   The recent research papers on control, management, and protection from the year 2021 to the year 2016 were compared in terms of their contributions and shortcomings and analyzed using VOS viewer software, as described in the sections below. This analysis was performed only for the authors' keywords, as they have the greatest analytical value because they represent the author's preferences.  The recent research papers on control, management, and protection from the year 2021 to the year 2016 were compared in terms of their contributions and shortcomings and analyzed using VOS viewer software, as described in the sections below. This analysis was performed only for the authors' keywords, as they have the greatest analytical value because they represent the author's preferences.

SBMG Versus Land-Based MG IEC/IEEE Standards
The standards for shipboard microgrids, such as IEC/IEEE 80005-1 and IEEE 2030.8, have several unique requirements and considerations compared to standards for landbased microgrids. Some of these differences include: • Environmental considerations: Shipboard microgrids are exposed to harsh environmental conditions, such as high humidity, saltwater, and vibrations, which can affect the performance of electrical components. The standards for shipboard microgrids consider these environmental factors and provide guidance for equipment selection and testing to ensure reliable operation [59]. • Safety requirements: Shipboard microgrids must adhere to stringent safety requirements, particularly regarding shock and fire hazards. Standards such as IEC/IEEE 80005-1 provide guidance for the design and testing of shipboard electrical systems to ensure they meet these safety requirements [60]. • Power quality considerations: Due to the sensitive electrical equipment on board ships, power quality is of the utmost importance. Standards for shipboard   The recent research papers on control, management, and protection from the year 2021 to the year 2016 were compared in terms of their contributions and shortcomings and analyzed using VOS viewer software, as described in the sections below. This analysis was performed only for the authors' keywords, as they have the greatest analytical value because they represent the author's preferences.

SBMG Versus Land-Based MG IEC/IEEE Standards
The standards for shipboard microgrids, such as IEC/IEEE 80005-1 and IEEE 2030.8, have several unique requirements and considerations compared to standards for landbased microgrids. Some of these differences include: • Environmental considerations: Shipboard microgrids are exposed to harsh environmental conditions, such as high humidity, saltwater, and vibrations, which can affect the performance of electrical components. The standards for shipboard microgrids consider these environmental factors and provide guidance for equipment selection and testing to ensure reliable operation [59]. • Safety requirements: Shipboard microgrids must adhere to stringent safety requirements, particularly regarding shock and fire hazards. Standards such as IEC/IEEE 80005-1 provide guidance for the design and testing of shipboard electrical systems to ensure they meet these safety requirements [60]. • Power quality considerations: Due to the sensitive electrical equipment on board ships, power quality is of the utmost importance. Standards for shipboard

SBMG Versus Land-Based MG IEC/IEEE Standards
The standards for shipboard microgrids, such as IEC/IEEE 80005-1 and IEEE 2030.8, have several unique requirements and considerations compared to standards for land-based microgrids. Some of these differences include:

•
Environmental considerations: Shipboard microgrids are exposed to harsh environmental conditions, such as high humidity, saltwater, and vibrations, which can affect the performance of electrical components. The standards for shipboard microgrids consider these environmental factors and provide guidance for equipment selection and testing to ensure reliable operation [59]. • Safety requirements: Shipboard microgrids must adhere to stringent safety requirements, particularly regarding shock and fire hazards. Standards such as IEC/IEEE 80005-1 provide guidance for the design and testing of shipboard electrical systems to ensure they meet these safety requirements [60]. • Power quality considerations: Due to the sensitive electrical equipment on board ships, power quality is of the utmost importance. Standards for shipboard microgrids, such as IEEE 2030.8, provide guidelines for maintaining stable power quality in the presence of variable loads and power sources [61].

•
Operational considerations: Shipboard microgrids have unique operational considerations compared to land-based microgrids. For example, shipboard microgrids may need to operate in the islanded mode for extended periods, and there may be limited access to maintenance resources during operation. Standards for shipboard microgrids provide guidance for these operational considerations to ensure reliable and safe operation [61].
Overall, while there may be some overlap between standards for shipboard and land-based microgrids, the unique environmental, safety, and operational considerations of shipboard microgrids require specific guidance and requirements. The applicable IEC/IEEE standards for SBMGs versus land-based MGs are shown in Table 2.

Classification of SBMGs
SBMGs can be classified according to their types, propulsion systems, and architectures. SBMG types may be AC, DC, or hybrid AC/DC. SBMGs have a variety of propulsion systems: the mechanical system was the first propulsion system, then it was developed to the electrical propulsion system, and, finally, the hybrid propulsion system was developed. SBMG networks can be gathered into various power system architectures, including radial, integrated, and zonal systems [66,67]. A comparison of SBMGs for each classification will be discussed in the following subsections.

SBMG Classification According to Distribution System Types
The SBMG can be classified according to its distribution system types as follows.

AC Shipboard Microgrid
The AC-SBMG was used when ships began to be electrified. In this type, the diesel generator is connected to the AC bus through breakers to deliver the power to the propulsion load and 50/60 Hz transformers are used to integrate the service loads, as shown in Figure 6a. The AC-SBMG ensures system continuity and improves fuel efficiency. However, there are many power quality issues, such as harmonic currents, frequency deviation, and unbalanced voltages due to high-power and propulsion load existence, making the frequency and voltage control of generators a vital issue. So, such a distribution system type is not a good solution for SBMGs compared to other types [68].

DC Shipboard Microgrid
The DC-SBMG has become more prevalent in recent years due to the existence of various new energy resources. They are compatible with the prime movers operating at their optimal speed, reducing fuel consumption and increasing fuel efficiency. The DC system might be pivotal for ensuring ships' electrical supply continuity, which is required for various marine operations [69]. In DC-SBMGs, all the sources are connected to AC/DC converters connected to the DC bus, which delivers the power to the load, as shown in Figure 6b. This configuration allows the high-speed generators and high-speed gas turbines to be used, making it possible to regulate generator speed without causing frequency issues [70]. However, SBMGs present the challenge of designing their protection systems. The lack of zero-crossing current makes the DC breaker disconnection more complex for large currents than the AC breakers. Though the DC-SBMG has protection challenges, it has a lot of merits over the AC-SBMG, as shown in Table 3. The table illustrates a comparison of DC and AC shipboard microgrids. Based on the information provided in the table, a potential novel criterion could be "Scalability", which refers to the ability of the shipboard microgrid to expand or contract its capacity and accommodate additional loads or sources. This criterion could be relevant for both AC and DC shipboard microgrids and could impact their suitability for future needs and expansion. AC shipboard microgrids typically use a centralized architecture with a large AC bus that distributes power throughout the ship. This makes them well-suited to handling large loads and accommodating additional loads as needed. However, adding additional generation sources can be more challenging, as the AC power must be synchronized with the existing system. This can require additional control systems and can limit the flexibility of the microgrid. DC shipboard microgrids, on the other hand, typically use a decentralized architecture, with multiple smaller DC buses distributed throughout the ship. This makes them more flexible and easier to expand as additional loads or generation sources are added. In addition, DC power does not require synchronization, which simplifies the control system and reduces the need for additional equipment. Overall, both AC and DC shipboard microgrids can be scalable, but the specific advantages and challenges will depend on the design of the microgrid and the ship's power needs.
Smart Cities 2023, 6, FOR PEER REVIEW 9 the frequency and voltage control of generators a vital issue. So, such a distribution system type is not a good solution for SBMGs compared to other types [68].

DC Shipboard Microgrid
The DC-SBMG has become more prevalent in recent years due to the existence of various new energy resources. They are compatible with the prime movers operating at their optimal speed, reducing fuel consumption and increasing fuel efficiency. The DC system might be pivotal for ensuring ships' electrical supply continuity, which is required for various marine operations [69]. In DC-SBMGs, all the sources are connected to AC/DC converters connected to the DC bus, which delivers the power to the load, as shown in Figure 6b. This configuration allows the high-speed generators and high-speed gas turbines to be used, making it possible to regulate generator speed without causing frequency issues [70]. However, SBMGs present the challenge of designing their protection systems. The lack of zero-crossing current makes the DC breaker disconnection more complex for large currents than the AC breakers. Though the DC-SBMG has protection challenges, it has a lot of merits over the AC-SBMG, as shown in Table 3. The table illustrates a comparison of DC and AC shipboard microgrids. Based on the information provided in the table, a potential novel criterion could be "Scalability", which refers to the ability of the shipboard microgrid to expand or contract its capacity and accommodate additional loads or sources. This criterion could be relevant for both AC and DC shipboard microgrids and could impact their suitability for future needs and expansion. AC shipboard microgrids typically use a centralized architecture with a large AC bus that distributes power throughout the ship. This makes them well-suited to handling large loads and accommodating additional loads as needed. However, adding additional generation sources can be more challenging, as the AC power must be synchronized with the existing system. This can require additional control systems and can limit the flexibility of the microgrid. DC shipboard microgrids, on the other hand, typically use a decentralized architecture, with multiple smaller DC buses distributed throughout the ship. This makes them more flexible and easier to expand as additional loads or generation sources are added. In addition, DC power does not require synchronization, which simplifies the control system and reduces the need for additional equipment. Overall, both AC and DC shipboard microgrids can be scalable, but the specific advantages and challenges will depend on the design of the microgrid and the ship's power needs.    The advancements in power distribution technology, with its ability to tap power from shore-based sources and RESs, have created more efficient and clean power systems onboard ships and vessels. RESs (such as wind and solar) and traditional energy sources (such as gas and oil) can be combined in a hybrid system to create a more sustainable maritime industry. A multi-energy hybrid power system can provide economical and eco-friendly energy for ships. Such a system provides an alternative energy source with the potential to overcome the limitations of using a single source [4]. In a hybrid system, there are two buses, a DC bus connected to a fuel cell or any other DC energy source and an AC bus connected to distributed generators, as shown in Figure 7, which represents the power system structure of the Viking Lady after the integration of a fuel cell by Wärtsilä [35]. maritime industry. A multi-energy hybrid power system can provide economical and ecofriendly energy for ships. Such a system provides an alternative energy source with the potential to overcome the limitations of using a single source [4]. In a hybrid system, there are two buses, a DC bus connected to a fuel cell or any other DC energy source and an AC bus connected to distributed generators, as shown in Figure 7, which represents the power system structure of the Viking Lady after the integration of a fuel cell by Wärtsilä [35].

SBMG Classification According to Propulsion Systems
As explained before, the propulsion systems used in ships are classified as mechanical, electrical, and hybrid propulsion systems. A description of each type and its main advantages and challenges will be presented in the following.

Mechanical Propulsion System
From oars and sails to mechanical propulsion, ships have significantly developed in the last two centuries. The primary way that ships were propelled was by steam engines, but this method was not always the most popular. Reciprocal engines and turbines were also used for air and water travel up until the early part of the 20th century. A mechanical propulsion system highly affects the design speed. The most efficient operating range of the diesel engine (DE) is between 80 and 100 percent of the top speed [4]. In this range, the fuel cost is reduced, and the engine emissions are minimized. This technology comes with only three power conversion stages: an engine, a gearbox, and a propeller, leading to lower conversion losses. Despite its benefits, mechanical propulsion faces several challenges, including limited maneuverability due to the engine's operating profile, increased stress on the engine leading to higher maintenance requirements, inefficiency and high emissions at low speeds, and lower dependability compared to electrical propulsion due to the risk of a breakdown in the drive train components. While different control strategies can mitigate some of these challenges, they cannot be entirely eliminated [4].

Electrical Propulsion System
Since the evolution of solid-state power electronics and digital controllers in the 1980s, it has become possible to electrify a ship with an electric propulsion system. Along with variable speed drives, field-oriented control and direct torque control have led to modern shipboard propulsion. In 1988, electric propulsion was introduced on the Queen

SBMG Classification According to Propulsion Systems
As explained before, the propulsion systems used in ships are classified as mechanical, electrical, and hybrid propulsion systems. A description of each type and its main advantages and challenges will be presented in the following.

Mechanical Propulsion System
From oars and sails to mechanical propulsion, ships have significantly developed in the last two centuries. The primary way that ships were propelled was by steam engines, but this method was not always the most popular. Reciprocal engines and turbines were also used for air and water travel up until the early part of the 20th century. A mechanical propulsion system highly affects the design speed. The most efficient operating range of the diesel engine (DE) is between 80 and 100 percent of the top speed [4]. In this range, the fuel cost is reduced, and the engine emissions are minimized. This technology comes with only three power conversion stages: an engine, a gearbox, and a propeller, leading to lower conversion losses. Despite its benefits, mechanical propulsion faces several challenges, including limited maneuverability due to the engine's operating profile, increased stress on the engine leading to higher maintenance requirements, inefficiency and high emissions at low speeds, and lower dependability compared to electrical propulsion due to the risk of a breakdown in the drive train components. While different control strategies can mitigate some of these challenges, they cannot be entirely eliminated [4].

Electrical Propulsion System
Since the evolution of solid-state power electronics and digital controllers in the 1980s, it has become possible to electrify a ship with an electric propulsion system. Along with variable speed drives, field-oriented control and direct torque control have led to modern shipboard propulsion. In 1988, electric propulsion was introduced on the Queen Elizabeth II to improve emissions reductions and fuel efficiency and give excellent maneuverability [78]. Since then, the trend has begun to move from mechanical to electric propulsion in many vessels. The electric propulsion system offers several benefits over traditional mechanical propulsion systems, including improved fuel efficiency and reduced noise emissions. Electric motors are simpler than mechanical engines, resulting in a longer lifespan and less maintenance. Additionally, with the proper control system, electric propulsion systems have high availability [4,79,80]. However, there are also challenges associated with electrical propulsion systems, such as lower energy efficiency and higher losses in conversion stages compared to mechanical systems [12,81].
In uncontrolled SBMGs, voltage and frequency swings can often occur under fault conditions, which may cause the switching off of the electrical systems and hence affect the system's reliability and availability.

Hybrid Propulsion System
Electric propulsion systems can reduce fuel consumption, but they are not costeffective for smaller vessels because of the extra costs required for electrical equipment and power conversion components. Ships powered by a hybrid propulsion system have the benefits of both electrical and mechanical propulsion. This improves the efficiency of electric power at low speeds and saves the fuel in diesel engines at high speed because electrical propulsion is used for low and intermediate speeds, while mechanical propulsion is used for higher speeds. The generator's mechanical drive engine allows the capacity to be generated either by the electric generator or generator sets. Hybrid designs often require trade-offs between electrical power output and physical size, or between durability and fuel efficiency.
The overall system structure of the ship can be divided into three different types: series, parallel, and hybrid series-parallel. In the series SBMG, the power from a combination of sources is transferred to the system load through a bus bar, as shown in Figure 8a. It has a variety of modes, such as a fuel-cell working mode, a generator-set working mode, and a combined power-supply working mode. A parallel power system, shown in Figure 8b, is a mechanical and electrical propulsion mix. Mechanical and electrical propulsion are combined through a coupling device to either operate independently or couple the operation of their components. A coupler can transfer mechanical power from the main engine to an operating motor/generator. On the electric propulsion side, energy is provided by various energy sources, such as wind, hydro, solar, wastewater heaters, and batteries, through a DC bus that delivers power to the load. Hybrid series-parallel SBMGs, shown in Figure 8c, provide an opportunity to get the best of both series and parallel SBMGs. Since the two different kinds of coupling devices exist for mechanical and electrical propulsion, they can be parallelized so that the main engine can drive the generator [1]. Hybrid propulsion systems are always a challenge. They require an optimal power management strategy to transfer electrical energy between mechanical drive motors and battery storage units. While implementing the control strategy, the main challenge is to balance all the system components.

SBMG Classification According to Power System Architectures
Radial, ring, and zonal systems are the main architectures related to the SBMG, as described in the following subsections.

Radial Architecture
Traditional SBMGs have radial structures that are recommended by IEEE Std. 1709-2010 [77], and they are designed to provide both propulsion and service loads by using separate generators for each load type (segregated architecture) [34]. However, with the

SBMG Classification According to Power System Architectures
Radial, ring, and zonal systems are the main architectures related to the SBMG, as described in the following subsections.

Radial Architecture
Traditional SBMGs have radial structures that are recommended by IEEE Std. 1709-2010 [77], and they are designed to provide both propulsion and service loads by using separate generators for each load type (segregated architecture) [34]. However, with the evolution of power electronics, it is becoming more common to integrate propulsion and service loads into a single power system [35], as shown in Figure 9a. On the other hand, when the ship is stopped or moving slowly, the propulsion power system generates excess unusable power. Therefore, the overall system efficiency is very low, as the service loads require immense power in modern ships. In general, this structure is uneconomical and has less efficiency.

SBMG Classification According to Power System Architectures
Radial, ring, and zonal systems are the main architectures related to the SBMG, as described in the following subsections.

Radial Architecture
Traditional SBMGs have radial structures that are recommended by IEEE Std. 1709-2010 [77], and they are designed to provide both propulsion and service loads by using separate generators for each load type (segregated architecture) [34]. However, with the evolution of power electronics, it is becoming more common to integrate propulsion and service loads into a single power system [35], as shown in Figure 9a. On the other hand, when the ship is stopped or moving slowly, the propulsion power system generates excess unusable power. Therefore, the overall system efficiency is very low, as the service loads require immense power in modern ships. In general, this structure is uneconomical and has less efficiency.

Ring Architecture
The ring distribution system, shown in Figure 9b, is also used in a few cases for SBMGs. It consists of the bus-tie switches that connect multiple DC buses that are closed in normal operation. This configuration has higher reconfigurability and survivability

Ring Architecture
The ring distribution system, shown in Figure 9b, is also used in a few cases for SBMGs. It consists of the bus-tie switches that connect multiple DC buses that are closed in normal operation. This configuration has higher reconfigurability and survivability than the radial configuration. When a fault in the distribution bus occurs, the nearest circuit breakers are automatically disconnected, and the rest of the load centers keep working as normal. In this configuration, the loads have only one link to the bus, making it more susceptible to faults in critical loads. The ring architecture is a transition between radial and zonal distribution, and fewer power systems use it.

Zonal Architecture
The extreme physical and electrical separation, the limitations of the generator's output, and its inertia have made the system inherently fragile, causing it to fail when exposed to high loads and strain. A reconfiguration strategy called zonal structure was developed to address the ship's key risks and vulnerabilities and maintain it in readiness to fulfill its mission, as shown in Figure 9c. This reconfiguration is more complex when the zonal distribution structure saves weight and space [82].

Control Techniques in SBMGs
The control techniques in the SBMG are mainly classified as dynamic positioning control (DPC) techniques and converter control techniques. Figure 10 illustrates the classification of control techniques applied to the SBMG. The following subsections discuss the control methods for the SBMG and present a comparison of the contributions and shortcomings of the most recent research works.

Dynamic Positioning Control (DPC)
A DPC system is used in marine engineering to maintain a ship's position and heading. In the presence of storm waterspouts or hurricane hits, the ship's position may be altered from its desired position. A DPC system can adjust the ship's positioning regardless of the influence of waves or current conditions. It automatically compensates for these forces and keeps the ship in a stable position with the help of the propulsion systems. DPCs have fantastic propulsion systems, such as the thruster subsystem. This part of the ship comprises four principal propellers, two in the front and two in the back, and eight bow and stern thrusters, all controlled by the DPC. All of these thrusters move water such that it provides forward movement for the ships [48]. In case of the worst single failure, it

Dynamic Positioning Control (DPC)
A DPC system is used in marine engineering to maintain a ship's position and heading. In the presence of storm waterspouts or hurricane hits, the ship's position may be altered from its desired position. A DPC system can adjust the ship's positioning regardless of the influence of waves or current conditions. It automatically compensates for these forces and keeps the ship in a stable position with the help of the propulsion systems. DPCs have fantastic propulsion systems, such as the thruster subsystem. This part of the ship comprises four principal propellers, two in the front and two in the back, and eight bow and stern thrusters, all controlled by the DPC. All of these thrusters move water such that it provides forward movement for the ships [48]. In case of the worst single failure, it is essential to check if there is enough thruster capacity and power to maintain the desired ship position [83]. Additionally, estimating the sea state parameters is necessary to help control methods and the decision-making process [84]. The DPC uses both feed-forward and closed control loops to achieve the best performance possible. The recent research related to DPC methods is shown in Table 4. Based on the table, it can be concluded that there are several shortcomings and limitations associated with the proposed control schemes for autonomous surface vessels. These limitations include the lack of handling of input saturation problems, the use of only position and heading measurements for control, and the failure to consider actuator losses and thruster saturation constraints. Other limitations include not considering stochastic disturbances and environmental disturbances in trajectory tracking. Additionally, some of the proposed control schemes require complex calculations, which may be challenging to implement in practice. Despite these limitations, it is important to note that developing autonomous surface vessel control schemes is a rapidly evolving field, and researchers are continually exploring new methods to address these challenges. Thus, future research may lead to the development of more robust and effective control schemes that can overcome the current limitations and shortcomings associated with autonomous surface vessel control. Figure 11 represents the co-occurrence analysis for these studies. The analysis combines the author's keywords in a single group called "dynamic positioning system" which contains the main keywords, namely, "vectorial backstepping", "dynamical system", "environmental disturbance", "fuzzy control", "ship handling", "control allocation", and "uncertainty analysis". Based on the given set of keywords, it appears that the main focus of the analysis is on the dynamic positioning of marine surface vessels, with a strong emphasis on control systems, optimization, and performance assessment. The keywords also suggest the use of various learning algorithms and techniques, such as reinforcement learning, deep learning, and neural networks, to enhance the performance of the control systems. There is also a significant emphasis on uncertainty analysis, fault detection, and fault tolerance in the design of control systems. The keywords suggest that there is ongoing research in the field of marine surface vessel control systems and that there is a need to improve their energy efficiency and environmental impact. The load dynamics effect is not investigated Figure 11. Co-occurrence analysis for dynamic positioning control articles.

Converter Control Methods
Due to the developments in power electronics, two types of converters are used in SBMGs: DC/DC and AC/DC converters. There are many types of AC/DC converters, such

Converter Control Methods
Due to the developments in power electronics, two types of converters are used in SBMGs: DC/DC and AC/DC converters. There are many types of AC/DC converters, such as pulse width modulation (PWM) force-commutated rectifiers, diode rectifiers, DC/DC converters cascaded to diode rectifiers, and thyristor phase-controlled rectifiers [104]. They can interface with different energy sources, provide power flow control, and regulate the SBMG's voltage and current. This ensures the stability, reliability, and efficiency of any DC SBMG. Several challenges are encountered while controlling the bidirectional DC-DC converters in DC-SBMGs. Some of these difficulties are constant power load (CPL), pulsed power load (PPL), and power quality [105]. Static var compensators solve these problems by using fixed capacitors-thyristor-controlled reactors and thyristor-switched capacitors [106]. They can effectively reduce power quality issues but need an appropriate control method for this. Many researchers have solved such problems by providing different control methods, as shown in Table 5. Based on the table, it can be concluded that model predictive control (MPC) and load frequency control (LFC) are the two most commonly used algorithms in power system control. These algorithms offer various advantages, such as improving power reliability and efficiency, mitigating the negative effects of pulsed power loads, and minimizing frequency and power oscillations. Additionally, MPC has been used to dampen steady-state deviations, reduce HESS losses, and provide stability for DC SBMGs. LFC, on the other hand, has shown better transient and steady-state performance than all other control methods. Figure 12 represents the co-occurrence analysis for these studies. The analysis divides the author's keywords into six clusters with the main keywords for each cluster. We can conclude that the topics of energy storage, control and optimization, transportation, emissions and pollution, and miscellaneous topics are the most common research areas from the given keywords. Furthermore, it is clear that these topics are highly interrelated, and advancements in one area can have a significant impact on others. From Figure 12, it can be concluded that the integration of RESs and ESSs in ship power systems increases the complexity of the control systems, and there are various methods used to control SBMGs, as shown in Table 5.
Smart Cities 2023, 6, FOR PEER REVIEW 2 complexity of the control systems, and there are various methods used to control SBMGs as shown in Table 5.

Uncertainties in SBMGs
Ships are exposed to different unexpected problems, including severe environmenta conditions, change in position according to the movement of the waves, the condition o the sea and the wind, and the existence of time-varying uncertain CPL in SBMG. Due to such uncertainty problems, SBMGs face many stability issues that are discussed below: Weather-condition uncertainties: Uncertainties during the navigation route due to weather conditions, such as wind (speed and direction) and waves (height and length) lead to uncertain navigation resistance. The ship is exposed to different propulsion load under various navigation uncertainties [139,140].
PV system uncertainty: The PV output varies with ship movement even if the sola radiation is constant, such that the PV system may undergo partial shading, lessening it efficiency [141].
Wind uncertainty: The wind output varies with the change in weather condition faced by the ship.
Uncertain time-varying loads: DC SBMGs have active loads, such as actuators and ESSs, that are interconnected. These loads are usually controlled by converters. If the load have high bandwidth and control performance, they consume power that is independen of the bus voltage. These loads are classified as CPL and behave like incremental negativ     -Apply a method that enhances the system reliability and quality of the control -Apply a genetic algorithm (GA) to obtain optimal control system parameters -Apply a hierarchical emergency control -Dynamic system model is not investigated

Uncertainties in SBMGs
Ships are exposed to different unexpected problems, including severe environmental conditions, change in position according to the movement of the waves, the condition of the sea and the wind, and the existence of time-varying uncertain CPL in SBMG. Due to such uncertainty problems, SBMGs face many stability issues that are discussed below: Weather-condition uncertainties: Uncertainties during the navigation route due to weather conditions, such as wind (speed and direction) and waves (height and length), lead to uncertain navigation resistance. The ship is exposed to different propulsion loads under various navigation uncertainties [139,140].
PV system uncertainty: The PV output varies with ship movement even if the solar radiation is constant, such that the PV system may undergo partial shading, lessening its efficiency [141].
Wind uncertainty: The wind output varies with the change in weather conditions faced by the ship.
Uncertain time-varying loads: DC SBMGs have active loads, such as actuators and ESSs, that are interconnected. These loads are usually controlled by converters. If the loads have high bandwidth and control performance, they consume power that is independent of the bus voltage. These loads are classified as CPL and behave like incremental negative impedances. Hence, the existence of time-varying uncertain CPL in SBMGs can threaten the stability of SBMGs [142,143].
Dynamic interactions of power converters: The complex interactions between power converters in SBMGs cause dynamic changes in ship performance in terms of voltage, frequency, and power fluctuations. Table 6 illustrates the most recent research that discusses the uncertainty problems and the techniques used to solve them.

Energy Management Systems (EMSs) in SBMGs
The main purpose of the energy management methods applied to the SBMG is to reduce fuel consumption, minimize running costs, ensure safety and sustainability, reduce downtime, improve efficiency, and provide fuel savings. A ship's EMS can save energy, control propulsion machinery and generators, perform load shedding, and provide a secure environment for the crew, which results in increasing the SBMG's reliability [37]. An EMS requires a good forecasting approach that addresses the energy demand and supply needed to optimize performance and costs while reducing the environmental impact [43]. Using RESs in SBMGs has many benefits, such as decreasing the amount of required energy, reducing gas emissions, and lowering the noise compared to conventional power plants. They can positively affect some other factors, such as climate change, due to the lack of heat production by RESs and wastewater due to the lack of water for cooling [153][154][155][156][157].
Smart EMSs communicate between the different sources and customer demands to achieve the best power matching and/or reduce the cost considering multiple constraints. Energy management techniques can be broadly classified into three categories based on the control architecture used: centralized, decentralized, or hierarchical. In centralized control, a single entity manages the entire microgrid, including generation, storage, and consumption. This approach provides a high level of control but can be costly and complex to implement. Decentralized control involves dividing the microgrid into smaller subsystems, each with its own control mechanism. This approach is less complex and more flexible than centralized control but may not provide optimal performance. Hierarchical control combines the advantages of both centralized and decentralized control, where each subsystem has its own control mechanism but there is also a higher-level control that coordinates the actions of the subsystems. This approach provides flexibility and scalability while ensuring optimal performance. The hierarchical control scheme consists of three layers: primary, secondary, and tertiary. The primary layer handles local control and maintains the balance between generation and consumption within individual microgrid components. The secondary layer coordinates microgrid components to optimize the system's performance. The tertiary layer focuses on global control and optimization of the entire microgrid system. Hierarchical control offers a flexible and scalable solution that integrates multiple energy sources and enables decision making at different levels of the microgrid system [158]. These classifications are explained in Figure 13. control, a single entity manages the entire microgrid, including generation, storage, and consumption. This approach provides a high level of control but can be costly and complex to implement. Decentralized control involves dividing the microgrid into smaller subsystems, each with its own control mechanism. This approach is less complex and more flexible than centralized control but may not provide optimal performance. Hierarchical control combines the advantages of both centralized and decentralized control, where each subsystem has its own control mechanism but there is also a higher-level control that coordinates the actions of the subsystems. This approach provides flexibility and scalability while ensuring optimal performance. The hierarchical control scheme consists of three layers: primary, secondary, and tertiary. The primary layer handles local control and maintains the balance between generation and consumption within individual microgrid components. The secondary layer coordinates microgrid components to optimize the system's performance. The tertiary layer focuses on global control and optimization of the entire microgrid system. Hierarchical control offers a flexible and scalable solution that integrates multiple energy sources and enables decision making at different levels of the microgrid system [158]. These classifications are explained in Figure 13.

Meta-Heuristic Techniques
Evolutionary-based  Figure 13. Classification of management methods. Table 7 gives a comparison of the most recent studies that represent and employ the EMS in the SBMG in terms of the contributions, the objectives and constraints of each paper, and the shortcomings that indicate the objective functions that have not been taken into consideration, besides the shortcomings of the contributions of each piece of research. Table 7 mentions using different energy storage systems, such as batteries, fuel cells, and cold ironing, to reduce emissions and increase efficiency. Additionally, the text highlights the importance of analyzing load profiles and implementing real-time energy management systems to ensure the safe and efficient operation of ships. The listed articles have  Table 7 gives a comparison of the most recent studies that represent and employ the EMS in the SBMG in terms of the contributions, the objectives and constraints of each paper, and the shortcomings that indicate the objective functions that have not been taken into consideration, besides the shortcomings of the contributions of each piece of research. Table 7 mentions using different energy storage systems, such as batteries, fuel cells, and cold ironing, to reduce emissions and increase efficiency. Additionally, the text highlights the importance of analyzing load profiles and implementing real-time energy management systems to ensure the safe and efficient operation of ships. The listed articles have various shortcomings, such as not investigating the impacts of integrating renewable energy sources with conventional sources, not considering power losses and battery lifetime, not including optimal sizing of energy storage systems, and not investigating uncertainties in wind and wave energy. Other common issues include not considering energy efficiency indicators, regulatory constraints, and system costs and not investigating the behavior of microgrids and fuel cells. Figure 14 represents the co-occurrence analysis for these studies. The analysis divides the author's keywords into ten clusters, with the main keywords in each cluster. From Figure 14, it can be concluded that the existence of multiple power sources in SBMGs increases the need for optimization problems with constraints to achieve the objective functions, the main one being minimizing the cost. One of the most prominent themes is related to energy management and efficiency, which includes sub-topics such as demand-side management, renewable energy resources, energy storage systems, and energy efficiency. Another important topic is emission control, which includes sub-topics such as greenhouse gases, emissions regulation, and carbon capture. Electric ship equipment and all-electric ships are other significant topics that emerged from the analysis. These topics include subtopics such as electric propulsion, energy storage systems, hybrid energy storage systems, and energy management systems. Additionally, there are some other sub-topics, such as fuel consumption, energy generation, and power generation, which are also relevant to the analysis. In conclusion, the co-occurrence analysis shows that there are several interrelated topics that are important for the research and development of efficient and environmentally friendly ship propulsion systems. Figure 14 represents the co-occurrence analysis for these studies. The analysis divides the author's keywords into ten clusters, with the main keywords in each cluster. From Figure 14, it can be concluded that the existence of multiple power sources in SBMGs increases the need for optimization problems with constraints to achieve the objective functions, the main one being minimizing the cost. One of the most prominent themes is related to energy management and efficiency, which includes sub-topics such as demandside management, renewable energy resources, energy storage systems, and energy efficiency. Another important topic is emission control, which includes sub-topics such as greenhouse gases, emissions regulation, and carbon capture. Electric ship equipment and all-electric ships are other significant topics that emerged from the analysis. These topics include sub-topics such as electric propulsion, energy storage systems, hybrid energy storage systems, and energy management systems. Additionally, there are some other subtopics, such as fuel consumption, energy generation, and power generation, which are also relevant to the analysis. In conclusion, the co-occurrence analysis shows that there are several interrelated topics that are important for the research and development of efficient and environmentally friendly ship propulsion systems.

Protection of SBMGs
Protection systems in maritime applications are important to keep people and property safe. If a protection system fails, it can lead to disastrous consequences, such as electrical faults, blackouts, or general hassle. A maritime system is usually well-equipped with a protection system to avoid these undesirable effects. An effective protection system should be sensitive, selective, quick-operating, reliable, simply constructed, and economical [39]. However, the DC-SBMG's protection system has a lot of challenges, including [33,[207][208][209][210][211]:

Ref. Contributions Objectives and Constraints Shortcomings
[168] -Perform a two-stage joint scheduling model to optimally coordinate the voyage scheduling and power generation of an all-electric ship (AES) to address the variation in the electricity price of the side during the navigation route -Apply a mixed-integer linear program (MILP) with multi-objective differential evolution (MODE) to coordinate the generation and voyage scheduling for AES and improve the reliability and energy efficiency of the SBMG  Table 7. Cont.

Ref. Contributions Objectives and Constraints Shortcomings
[172] -Apply a DRL scheme to optimally schedule the power of a ferry boat that uses a fuel-cell and battery ESS to solve the EMS problem -Improve loss of load expectation and reliability index

Ref. Contributions Objectives and Constraints Shortcomings
[177] -Apply a low-complexity near-optimal algorithm based on benders decomposition (LNBD) to optimally manage the power for failure mode considering the mid-time scheduling and the faults at bus and generators to improve the system performance for supplying the demand power

Ref. Contributions
Objectives and Constraints Shortcomings [192] -Apply adaptive multi-context cooperatively coevolving particle swarm optimization (AM-CCPSO) to provide a solution for the shipping company's operational cost control considering the emission regulation and upcoming tighter emissions regulations -Investigate the optimal operation and cooperation of a hybrid energy system and on-land shore power to achieve cost savings

Protection of SBMGs
Protection systems in maritime applications are important to keep people and property safe. If a protection system fails, it can lead to disastrous consequences, such as electrical faults, blackouts, or general hassle. A maritime system is usually well-equipped with a protection system to avoid these undesirable effects. An effective protection system should be sensitive, selective, quick-operating, reliable, simply constructed, and economical [39]. However, the DC-SBMG's protection system has a lot of challenges, including [33,207- The occurrence of miscoordination between the primary and secondary protection due to the short time required for fault clearing and the circuit breaker used in the DC-SBMG.
AC circuit breakers often cannot be used in DC circuits because they require current zero crossing. The emerging hybrid circuit breaker has been proposed to solve this problem but still provides a low protection level. If the fault current is large enough, it can damage the freewheeling diodes. The ability of a diode to withstand these faults is defined by the amount of adiabatic heating that occurs during the fault. Solid-state circuit breakers and intelligent electronic devices (IEDs) were used in [25]. The IED is a crucial device used in electrical networks where fault currents are detected and localized using the direction of the current and the differences between IEDs. Once a fault has been detected, a solid-state circuit breaker isolates it to prevent high voltage from being released. Therefore, IEDs and solid-state circuit breakers are necessities to protect DC circuits [33].
However, the DC zonal SBMG may require small time-coordinating protection measures among its various components. This is because the semiconductors in the power converters, such as diodes, IGBTs, and thyristors, have low thermal capabilities, i.e., there is a lag time between fault detection and clearance [212]. Therefore, the DC protection has to take place within a few seconds.
In designing a DC-SBMG, one of the main constraints is the lack of standards and guidance on implementing comprehensive fault management. Fault management is the ability of the SBMG to perform system reconfiguration to deliver the power to the critical loads instead of interrupting these loads. One of the significant differences between marine systems and land-based ones is the load profiles. Marine engines are sensitive to power outages, causing their loads to be more critical; hence, they need to be reconfigured quickly [38]. In this context, SBMG fault management consists of three stages. The first is the detection and localization of the fault. The second stage is the isolation of the fault. The third stage is the reconfiguration of the isolated system to feed the critical load. Power converters need to avoid a complete failure until the fault clearance occurs using an appropriate FTC approach. Figure 15 illustrates the fault-management methods in DC-SBMGs. Table 8 presents the most recent techniques used in DC protection for SBMGs for fault management. Based on the contributions presented in Table 8, it can be concluded that various fault-detection, diagnosis, isolation, and protection techniques have been proposed for DC SBMGs. Most of the contributions focus on fault detection and isolation methods, with some proposing fault-localization and reconfiguration solutions. However, fault reconfiguration is not investigated in most of the contributions, which can lead to prolonged downtime in the event of a fault. Additionally, the system's dynamics are not investigated in some of the contributions, which can impact the effectiveness of the fault-detection and diagnosis techniques. Overall, further research is needed to address the issues identified in the contributions and improve the fault management of DC SBMGs.  Figure 15. DC-SBMG fault-management methods. Table 8. Comparison of the contributions and issues of the most recent research related to faultmanagement techniques [213][214][215][216][217][218][219][220][221][222][223].

Refs.
Contributions Issues [213] Presents fault diagnosis based on machine-learning Noise-Assisted Multi-variate Empirical Mode Decomposition and Multi-level Iterative-LightGBM (NA-MEMD) and (MI-LightGBM) Fault reconfiguration is not investigated [214] Presents static and dynamic protection systems Discusses the short-term dynamics for a zonal SBMG after fault occurrence considering load-shedding actions Models the propulsion power converter's electro-mechanics The system's dynamic is not investigated [215] Presents fault detection and isolation based on nonintrusive load monitoring (NILM) Fault reconfiguration is not investigated [216] Presents a fault diagnosis method based on the wavelet-based filtering approach Minimizes the probability of misdetection Fault reconfiguration is not investigated [217] Presents a scalable solid-state bus-tie Switch that can be easily scaled for current and voltage Investigates fault-detection methods Reduces the electrical and thermal stresses by using multiple units rather than a single unit Fault localization and reconfiguration are not investigated [218] Presents a new solid-state bus-tie switch (SSBTS) Parallel connecting multiple units of the topology to increase power and voltage ratings Fault localization and reconfiguration are not investigated Table 8. Cont.

Refs. Contributions Issues
[219] Presents a solid-state DC circuit breaker Detects and isolates the fault in significantly less time Fault reconfiguration is not investigated [220] Explores three different protection schemes for DC faults in SBMGs, namely, a six-pulse thyristor rectifier, a six-pulse diode rectifier, and a two-level active rectifier Fault reconfiguration is not investigated [221] Presents a fault-detection algorithm based on overcurrent Recloses the Z-source breaker after the fault if necessary by control Integrates Z-source DC circuit breakers into a zonal MVDC SBMG Provides solutions for two Z-source breakers to work in parallel and supply current for the same load center Explores the SCR's gate control Fault localization is not detected [222,223] Presents a fault-detection and -localization method based on a distance scheme Locates different types of faults in both forward and reverse directions Fault reconfiguration is not investigated Figure 16 represents the co-occurrence analysis for these studies. The analysis divides the author's keywords into four clusters, with the main keywords in each cluster. Based on the co-occurrence analysis of the provided keywords, it can be observed that the majority of the keywords are related to fault detection and protection in electric power distribution systems, particularly in shipboard applications. Techniques such as distance-protection schemes, fault localization, and fault-detection algorithms are commonly mentioned, along with advanced monitoring and diagnostic methods, such as machine learning and signal processing. Other relevant topics include power system protection, load management, and reconfiguration, as well as the challenges and practical problems associated with implementing these techniques in complicated structures, such as marine power plants and shipboard microgrids. Overall, the analysis highlights the importance of reliable and high-performance electric distribution systems in ensuring the safe and efficient operation of ships and other marine vessels.
Smart Cities 2023, 6, FOR PEER REVIEW 41 majority of the keywords are related to fault detection and protection in electric power distribution systems, particularly in shipboard applications. Techniques such as distanceprotection schemes, fault localization, and fault-detection algorithms are commonly mentioned, along with advanced monitoring and diagnostic methods, such as machine learning and signal processing. Other relevant topics include power system protection, load management, and reconfiguration, as well as the challenges and practical problems associated with implementing these techniques in complicated structures, such as marine power plants and shipboard microgrids. Overall, the analysis highlights the importance of reliable and high-performance electric distribution systems in ensuring the safe and efficient operation of ships and other marine vessels.

Real Ships in The World
There are many types of ships which vary in their features and functionalities. Ships have been electrified to minimize GHG emissions. Table 9 shows the shipboard microgrids implemented across the world, with their names, types, available power supplies, storage

Real Ships in The World
There are many types of ships which vary in their features and functionalities. Ships have been electrified to minimize GHG emissions. Table 9 shows the shipboard microgrids implemented across the world, with their names, types, available power supplies, storage systems, and the years in which they started sailing. Table 9. Examples of the existing real ships in the world [14,36].

Research Trends and Recommendations
Despite the many research papers in the SBMG field, there is still a significant need for further research. The research gaps regarding the control, uncertainties, management, and protection of SBMGs can be summarized as follows: In terms of SBMG control: The problems of voltage and frequency deviations are investigated using different control methods. However, most of these methods are not accurate enough to give the desired voltage and frequency levels; voltage deviation rep-resents a vital issue in SBMGs due to the presence of PPLs and CPLs. So, employing artificial-intelligence techniques with such control schemes would be an excellent solution for the future. Moreover, applying machine-learning and deep-learning approaches with the control methods can enhance their performance in achieving optimal voltage and frequency levels. In addition, performing appropriate efficient control methods while eliminating the effect of packet losses is a new future research trend.
In terms of SBMG uncertainties: The unknown loads and the uncertain output powers of both PV and wind due to severe environmental conditions facing ships are significant issues. The investigated research focused on solving only one problem regarding uncertainties. In the future, it is recommended to apply methods and techniques that use intermittent platforms for data processing, analysis, and storage, such as cloud computing, Fog, and the Internet of Things (IoT). These methods can help in solving different uncertainty problems simultaneously.
In terms of SBMG energy management: In order to optimally share the power to the demand load, ensuring high system stability, it is recommended to incorporate the IoT with energy management strategies to facilitate communication between each element in the SBMG. Applying new artificial intelligence techniques can improve the system's performance and they can be used to optimally manage the system while considering the effect of demand response.
In terms of SBMG protection: Fault management is a big issue for SBMGs, as PPLs affect fault-detection and -localization methods. So, it is recommended to use smart protection strategies depending on deep-learning strategies to detect faults, in addition to using IoT to communicate between detection and isolation processes. Designing an efficient protection scheme based on artificial intelligence and optimization algorithms while using a communication channel with the minimum delay time is a new trend for SBMGs. Moreover, coordinated protection and reconfiguration are necessary to achieve higher survivability.
Additionally, in the future, it will be possible to form combined sea microgrids by integrating multiple SBMGs using wireless technology.

Conclusions
This paper has provided a comprehensive literature review of the classifications, control, uncertainties, management, and protection of SBMGs. It has explored the developments of the distribution, propulsion, and power system architectures. Dynamic positioning and converter control techniques have also been discussed, along with uncertainty issues and management optimization techniques. Fault detection, isolation, and reconfiguration techniques have also been presented, and a co-occurrence analysis was performed to identify the most recent trends in control, management, and protection. The main conclusions of this review are: • Dynamic modeling of SBMGs, considering all uncertainty issues, is essential for their operation. • A hybrid AC/DC distribution system with an integrated power system in a zonal structure is recommended for a more reliable and flexible power system. • A hierarchical control framework is better suited for regulating voltage and frequency deviations in complex SBMGs. • Particle swarm optimization (PSO) and genetic algorithms (GAs) are more effective for multi-objective optimization with multiple constraints using machine learning. • Machine-learning methods with communication for fault diagnosis and breakerless topologies for fault isolation are recommended for better protection systems. • Fifth-generation wireless communication is suggested to reduce delay time and sensor losses in control, management, and protection processes.