Lakhasly

This paper evaluates the transition from alternating current (AC) to direct current (DC) based electrical distribution systems for marine vessels equipped with diesel-electric hybrid propulsion. Historically, marine vessels adopted AC systems due to their prevalence in land-based infrastructure. Current hybrid diesel-electric systems generate AC power to a common AC bus, supplying propulsion drives and service loads. However, growing interest in DC-based systems stems from potential improvements in electrical performance and fuel consumption. This study provides an overview of typical AC systems, presents modifications for a DC-based system, and quantifies the benefits in terms of power factor, total harmonic distortion (THD), voltage regulation, and fuel efficiency. It also proposes a genetic algorithm (GA) based technique to improve fuel efficiency through optimized generator scheduling.

Introduction and System Overview: The early adoption of electricity on marine vessels, dating back to the SS Columbia with its DC lighting system, gradually shifted to AC systems by the early 1900s, aligning with land-based distribution trends. AC systems, typically operating at 440 V to 11 kV, became standard. However, environmental concerns have spurred research into improving fuel efficiency through various means, including propulsion system power management and integration of renewable energy sources. Modern marine vessels frequently use electric or diesel-electric propulsion systems, which offer advantages such as improved generator and propulsion drive efficiency at low loads, faster dynamic response, and reduced equipment weight, volume, and placement flexibility. The increasing proliferation of renewable energy resources, which often interface with DC buses, further drives interest in DC distribution systems for both land-based microgrids and marine applications, as DC systems inherently avoid synchronization issues and offer potential gains in efficiency and reduced equipment size. While DC protection devices were historically a challenge, recent commercial developments in DC circuit breakers have made DC systems feasible for ships with power requirements up to 12 MW, such as offshore supply and diving support vessels. This study fills a knowledge gap by examining operational efficiency gains of DC systems compared to similarly-rated AC systems, using an experimental testbed and a diving support vessel case study, including a novel GA-based generator scheduling method.

Electrical System Architecture: The diving support vessel under study uses an AC system with a total generation capacity of 12 MVA, supplying four electrical propulsion drives and hotel/service loads via two 690 V AC buses. Propulsion drives receive three-phase AC, which is then fed through a three-winding transformer (star-star-delta) followed by a 12-pulse diode bridge rectifier, before powering a variable speed drive. Service loads are supplied via step-down transformers from the 690 V AC bus to 230 V single-phase or 440 V three-phase.

The proposed DC system directly replaces the AC architecture by moving from a 690 V AC bus to a 1 kV DC bus. The existing generators remain, but Active Front-End (AFE) converters are introduced to convert AC to DC. Propulsion drives then source power directly from the 1 kV DC bus, eliminating the need for three-winding transformers and 12-pulse rectifiers. Service loads require inverters to convert DC back to AC, replacing the step-down transformers. This DC architecture also facilitates easier integration of energy storage elements via DC-DC converters. Implementing a DC system is most feasible for new marine vessels due to extensive modifications required for retrofitting, including removal of 12-pulse transformers, installation of AFE rectifiers, replacement of AC-source propulsion drives with DC-source variants, installation of power inverters, and replacement of AC-rated cables and protection devices with DC-rated equivalents. While the total cost for a DC system is estimated to be 20% higher, primarily due to AFE converters, these costs are projected to be offset by fuel savings over the vessel's operational lifetime.

An experimental testbed was constructed to compare AC and DC systems using the same generator and propulsion motor under variable loads. This single-generator, single-propulsion motor setup effectively validates electrical performance irrespective of the number of generators or motor drive size.

Electrical Performance: In AC systems, the 12-pulse rectification system, while reducing 5th and 7th harmonics, still exhibits high current THD, particularly at low loads, leading to reactive power production and energy losses. In contrast, DC systems employing AFE converters offer active control over power flow, significantly minimizing current harmonics and improving THD.

• Power Factor (PF): The DC system consistently demonstrates a higher power factor across all load levels (0–6 MW) compared to the AC system. At low loads, the minimum power factor for the AC system is 0.7, while the DC system maintains over 0.8. The use of active rectifiers in the DC system, which employ pulse-width modulation, allows for better control of current flow, thereby improving the power factor.
• Total Harmonic Distortion (THD): The input current THD is lower for the DC system (8.64%) compared to the AC system (17.35%). AFE converters' ability to control input current quality leads to less distortion, preserving generator integrity and overall system efficiency by reducing harmonic-induced losses.
• DC-link Voltage Regulation (VR): The DC-link voltage regulation is significantly improved in the DC system at 0.12%, compared to 10.14% in the AC system. AFE converters actively regulate the DC-link voltage to its set-point, preventing voltage drops that would otherwise increase line currents, leading to higher I²R losses and potential overloading of electrical equipment.

Overall, the AFE-based DC system offers superior power quality through better input power factor, lower THD, and tighter voltage regulation. These improvements translate into reduced reactive power generation, decreased energy losses, enhanced efficiency of the diesel generator-electric propulsion drive system, and improved safety and reliability of the on-board electrical network.

Generator Scheduling and Fuel Consumption: Specific fuel consumption (SFC) curves show that a diesel generator's fuel efficiency depends on its rotational speed and power output. In AC systems, generators operate at a fixed speed due to synchronization requirements, meaning efficiency is solely power-dependent, typically increasing as power approaches the rated capacity. Conversely, in a DC system, the same diesel generators can operate at variable speeds, leveraging additional degrees of freedom to achieve lower SFC across all power levels (as shown in Fig. 10).

Current marine vessels often use symmetrical loading, where total load is evenly divided among online generators, which is sub-optimal for fuel efficiency. For DC systems, the variable speed capability offered by AFE converters allows for more efficient operation. This paper proposes a genetic algorithm (GA) to optimize asymmetrical generator scheduling, minimizing total fuel consumption by efficiently allocating load powers to individual generators, subject to constraints like minimum load (typically 20% of rated power).

A case study on a diving support vessel (DSV) with four 3000 kVA diesel generators compares fuel consumption. In normal operation, symmetrical loading of AC systems showed that a shift to symmetrically loaded DC systems yielded substantial fuel savings, ranging from 1.4% to 18.5%. While optimized asymmetrical loading provided only marginal savings (max 0.3%) in AC systems, it significantly enhanced fuel efficiency in DC systems, maintaining average savings within the 1.4–18.5% range compared to symmetrically loaded AC. For dynamic positioning (DP) mode, where all generators are online for fast dynamic response, optimized asymmetrical loading provided insignificant savings in AC systems. However, moving to a DC system alone delivered 7.8% to 18.5% fuel savings, which further increased to 12.8% to 18.5% with optimized asymmetrical loading, particularly beneficial at low loads. Based on the vessel's operational profile (transit:manoeuvring:DP ratio of 3:1:8), an annual fuel saving of 7% is projected. The core reason for these savings in DC systems is the ability to operate generators at variable speeds. The GA optimization method is applicable to other marine vessels, provided their SFC curves and operational profiles are known.

Discussions and Conclusion: The shift to DC-based systems offers clear benefits in both electrical performance and fuel efficiency. Key topological changes involve replacing 12-pulse transformers and rectifiers with AFE converters and modifying the bus system to DC, requiring DC-rated auxiliary devices. Acknowledging the increased difficulty in designing DC protection devices, recent industrial advancements in high-voltage DC circuit breakers offer promising solutions.

In conclusion, while hybrid diesel-electric marine vessels predominantly use AC systems due to historical prevalence, DC-based systems offer significant advantages. Experimental results confirm that DC systems with AFE rectifiers are superior to 12-pulse AC-based systems in power factor, input current total harmonic distortion, and DC-link voltage regulation. Furthermore, conventional symmetrical generator loading is sub-optimal for fuel efficiency. Optimized asymmetrical generator loading significantly improves fuel efficiency, especially in DC-based systems where diesel generators can operate at variable speeds, providing an additional degree of freedom not available in AC systems. Consequently, future marine vessels can achieve enhanced cost-efficiency and improved electrical performance through the adoption of DC-based systems coupled with optimized asymmetrical generator loading schemes.

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Alternating current and direct current-based electrical systems for marinevessels with electric propulsion drivesMerlin Chaia,⁎, Dastagiri Reddy Bonthapallea, Lingeshwaren Sobrayena, Sanjib K. Pandaa,Die Wub, XiaoQing Chenba National University of Singapore, Electrical Machines and Drives Laboratory, E3-04-04, 2 Engineering Drive 3, 117581, Singaporeb ST Marine Ltd, 7 Benoi Road, 629882, SingaporeHIGHLIGHTS• Alternating current(AC)- and direct current (DC)-based marine vessels are presented.• Improvements in electrical performance achieved by DC-based systems is derived.• A genetic algorithm-based technique to improve fuel efficiency is proposed.• Quantitative benefits of moving to DC-based systems for marine vessels are studied.ARTICLE INFOKeywords:Marine vesselDiesel-electricHybridACDCFuel efficiencyABSTRACTThe initial widespread electrification of marine vessels primarily used alternating current (AC)-based systems asthey were prevalent in the electrical distribution infrastructure for land-based systems. At present, there aremarine vessels that operate based on a hybrid diesel-electric system, in which the on-board diesel enginesgenerate AC power to a common AC bus, which in turn supplies power to the electrical propulsion drives andother service loads. Recently, there has been an active interest in a transition to direct current(DC)-basedelectrical distribution system with diesel-electric hybrid propulsion systems for marine vessels due to potentialimprovements in electrical performance and fuel consumption. This paper evaluates the improvement in themove to DC-based distribution system for marine vessels in terms of electrical performance and fuel efficiency.An overview of a typical AC electrical system currently in use is provided, and modifications of the system to aDC-based system is presented. The power factor, total harmonic distortion, and voltage regulation are discussed.Both symmetrical and asymmetrical generator scheduling are examined and the potential fuel savings for anexample diving support vessel is presented. A discussion on the recommendations to shift towards DC-basedsystems is then provided based on the findings of this paper.1. Introduction1.1. Diesel-electric marine vesselsThe use of electricity for energy transfer has been well-documentedand was enabled through the production of commercial electromechanical generators [1]. Although the use of both alternating current(AC) and direct current (DC) were available, it was soon realized thatAC systems were more feasible due to the invention of the powertransformer that can easily step up AC voltages to allow for high-voltage, low-current transmission systems, which were more efficient atthat time.The first known electrical system implemented on-board a marinevessel was a DC system designed for a commercial vessel, the SSColumbia. However, this system was primarily for electrical lighting,with the propulsion system still mechanical-based. The first electricalpropulsion drive was first implemented for naval vessels, i.e. the Vandaland the USS Jupiter, in the early 1900s [2]. As land-based electricitynetworks trended towards the use of AC systems in distribution ofelectricity, so did the marine industry due to easy access to electricalequipment and knowledgeable manpower. The initial DC system wasdiscontinued thereafter for newer ships, and AC systems withhttps://doi.org/10.1016/j.apenergy.2018.09.064Received 1 November 2017; Received in revised form 23 April 2018; Accepted 6 September 2018⁎ Corresponding author.E-mail addresses: [email protected] (M. Chai), [email protected] (D.R. Bonthapalle), [email protected] (L. Sobrayen),[email protected] (S.K. Panda), [email protected] (D. Wu), [email protected] (X. Chen).Applied Energy 231 (2018) 747–7560306-2619/ © 2018 Elsevier Ltd. All rights reserved.Tdistribution voltages of 440 V, 690 V, 3.3 kV, 6.6 kV, and 11 kV arecommonly used.Due to environment concerns [3], there has been ongoing researchon the greenhouse gas emissions produced by marine vessels [4]. Mostefforts, however, has been largely focused on energy efficiency improvement attained through design changes in the marine diesel engine[5,6] or the fuel used [7,8]. There is also the possibility of improvingfuel efficiency through power management of the propulsion system[9]. The inclusion of renewable energy sources, such as solar, on-boardmarine vessels has also been proposed [10].1.2. State-of-the-artThe use of electric or diesel-electric propulsion systems in marinevessels are now commonplace[11]. Some marine vessels, e.g. platformsupply vessels, diving support vessels, and containerships, use dieselelectric propulsion systems [12]. These vessels have both mechanicalpropulsion drives used during long-range, constant-speed operationsand electrical propulsion drives that are used for low-speed maneuvering operations.Newer cruise ships tend to use electric propulsion system, in whichelectrical generators are used to produce electricity that can then beutilized by electric propulsion drives without the need for mechanicalcoupling between the generators and propulsion drives. It should benoted that although the propulsion systems are electric-based, thegeneration of electricity still requires diesel generators. There is ongoing research on the integration of energy storage devices such asbatteries and flywheels in marine vessels[13] to complement the dieselgenerators. However, even with inclusion of renewable energy resources, the use of diesel generators is still required to produce electricity[14]. This is due to the high power demand on-board marinevessels.The use of electric propulsion drives to replace mechanically-linkeddiesel propulsion drives has been gaining traction due to their advantages[15], which include:• Improved efficiency of the generators• Improved efficiency of propulsion drives at low load and low speed• Faster dynamic response• Reduced weight and volume of electrical equipment• Flexibility in equipment placementWith electric propulsion drive, an electrical distribution system isrequired to transfer energy generated by the diesel generator to thepropulsion drive. At present, marine vessels, similar to land-based distribution system, primarily run on an AC distribution system. In landbased microgrid systems, such as buildings[16], there is also an interestin a shift to DC-based distribution system[17]. This is primarily drivenby the increasing proliferation of renewable energy resources[18],which conventionally require a DC bus for integration purposes.However, the adoption of DC distribution systems is not widespread. This is partly due to a historical lack of DC protection devices.Recent developments by ABB[19] and GE[20] have produced commercially available DC circuit breakers, with maximum ratings of12 MW. However, as it has been found that smaller ships tend to havehigher greenhouse emissions[21], these DC circuit breakers are applicable for smaller ships with lower power requirements. For example,offshore supply vessels and diving support vessels typically have powerrequirements in the 10 MW range.The integration of renewable energy resources has also been explored for marine vessels, such as the use of solar photovoltaic panels[22] and wind-based generation sources[23]. These also require the useof a DC bus for ease of integration as DC systems do not have synchronization issues. In addition, there are potential improvements inefficiency, fuel savings, and reduction in weight and volume of electrical equipment[24,25] that can be attained via a shift to a DCdistribution system. Recent research on DC distribution systems onboard marine vessels has used land-based DC smart grid technology as acomparison tool[24], and have focused on the design of the power architecture and energy storage integration[25]. However, there is a lackof research in the operational aspects of the DC system.This study aims to address a knowledge gap in the operational efficiency gains that can be attained through the replacement of ACsystems with similarly-rated DC systems. Specifically, this study examines the operational details of a diving support vessel that currentlyoperates with an AC distribution system, and proposes an equivalent DCdistribution system that can be installed in future iterations of the samemarine vessel. This is performed through the use of an experimentaltestbed that compares both AC and DC systems with the same generatorand propulsion load, which has not been found in literature elaborately.The required equipment changes and the improvement in electricalperformance resulting from the changes are examined quantitativelythrough the use of an experimental testbed that is a scaled-down modelof the actual electrical system. A novel generator scheduling methodusing genetic algorithm is also proposed, and the fuel savings in multiple operation modes are detailed. Through this study, the benefits ofusing DC systems in marine vessels during the operation of the marinevessel can be quantified.2. Electrical system of marine vessel under study2.1. Current AC architectureThe AC system currently installed on-board the marine vessel understudy, which is a diving support vessel, is shown in Fig. 1. For thisvessel, the total electrical generation capacity is 12 MVA, which supplies four electrical propulsion drives as well as the hotel/service loadson-board the vessel. There are two 690 V AC buses, which can beconnected by closing the bus tie in between the two buses. This allowsfor transfer of power between the port and starboard generators to theelectrical propulsion drives and service loads on either side of thevessel.The electrical propulsion drives obtain power from the 690 V ACbus. The three-phase AC supply is then fed through a three-windingtransformer with a star-star-delta configuration, following by a 12-pulsediode bridge rectifier. The electrical propulsion motor is then driven bya variable speed drive, which allows for variable speed and variabletorque operation.The service loads are also supplied from the 690 V AC bus, whichhas a step down transformer reducing the AC voltage from 690 V to amore usable 230 V single-phase or 440 V three-phase supply. Theseloads may include lighting, navigational equipment, auxiliary machinery, and cooling pumps.2.2. Future DC architectureThe proposed DC system that can directly replace the current ACsystem is shown in Fig. 2. The main different between the DC and ACsystems is the move from a 690 V AC bus to a 1 kV DC bus. As thegenerators used remain the same, active front-end (AFE) converters arerequired to convert the produced AC voltages to DC. As the bus voltageis now DC, the electrical propulsion drives no longer require threewinding transformers and 12-pulse rectifier to convert AC to DC; thevariable speed drive for the electrical propulsion motor can now sourcepower directly from the 1 kV DC bus.For the service loads, inverters to convert DC to AC are required.These directly replace the step-down transformers that were in the ACsystem. Single-phase and/or three-phase loads remain able to sourcepower from the DC bus. With the DC bus, the connection of energystorage elements such as batteries are now viable. However, DC to DCconverters are required to control the power flow between the energystorage element and the DC bus.M. Chai et al. Applied Energy 231 (2018) 747–756748Due to the differences between AC and DC architectures, the implementation of DC systems can only be feasibly performed for a newmarine vessel. A retrofit of an existing AC system-based marine vesselwill require a large number of changes, i.e.:• Removal of 12-pulse transformers• Installation of active rectifiers for each diesel generator• Replacement of AC-source electrical propulsion drives with DCsource electrical propulsion drives• Installation of power inverter to enable 230 V AC bus that is required for service loads• Replacement of electrical cables from 3ϕ AC-rated cables to DCrated cables• Replacement of protection devices from AC-rated to DC-rated circuitbreakers and relaysIn this manuscript, the economic evaluation of retrofitting an existing marine vessel with AC system is not considered due to thesignificant number of modifications required. It has been estimatedthrough quotations from equipment manufacturers that the total costfor the DC system is 20% higher than that of the current AC system.This is primarily due to the higher cost of active rectifiers. However, thesavings in fuel costs is projected to be for the operational profile of themarine vessel using data collected on-board the marine vessel over aperiod of a year.2.3. Experimental testbedTo facilitate the comparison between AC and DC systems in marinevessels, a laboratory-based experimental testbed has been constructed,as shown in Fig. 3. The experimental testbed is able to operate underboth AC and DC systems while using the same generator and propulsionmotor with variable load levels. A photo of the experimental testbed isalso shown in Fig. 4.A notable difference between the experimental testbed and theelectrical layout shown in Figs. 1 and 2 is in the number of generatorsFig. 1. AC system for a marine vessel.Fig. 2. DC system for a marine vessel.M. Chai et al. Applied Energy 231 (2018) 747–756749and propulsion drives. However, the electrical performance is dependent the type of converters used; it not dependent on the number ofgenerators and propulsion motors or size of the propulsion motor drivesystem. Therefore, the electrical performance of both AC and DC systems can be validated through a single-generator, single propulsionmotor testbed.The specifications for the testbed are listed in Table 1. All experimental results presented in this manuscript are recorded using a Yokogawa DLM2024 mixed signal oscilloscope and a Hioki PW3360-21power logger.3. Electrical performanceAn n-pulse rectification system is able to reduce the ± ( 1) n2 harmonics in the AC currents. For the 12-pulse rectification system used inthe diving support vessel, this allows for reduction of the 5th and 7thharmonics which normally arise from the use of passive rectifiers. Thisreduction is mandatory to fulfill the marine standards for total harmonic distortion (THD) on the AC bus. However, even with the harmonics reduction by the three-winding transformer and 12-pulse rectifier, the current THD remains high, especially at low loads.As can be observed in Fig. 5, the source current Is, is distorted byharmonics. The ideal waveform shape for Is is typically sinusoidal, i.e.same waveform shape as the source voltage, Vs, in order to minimizereactive power production. Reactive power production correlates toenergy losses, which reduce the fuel efficiency of the marine vessel.For the DC system, use of AFE converters is beneficial, being activeconverters which can control active and reactive power flow. There areFig. 3. Single line diagrams for AC and DC system experimental testbeds.Fig. 4. Experimental testbed for AC and DC systems in marine vessels.Table 1Specifications for experimental setup.Parameter ValueElectrical frequency 50 HzTransformer power rating 10 kVATransformer ratio 415 V-415 V-415 V Y-Δ-YMotor drive type Variable frequency driveMotor drive power rating 7.5 kVAMotor drive input voltage 500–800 VDCAC generator power rating 7.5 kVAAC generator voltage rating 3ϕ, 415 VAC generator current rating 10.4 AAC generator excitation 60 V, 2.7 AM. Chai et al. Applied Energy 231 (2018) 747–756750existing and well-established research for their control methods [26,27]that can minimize the current harmonics and improve the THD.Fig. 6 shows the input voltages and currents for an AFE converterwith the same load profile as in the case in Fig. 5. The input currentsdemonstrate lower distortion as compared to the case in Fig. 5. Whilehigher order frequency harmonics are present, these are easily filteredusing the low-pass LC- or LCL-based filters [28].Fig. 7 shows the power factor of the input waveforms over variousload levels, where the definition of power factor is:= = PF θ PS cos( ) | | (1)wherePF: power factorθ: angle between real and apparent powerP: real power (W)S: apparent power (VA)The relationship between real, reactive, and apparent powers is= + S P jQ . In ideal conditions, the value of real power should be equalto the value of apparent power, resulting in unity power factor (i.e.= power factor 1). This also means that no reactive power is present.However, due to real-world non-ideal conditions, there are inherentreactive and/or non-linear components in the power converter thatresults in a power factor of less than 1.As observed in Fig. 7, the power factor in the DC system isconsistently higher than the AC system over the entire load range of0–6 MW. The difference in power factor is particularly high at low-loadconditions, where the minimum power factor of the AC system is 0.7while the power factor of the DC system is > 0.8.The improvement in power factor for the DC system is achievedthrough the use of active rectifiers. In the AC system, the conduction ofcurrent through the 12-pulse rectifiers only occur when the instantaneous DC voltage is lower than the instantaneous AC voltage;whereas in the DC system, pulse-width modulation of the active rectifiers can control the current flow from the generators[28]. With appropriate modulation techniques such as space vector modulation, theharmonic currents drawn from the diesel generators are reduced, henceimproving the power factor.Fig. 8 presents the total harmonic distortion (THD) in the inputcurrents, where THD is calculated as follows:= ∑ =∞THDII in n2 212 (2)whereI1: fundamental component of the input currentIn: nth harmonic component of the input currentThe THD is observed to be lower for the DC system compared to theAC system. The ability of the AFE converter to control the quality of theinput current allows for lower distortion in the input currents, henceimproving the quality of the current drawn from the generators.Excessive distortions in the input current waveform result in fasterdeterioration of the on-board generators, which acts to convert dieselfuel to electrical power. The harmonics also generate losses that decrease the efficiency of the overall system, thus reducing the fuel efficiency of the marine vessel.Fig. 9 shows the DC-link voltage that is supplied by the 12-pulseFig. 5. Typical waveforms seen in a 12-pulse rectifier system.Fig. 6. Source voltages and currents for AFE converter system.Fig. 7. Comparison of power factor for AC and DC systems.Fig. 8. Comparison of THD for AC and DC systems.M. Chai et al. Applied Energy 231 (2018) 747–756751converter and the active front-end rectifier in the AC and DC systemsrespectively. The DC-link voltage regulation of the AC and DC systemscan be calculated:= − ×VR V VV| || |100% nl flfl (3)whereVR: DC-link voltage regulationVnl: no-load DC-link voltageVfl: full-load DC-link voltageThe voltage regulation with the use of 12-pulse rectifier was foundto be 10.14% in the AC system, while the voltage regulation is significantly improved to 0.12% in the DC system with the use of AFEconverters. This is due to the ability of AFE converters to activelyregulate the DC-link voltage to its set-point. The ability to regulate theDC-link voltage is important, as the drop in voltage due to poor regulation results in the increase in line currents in order to produce thesame power output. This may lead to overloading issues that can heatand damage essential electrical equipment on-board the marine vessels.Through the comparisons shown from Figs. 7–9, it can be concludedthat the AFE-based DC system demonstrates better power quality interms of input power factor, THD, and voltage regulation. In the marinevessel, these three parameters are key factors towards improving theperformance of the marine vessel. The improved performance in powerfactor and THD will reduce unnecessary generation of reactive power,and hence improving the efficiency of the overall diesel generatorelectric propulsion drive system. With stiff voltage regulation throughthe use of active rectifiers, the current sourced by the electrical propulsion drive can be reduced, as current is inversely proportional tovoltage for a fixed power demand. This not only reduces I R2 losses inthe transmission network, but improves the overall safety and reliability of the on-board electrical network.4. Generator scheduling4.1. Specific fuel consumptionFig. 10 shows the specific fuel consumption curves for a 13 MVArated marine vessel in both AC and DC modes. This has been extractedfrom the brake specific fuel consumption contour plot for a 3 MW dieselgenerator currently installed on-board the marine vessel under study[29].The fuel efficiency of diesel generators is a function of the rotational speed and the power produced. The specific fuel consumptioncurves are usually produced by the original equipment manufacturersthrough empirical testing of the diesel generators.However, in an AC system, the diesel generators operate at fixedspeed operation due to the connection of multiple generators onto acommon AC bus. This means that all generators that share the samepoint of common coupling operate at the same speed and also have tobe in-phase with one another. Therefore, the fuel efficiency of the dieselgenerator is only dependent on the power produced - the fuel efficiencyis observed to increase as the power produced approaches the ratedpower of the diesel generator.On the other hand, the same diesel generators operating in a DCsystem can make full use of all degrees of freedom, as it is possible tooperate in variable speed operation. By observing the two curves inFig. 10, it can be seen that the specific fuel consumption curve in a DCsystem is lower than that in an AC system for all power levels. Thediesel generator is found to have the lowest fuel consumption whenproducing the rated power, 3000 kW, in AC systems and 1800 kW in DCsystems.4.2. Symmetrical loadingThe current allocation of power generation between the generatorsis performed on an symmetrical loading basis, i.e. total load demand isdivided by the total number of online generators:= P Pn k total(4)wherePtotal: total power demand of the marine vesselPk: power produced by the kth generatorIt has been found that in terms of fuel efficiency, symmetricalloading of diesel generators has been found to result in not operating atthe most efficient operating point. At present, there is ongoing researchto optimize the generator scheduling to improve the overall fuel efficiency marine vessels [30].4.3. Asymmetrical loadingWith the use of AFE converters in the DC systems, the generators nolonger have to rotate at specific speeds to synchronize with the commonDC bus, unlike AC systems which have fixed AC bus electrical frequencies that are typically 50 Hz or 60 Hz. With this extra degree offreedom, the generators are able to run at more efficient modes, thusimproving the fuel efficiency of the vessel.The use of evolutionary algorithms, such as genetic algorithm (GA)[31–33] and particle swarm optimization [34,35] has been proposedFig. 9. DC-link voltage regulation for AC and DC systems.Fig. 10. Specific fuel consumption curves for AC and DC systems.M. Chai et al. Applied Energy 231 (2018) 747–756752for various industrial applications. These have been found to be able toconverge onto global minimum or maximum of multi-objective functions.In the case of marine vessels, the total number of online generatorsis the number of dimensions that have to be taken into account, e.g. fora marine vessel with four online generators, the optimization criteriabecomes a 4-dimensional problem.This optimization criteria can be describes as follows:∑= × =FC PSFC P Pmin { ( ) [ ( ) ]} total knkk k1 (5)subject to ∑ = = P P knk total 1 where:FC P( )total: total fuel consumption for a given total load demandPtotal: total load demandSFC P ( )k k : specific fuel consumption function for generator, kPk: load power of generator, kn: total number of on-board diesel generatorsThe objective of Eq. (5) is to minimize the total fuel consumption forthe marine vessel, which in turn is dependent on Ptotal and the specificfuel consumption (SFC) curve.GA is utilized to solve the optimization criteria in Eq. (5), withparameters listed in Table 2. The methodology of GA is to generate arandom population of solutions which are then evolved towards solutions that fit the minimization criteria. Each generation of solutions areevaluated for fitness of solution and are either discarded or furtherevolved to form a new generation. GA terminates when the maximumnumber of generations has been reached or the generated solution hasreached the termination criteria. By using GA, the optimization processcan converge to a global optimum.For diesel generators, there is an additional criteria that specifies theminimum load of each diesel generator; this is typically 20% of its ratedpower.4.4. Comparison of fuel consumptionA diving support vessel (DSV) is used in a case study to compare thefuel consumption with AC and DC systems, and also to calculate theadditional fuel savings that result from the use of asymmetrical loadingof the diesel generators. The DSV has a total of 4 on-board dieselgenerators, with all 4 rated at 3000 kVA. The minimum loading of eachgenerator is therefore 600 kVA, and the maximum is assumed to be therated power of 3000 kVA.The distribution of generator loading after the application of theproposed optimization is shown in Figs. 11 and 12. As can be observed,the distribution of generator loading differs between AC and DC systems in both normal and dynamic positioning modes. An observation tonote is that for the marine vessel under study, the application of theproposed optimization does not change the distribution of generatorloading for the AC system in dynamic positioning mode. This is due tothe fixed number of generators in this mode, and the fact that the fuelconsumption curve for AC systems as shown in Fig. 10 decays exponentially. The optimal distribution of generator loading is hence anequal division among all four generators.In normal operation, the number of online generators is based onthe total electrical load demand; the minimum number of diesel generators that can meet this demand is typically used. There are also casesnear multiples of the rated power of the generator, i.e. 6 MVA and9 MVA, where the number of online generators is increased by one. Forexample, for a total load demand of 8.5 MVA in DC systems, the totalnumber of generators used is four, although technically three generators have sufficient capacity to meet the load demand. This is discovered through the application of GA optimization.The DSV can also operate in dynamic positioning (DP) mode, inwhich all 4 diesel generators are always online regardless of the totalload demand. In DP mode, the electrical propulsion drives on-board thevessels are used to maintain the position of the marine vessel whiledivers work underwater for maintenance or other operations. Althoughoperation in DP mode is less fuel efficient, all 4 generators are online sothat fast dynamic response can be achieved.Fig. 13a and b shows the total fuel consumption of the DSV in bothnormal and DP modes respectively. In both modes, symmetrical andasymmetrical generator loading have not been observed to have significant impact in the fuel consumption of the DSV with an AC system.The fuel savings achieved by the shift from an AC system to an DCsystem alone is substantial. This is without the use of optimizationtechniques for asymmetrical generator scheduling, and the symmetricalloading of diesel generators is maintained. As shown in Fig. 14a, thefuel savings ranges between 1.4% and 18.5% compared to the currentlyimplemented AC system in normal operation.With the optimization of asymmetrical generator scheduling appliedto both AC and DC systems, it can be observed that fuel savings issignificantly higher for the DC system, as shown in Fig. 14. With the ACsystem, fuel savings is largely insignificant, with only specific generatorloads achieving a maximum of 0.3% fuel savings at specific load conditions. With the optimized asymmetrically-loaded DC system, theaverage fuel savings across the entire load range improves to a higheraverage value with the same 1.4–18.5% range compared to the symmetrically-loaded DC system.In dynamic positioning operation, the application of asymmetricalloading with optimization results in insignificant fuel savings. On theother hand, the fuel savings resulting from the move to a DC system issubstantial, ranging between 7.8% and 18.5% under normal operation,as shown in Fig. 14b. When optimization in asymmetrical loading isapplied, the fuel savings further increase to 12.8–18.5% compared tothe symmetrically-loaded AC system. This is particularly more significant under low loads when the optimization can more appropriatelyallocate load generation to all online generators to achieve overallhigher fuel efficiency.Data has previously been collected on-board the marine vesselunder study to extract its operational profile. The operational profile ofthe vessel has three modes: transit, manoeuvring, and dynamic positionwith the time ratio spent in each mode being 3:1:8. With the fuelconsumption profile detailed in Fig. 14, the expected fuel savings over ayear for the marine vessel has been projected to be 7%.It is therefore obvious that a shift to a DC system results in fuelsavings due to the ability to operate the generator at variable speed,which provides an additional degree of freedom that is not available inAC systems. This is regardless of the use of either an symmetricalsharing technique, or an optimized asymmetrical generator loadingtechnique for multiple generators.It should be noted that the optimization criteria proposed in Eq. (5)can be applied to other marine vessels. While the results shown inFigs. 13 and 14 are specific to the diving support vessel under study, thesame optimization criteria can be applied to other marine vessels toproject the fuel consumption and savings. As with the diving supportvessel, the brake specific fuel consumption contour plots and the operational profile of the marine vessel have to be obtained for an accurate calculation of the fuel consumption and savings.Table 2Parameters for optimization using genetic algorithm.Parameter ValueMinimum load 600 kVAMaximum load 3000 kVANumber of generators ⌈ ⌉ P/3 MVAtotalMax. number of generations 100Termination criteria for change in cost function ⩽ FC PΔ( )1 μtotal M. Chai et al. Applied Energy 231 (2018) 747–7567535. DiscussionsThis paper compares the use of AC- and DC-based systems on-boardmarine vessels in terms of electrical performance and fuel efficiency,and can be summarized as follows:From Table 3, it can be observed that a move to DC-based systemcan produce benefits in both electrical performance and fuel efficiency.In terms of topological changes, the main component change is thereplacement of the 12-pulse transformers and rectifiers with AFE converters. The common bus system also be modified to be DC-based, andauxiliary devices, e.g. circuit breakers, voltage and current sensors, alsohave to be replaced with appropriately rated parts that are able tomeasure and interface with DC voltages and currents.An issue with DC-based systems is the increased difficulty in designing protection devices. With AC-based systems, the current waveforms naturally have zero-crossings allow for easier extinguishing of thecurrent. In DC-based system, the constant non-zero voltage and currentswill be interrupted at full current flow. However, there has been recentindustrial and academic research that has developed high voltage DCcircuit breakers for high-voltage transmission purposes [19].6. ConclusionAt present, hybrid diesel-electric marine vessels mainly use ACbased systems due to the prevalence of AC power distribution networksin land-based systems. There has been an interest in a transition to DCbased systems due to its potential benefits in terms of electrical performance and fuel efficiency. It has been demonstrated that the currentdrawn from the AC bus contains low-order harmonics for typical 12-pulse AC-based systems. For DC-based systems, the use of AFE rectifierseliminates these low-order harmonics. Experimental results have confirmed that the electrical performance in terms of power factor, inputcurrent total harmonic distortion, and DC-link voltage regulation issuperior in DC-based systems.The use of the conventional symmetrical generator loading technique was shown to be sub-optimal for fuel efficiency. Optimizedasymmetrical generator loading can improve the fuel efficiency of themarine vessel, but can only do so slightly in current AC-based systems.In DC-based systems, the diesel generators can operate at variablespeed, which provides them an additional degree of freedom that allowsthem to operate with increased fuel efficiency. The gains in fuelFig. 11. Generator load distribution for AC and DC systems in normal mode with proposed optimization.Fig. 12. Generator load distribution for AC and DC systems in dynamic positioning mode with proposed optimization.M. Chai et al. Applied Energy 231 (2018) 747–756754efficiency is even more significant when optimized asymmetrical generator scheduling techniques are applied.From the perspective of both electrical power quality and fuel efficiency, marine vessels of the future can be cost-efficient with improved electrical performance through the use of DC-based systemswith optimized asymmetrical generator loading schemes.AcknowledgementsThis work was supported by Singapore Maritime Institute incollaboration with Singapore Technologies Marine Ltd (SMI-2013-MA08) and has been carried out at the National University of Singapore asa part of the research on Intelligent Power Management System forElectric Propulsion-based Marine Vessels for Improving Reliability,Operational Cost, Performance and Efficiency Operating underDifferent Operating Conditions.References[1] Guarnieri M. 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