IEEE TRANSACTIONS ON TRANSPORTATION ELECTRIFICATION...

IEEE TRANSACTIONS ON TRANSPORTATION ELECTRIFICATION 2016 1

Past, Present and Future Challenges of the MarineVessel’s Electrical Power System

Espen Skjong, Rune Volden, Egil Rødskar, Marta Molinas, Member, IEEE, Tor Arne Johansen, Senior Member,IEEE, and Joseph Cunningham

Abstract—The evolution of the use of electricity in marinevessels is presented and discussed in this article in an historicalperspective. The historical account starts with its first commercialuse in the form of light bulbs on the SS Columbia in 1880 forillumination, going forward through use in hybrid propulsionsystems with steam turbines and diesel engines and then transi-tioning to the present with the first fully electric marine vesselbased entirely on the use of batteries in 2015. Electricity useis discussed not only in the light of its many benefits but alsoof the challenges introduced after the emergence of the marinevessel electrical power system. The impact of new conversiontechnologies like power electronics, battery energy storage, andthe DC power system on overall energy efficiency, power quality,and emission level is discussed thoroughly. The article guidesthe reader through this development, the present and futurechallenges by calling attention to the future research needs andthe need to revisit standards that relate to power quality, safety,integrity, and stability of the marine vessel power system, whichare strongly impacted by the way electricity is used in the marinevessel.

Index Terms—Marine vessel electrical power system, diesel-electric propulsion, steam turbine, power electronics, batteryenergy storage, power quality, harmonics

I. INTRODUCTION

STARTING with the earliest records of a commerciallyavailable shipboard electrical system which date back to

the 1880s with the onboard dc system of the SS Columbia; theinvention of the AC induction motor, the transformer, and thediesel engine triggered new research and development towardthe end of the 19th century and the beginning of the 20th.In this period, the initial steps were made in research relatedto submarines, batteries, steam turbines, and diesel engines.The two most important developments before WWI were thefirst diesel-electric vessel (Vandal) in 1903 and the first navalvessel with electric propulsion in 1912 (USS Jupiter). Duringthe period of rising tension that preceded WWI the first cargovessels with turbo-electric propulsion were conceived anddeveloped in the United States and the United Kingdom. The

E. Skjong, R. Volden and E. Rødskar are with Ulstein Power & ControlAS, 6018 Alesund, Norway

E. Skjong, M. Molinas and T. A. Johansen are with the Department ofEngineering Cybernetics, Norwegian University of Science and Technology,7034 Trondheim, Norway

E. Skjong and T. A. Johansen are with the Centre for Autonomous MarineOperations and Systems (AMOS), Norwegian University of Science andTechnology, 7052 Trondheim, Norway

J. Cunningham is an adjunct lecturer, engineering development and history,at various institutions and associations.

Email: [email protected], [email protected],[email protected], [email protected],[email protected], [email protected]

outbreak of WWII stimulated new developments that broughtthe T2-tanker with turbo-electric propulsion into the picture.Nuclear powered vessels emerged in the late 1950s and thefirst passenger liner to use alternating current was inauguratedin 1960 (SS Canberra), 70 years after the invention of thealternating current motor. In the period 1956-1985, the powerelectronics revolution triggered by the innovative solid-statetechnology marked the beginning of a new era for marinevessels; the era of the all-electric vessel. As a result of that,Queen Elizabeth II was inaugurated in 1987 with the firstdiesel-electric integrated propulsion system. And in the lasttwo decades, the marine vessel community has witnessed thedevelopment of the first vessels having LNG as fuel. In January2015, marking the start of the era of the all-electric vessel,the world’s first purely battery-driven car and passenger ferryAmpere was been placed in use and is being regularly operatedin Norway. Fig. 1 guides the reader through the milestones inthe evolution of the the marine vessel electrical power systemfrom 1830 to 2015.

This new era of electric marine vessels does not come with-out challenges, however. In what follows, the paper highlightsthe different stages in the evolution of the marine vessel’sdevelopment and the impact of electricity use in this evolution.Following the historical account, the paper moves towardsmodern electric ship propulsion discussing the new challengesof moving towards hybrid AC/DC and pure DC power systems,the challenge of electrical stability, harmonic pollution, andpower quality in stand-alone microgrids like the marine vessel,the role of battery energy storage systems, and the movetowards emission free operation among others. Along withthese challenges, potential solutions and possible roads tofollow are presented.

II. EARLY STEPS OF THE MARINE VESSELELECTRIFICATION

The first recorded effort to apply electric power on a marinevessel occurred in the late 1830s after Moritz Hermann Jacobiof Germany invented a simple battery powered direct current(dc) motor which was installed experimentally on small boats[1]. It suffered from numerous imperfections and there was noimmediate adoption of electric propulsion for ships. The firstsuccessful application of electric power on ships was that ofgun firing circuits in the 1870s. The development of arc lampsfor illumination of streets and public spaces was followed byarc lamp searchlights on ships to illuminate attacking shipsand blind enemy gunners. Luxury liners were equipped withcall bells for the convenience of passengers [2].


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Fig. 1. Historical highlights of the development of the Marine Vessel’s Power System from 1830 to 2015 [2].

The development of incandescent lighting by Thomas Edi-son and others was followed by an installation on the passen-ger and freight vessel SS Columbia in 1880. That consistedof 120 lamps powered by a set of dynamos; the system wascrude with lead wires functioning as fuses and lamp intensitywas regulated only by the engine room crew’s adjustment ofthe generators according to the appearance of the lamps [3].Nonetheless, it led the US Bureau of Navigation to mandateadditional installation of electric lights. Soon after, electricmotors were installed in ventilation and gun firing circuits.In 1896, the USS Brooklyn was fitted with an 80 volt dcelectrical system to operate winches, deck machinery, and gunmounts [2]. Most installed shipboard power systems were dcas alternating current (ac) motors were not yet perfected. Thefirst successful electrically powered vessel was the Elektra,a passenger ferry with a capacity of 30 persons, built by theGerman firm Siemens & Halske in 1885. Measuring 11 meterslong by 2 meters wide, it was powered by a 4.5 kW motorsupplied by batteries [4].

A. Alternating Current Motor and Transformer

The development of ac motors based on the inductiveeffect of phase displaced conductors; primarily by NikolaTesla (US), Galileo Ferraris (Italy), and Michael OsipowitchDolvio-Dobrowolsky (Germany), made possible an alternative;however, the reliable dc motors developed by Frank Sprague(US) and others tended to favor the use of direct currentmotors. Regardless, ac research continued; George Westing-house took the lead in the United States. Often overlooked

was the Hungarian team of Karoly Zipernowsky, Otto Blathy,and Miksa Deri, (ZBD) whose closed core transformer ofhigh efficiency made practical ac power distribution and wasadopted by Westinghouse [5], [6].

Still, the issue of ac power factor (the useful power deliv-ered after losses due to inductive and capacitative reactance)constrained the adoption of ac for commercial power, railways,and on ships. However, dc systems were heavier and larger;thus in an effort to reduce weight ac systems were designedfor frequencies up to 400 Hz but the mechanical frequencyconverters of the time were cumbersome. Practical ac propul-sion was demonstrated in 1908, though without modern powerelectronics control was complex; effected by voltage andfrequency changes and variations in pole connections. It issaid that the complexity of ac systems led the British Navy toretain dc systems, even though Germany followed the lead ofthe US which had adopted ac systems in 1932 [2].

B. Turbo-electric Powered Vessels

In the early 1900s, Britain favored the development ofsteam turbine drive systems with reduction gears while theUS focused on electric drive with the first turbo-electric driveinstalled in 1908. Rated at 400 shaft horsepower, it wasinstalled on the Joseph Medill, a fireboat [1], [7]. Four yearslater, the collier USS Jupiter became the first naval vesselto adopt turbo-electric propulsion. That was an experiment,the ship also included a diesel engine and a direct coupledsteam turbine. The 3,500 hp turbo electric system supplied byGeneral Electric was deemed a success and the Navy decided


to convert all front line battle ships to electric power. TheJupiter went on to become the first aircraft carrier the USSLangley [8], [9].

The first battleship to adopt turbo-electric propulsion wasthe USS New Mexico. Launched in 1914, it was equippedwith a pair of 11.5MW 3,000/4,242 volt dual voltage, variablefrequency generators that powered four 7,500 hp 24/36 poleinduction motors and was capable of a speed of 21 knots.The shaft alley was shorter and thus less of a target, andfuel economy was improved substantially. All that came at theexpense of weight the electric motors and controls were heavythough reversal was accomplished easily by the switching ofcircuits with no need to change steam systems [2], [8].

The first passenger vessel to incorporate the new systemwas the Yorktown. Built as the Cuba in 1894, after a 1916wreck it was reconfigured as a turbo electric system in 1919[2], [10]. Electric drive was not limited to the US. In Swedenthe shipbuilder Rederiaktiebolaget Svea constructed in 1916 apair of cargo ships. One of them, the Mimer, was constructedwith steam power; the other, the Mjolner, was equippedwith electric drive; radial flow reaction turbines powered acinduction motors coupled to a single shaft through reductiongears [11]. Two years later the cargo ship SS Wulsty Castlewas constructed in Britain with a similar drive system.

While the steam engine was practical for land based powergeneration, the low efficiency of fuel consumption led to asearch for a better method. Rudolf Diesel, a German inventor,patented the diesel engine in 1892 and licensed production inSweden and Russia. In 1903, constant speed diesel enginescoupled to an electric transmission were installed in the Van-dal, a river barge sailing on the Volga River that transportedcoal to St. Petersburg and also to Finland [1], [2].

C. Submarines

The availability of electric power for illumination, commu-nication, and propulsion had fostered the concept of an all-electric ship. It was therefore logical to extend that concept tosubmarines for which a practical power source had remainedan elusive goal. There had been much experimentation withthe concept of underwater crafts during the 19th century;propulsion varied from manual to stored compressed air,even pressure from chemical reactants. In 1885, the Frenchdesigner Claude Goubet had introduced electric propulsionwith a pair of experimental submarines. By 1900, France,the United States, and Britain were exploring the submarineconcept, the latter two nations expanded on the work of JohnPhilip Holland. Most of those schemes focused on the internalcombustion engine for surface operation and the charging ofbatteries for use when submerged [2], [12].

D. Effect of World Wars I and II

Germany made extensive use of submarines to attack shipsduring WW I; subsequently the United States, Britain, andJapan engaged in an arms race. That was stopped by treaties inthe 1920s which limited or forbade entirely the construction ofnew, or the reconstruction of, existing vessels thus effectivelyhalting technical development. Subsequent agreements sought

to continue limits imposed on navies until Japan withdrewfrom the agreements in 1934. An arms race followed, and theUnited States commenced construction of battleships thoughelectric propulsion was not adopted due to concern for elec-trical system vulnerability to damage during battles and alsoa concern for the additional weight which could be betterutilized for weapons or armor [2], [13].

Electric propulsion was adopted by the United States forthe navy oiler, a tanker that supplied oil to ships at sea. Witha maximum power of 7,240 hp and a speed of 15 knots ithad a range of 12,600 miles. 481 were constructed duringthe war [2]. Diesel-electric submarines of various types wereconstructed in large numbers during World War II.

III. TOWARD MODERN ELECTRIC SHIP PROPULSION

The development of the mercury pool rectifier for powerconversion in the early 1900s produced a practical alternativeto mechanical power conversion; both as a rectifier and also asan inverter. Solid state power electronics emerged in the 1960sand 70s to enable significant advances in the power systems ofships. The first British passenger liner with alternating currentpropulsion was the SS Canberra in 1960. Equipped with three6,000 volt synchronous motors that produced 85,000 hp, themost powerful ever installed on a ship, they enabled it to cruiseat 27.5 knots. Separate generators supplied non-propulsionloads [14], [15].

The use of power electronics to maximize fuel efficiencybecame a trend in the 1980s. In 1984, the Cunard Line re-equipped the Queen Elizabeth II with nine German MANdiesel engines coupled to an electric transmission. The systemwas designed such that only seven engine sets were required tomaintain the design speed of 28.5 knots, which thus effecteda fuel savings of 35% [2].

Power electronics found extensive application in off-shorevessels such as Platform Supply Vessels (PSV) and otherservice ships. Dynamic Positioning (DP) systems requiredsophisticated control systems to maintain position in special-ized operations. Diesel electric propulsion was the standardmethod, though LNG was also adopted in the early 2000s.Nuclear reactors for steam turbine systems were developedinitially for submarines the USS Nautilus of 1954 beingthe first such vessel, the USS Long Beach the first nuclearpowered surface vessel followed in 1959. That same year thefirst passenger and cargo ship, the NS Savannah was launched[16], [17].

In the constant drive for greater fuel economy, hybrid driveships have been developed; the propulsion supplied by gasturbine direct drive or electric motors supplied by dieselengine-generator sets, the system configured as needed tomaximize fuel efficiency.

Fuel cells and Battery Energy Storage Systems (BESS)to adapt ships to renewable energy sources have emergedrecently. In January 2015, the world’s first fully electric pas-senger and car ferry, the MF Ampere was launched. Capableof accommodating 120 cars and 360 passengers, it makes a30 minute crossing between Oppedal and Lavik near Bergen,Norway. One MW of battery capacity supplies the ferry, the


battery sets charged when the vessels arrive at the docks.Operated by Norled AS, the ferry is a product of the Fjellstrandshipyard and Siemens AS [2].

IV. PROPERTIES AND CHALLENGES OF THE MARINEVESSEL’S POWER SYSTEM

Electrical power systems for marine vessels have existedfor more than 100 years, and history has shown a high levelof research and innovation, to bring the early applivations ofshipboard electricity of the 1880s to modern power systems.Vessels today often consist of an ever increasing electricalload: The majority of the propulsion systems and auxiliaryloads, such as weapon systems in naval vessels, hotel andservice loads in a cruise vessel, and station-keeping (DP)systems for subsea operations, are of an electrical type. Thepower is, in general, generated from prime movers using e.g.diesel and/or gas, or from nuclear plants with a turbo-electricconfiguration. In many modes of operation the power systemsneed to be reliable and exercise a high level of survivability.The Naval Sea Systems Command states the design philosophyfor naval power systems very well [18]–[20]:

The primary aim of the electric power system designwill be for survivability and continuity of the elec-trical power supply. To insure continuity of service,consideration shall be given to the number, size andlocation of generators, switchboard, and to the typeof electrical distribution systems to be installed andthe suitability for segregating or isolating damagedsections of the system.

This design philosophy does not only apply to naval ships.Vessels that exercise dangerous operations, such as DP op-erations near offshore structures, or operations in which ingeneral, any failure could have a high economical or environ-mental consequence, need power systems with high levels ofreliability and survivability and electrical stability.

On the commercial side the vessels should be fuel efficient,thus keeping the emissions (air pollution) to a minimmumand the fuel costs low. One of the most critical issues facingship owners and builders today is stricter regulations foremissions, such as the International Maritime Organization’s(IMO) MARPOL air pollution regulations [8], [21]. Due tothese stringent exhaust emission regulations, a lot of focushas been devoted toward technology such as exhaust catalysts,electronically injected common rail diesels, and waste-energyrecovery, such as heat-exchange systems. Also alternative fuel,such as LNG, has also found its way to a broader market.

Properties (and requirements) such as reliability, survivabil-ity, and continuity of electrical power supply, sustainability,and efficiency can all be related to the power system’s design,electrical stability, and manner of operation. In the followingsome of the aspects of the shipboard power system’s ongoingdesign trends, properties, and challenges will be discussed. Fora thorough introduction to the most common shipboard powersystem designs it is referred to [22].

A. AC vs DCThe early shipboard power systems were of a dc type, but

with the introduction of the ac motor this changed and ac

became the main trend in shipboard power system designs.One of the reasons for this was that the early dc systems(without power electronics) needed rotating devices to trans-form the power from one voltage level to another [23]. Theac power system has been the most used power system inmarine vessels, but now, with modern power electronics andother technological advantages, the discussion of using dc orac distribution in shipboard power systems has been broughtto the table and some of the key points whether to use acor medium-voltage dc (MVdc) are (adopted from [20], [24]–[26]):

• Impedance: MVdc power systems are capable of pro-viding greater energy dynamics than the classical acpower systems due to elimination of many componentsfor power conversion and optimizing the use of the cables(only ohmic resistance). The dc distribution doesn’t expe-rience skin effect in the power transmission, as is the casein ac transmission. Also, due to the lack of a fundamentalfrequency, the dc system does not have a power factor,and depending on the voltage levels, the weight of cablesmay decrease for a given power level. Unlike the dcsystem, the ac system has reactive currents that increasethe losses, which thus reduce the energy transportationcapability. Cable impedance in an ac system causes acurrent-dependent voltage drop along the cable, howeverthe impedance of the cable automatically limits the short-circuit currents. In dc systems only the (very low) ohmicresistance of the cables limit the short-circuit currents,thus all parts of the power system are equally effected bya short-circuit at an arbitrary position. This effect, andthe missing natural zero-crossing of the ac current makesit hard to break a connection (bus-tie/circuit breaker) oreven limit the dc current, which may endanger powerconverting devices that contain power electronics.

• Prime mover speeds: In dc systems the speeds ofthe prime movers can be altered, as the prime moverspeeds are largely decoupled from the power quality ofthe grid. Since frequency control is not a concern, theprime movers can run at optimized speeds (relative powerdemand with the objective of increased fuel efficiency)connected to generators with an arbitrary number ofpoles.

• Connection of parallelled power sources: In ac systemsparallelled power sources must be both voltage and phasematched before being connected to the power system. Ina dc system the phase matching is not needed, resultingin a faster power generation response time.

• Power Electronics Conversion System: In dc systems,medium or high frequency transformers (dc-ac-dc elec-tronic transformers) can be used resulting in a smallerfootprint. On the other hand, in ac systems the transform-ers make an easy and reliable adaption of the voltagelevels, however the conversion system often includes adc-link stage. Hence, using a cable connection instead ofthe internal direct connection of the dc-links between thesource- and load-side of the converter leads to a dc grid.Linking the dc-links from the converters directly will


demand a sufficiently high dc-link voltage in the orderof 10kV. Using back-to-back converters with internal dc-links, which are state of the art, this dc-link voltage canbe reduced by adapting to the high ac-side voltage bya transformer, at the cost of increased weight and spaceand reduced efficiency.

• Fault currents and circuit breaker technology: In dcsystems the fault currents can be controlled to levelsconsiderably lower than in ac systems. This is becausepower electronics can be used instead of conventionalcircuit breakers. Lower fault currents will also reducedamage during faults. On the other hand, the ac systemscan use much simpler circuit breaking technology thandc systems as electrical arcs clear at zero-crossing of thecurrent.

• Acoustic signature: The dc system does not have asignificant acoustic signature, as is the case with acsystems due to a common fundamental frequency. Thiscan be an important feature for naval vessels. However,the constant magnetic field created by dc current canleave a residual magnetic field in ferrous materials, whichcontributes to the overall ship magnetic signature. Thistends to be, among other things, a disadvantage withregards to mines and sensor/equipment interference.

• Weight and space: In dc systems, high-speed gas tur-bines can be used in conjunction with high-speed gener-ators, without the need for reduction gears for frequencycontrol, which is often the case in ac systems. A matedhigh-speed gas turbine and generator enables a shortergenerator with reduced footprint. This is desirable dueto space and weight savings. For constant power, the dcsystem needs only two conductors compared to the acsystem, which needs three. Removing one conductor isbeneficial due to weight savings.

New technological advances, such as the modular multilevelconverters (MMC) can, in special configurations, solve manyof the issues and challenges of dc power grids, thus makingthe dc system a more interesting solution in shipboard powersystems than before. Even though the MVdc solution maylead to reduced weight, increased efficiency, and offer high-energy transport capability at low losses, challenges suchas short-circuit currents, dc-breaker technology, and systemstandardization must be solved [24]. The different powersystem solutions, whether it is pure ac, a hybrid between acand dc or pure dc, have different properties, advantages anddisadvantages. The choice of power system (pure ac, ac/dcor pure dc) will be strongly dependent on, among others,available technology and different components from differentmanufacturers, developer and customer preferences, most eco-nomical solution, type of equipment connected to or poweredby the power system, possibilities for energy storage, spaceand weight requirements, the level of redundancy and rules andregulations from classification entities. These aspects, alongwith an economical point of view, will influence in shapingthe power system solution.

B. Marine Vessel Power Systems and Microgrids

Microgrids are electrically and geographically small powersystems capable of operating connected to, or islanded from,a national grid [27]. In islanded mode, the microgrid hasstrict requirements imposed such as energy independence andservice quality for an extended period. Marine vessel’s powersystems are indeed microgrids; they are isolated (and islanded)while at sea) and part of a terrestrial grid while docking andconnected to shore power. Shipboard power systems have alot in common with terrestrial stand-alone microgrids; manyof the methods and a lot of equipment and components arethe same [28]. In addition many control strategies and designprinciples used in microgrids may be applicable for shipboardpower systems, and also the other way around. Examplesof such control strategies and design principles are voltageand frequency control schemes, power quality improvementstrategies, power sharing methods for multiple distributedgenerators, and energy management systems [29]–[33]. Athorough overview of technical cross-fertilization between ter-restrial microgrids and ship power systems is presented in [27].Some of the main differences between a shipboard micrigridand larger terrestrial (commercial) grids are summarized in thefollowing [20], [27]:

• Frequency: The shipboard power system’s fundamentalfrequency cannot be assumed constant. Due to limitedrotational inertia of the prime movers and the generators,rapid load changes can cause fast acceleration and de-celeration of the motor shafts, which causes frequencyfluctuations. Such fluctuation may last for a couple ofseconds until the speed of the shafts reach a steady statethat coincides with the reference frequency.

• System analysis: In analysis of a commercial grid allthe system’s time constants are quantified and used toanalyze the problem by time-scale separation. However,such analysis is not easy to conduct in a shipboard powersystem due to the principal time constants for motordynamics, electrical dynamics, and controls which all liein the same time range of milliseconds to seconds.

• Planning of power generation: In a commercial gridthe power delivered by each generating unit is scheduled.The difference between consumed and produced power isregulated through equipment acting as swing generators.This is not the case in a shipboard power system asall the generators share the active and reactive powerthrough fast exchange of load-sharing information, whichamplifies the parallelled generators’ dynamics. Hence,instead of generator scheduling the shipboard powergeneration exhibits load sharing, which is often realizedby generator droop control.

• Electrical distances and load flow: In the commercialpower sector it is important to model the electricaldistances (transmission lines) in the power distribution toachieve the right dynamics and proper voltage regulation.This is not the case in a marine vessel’s power systemas the electrical distances are short, thus trivializing theload-flow problem. The short electrical distances result inlow impedance which increases the coupling between the


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(a) Conventional electric-drive power system: Separated, or segregated,power generation for propulsion and auxiliary loads.

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(b) Integrated power system (IPS): Integrated power generation for propul-sion and auxiliary loads.

Fig. 2. Simplified drawing illustrating the main structural difference betweenconventional and integrated power systems.

different parts of the power system. Hence, to strengthencoupling between devices and subsystems the assuranceof stability needs proper attention.

• System’s size and extent: Due to the shipboard powersystem’s limited extent, a higher level of centralizedcontrol can be applied than in commercial systems. Theshorter electrical distances also facilitate easier synchro-nization of data and measurement retrieval than in acommercial grid.

• Load profile: In a shipboard power system the loadprofile is often rapidly changing due to the power de-mand from the propulsion system and other high-ratedsystems and equipment. Hence, the power (both activeand reactive) is changing more rapidly in a shipboardpower system than in commercial distribution systems.

• Single line faults: A shipboard power system is designedto continue operation with a single phase (line) to ground.For safety reasons such medium voltage systems alwaysinclude high impedance grounding systems.

• Environmental effects: A shipboard power system mustbe able to operate in a tough environment, which ischaracterized by vibrations, shock and motion dynamics,and should survive salinity and moisture.

C. Integrated Power System (IPS) and Grid Design

In an Integrated Power System (IPS), or integrated-electricship, all the required power, for the vessel’s propulsion andauxiliary (service) loads, is generated and distributed by thesame main generators. In comparison, in a conventional (seg-regated) electric-drive vessel power system, the propulsion

and the auxiliary loads are separately powered by dedicatedgenerators [22]. Fig. 2 illustrates the main structural differencebetween the conventional (segregated) power system and theIPS.

The propulsion system in a conventional power systemwas originally a mechanical-drive system with reduction gearsconnecting the prime movers to propeller shafts. Many vesselswere converted to electric propulsion to gain faster response,which resulted in the separated conventional electric-drivepower system. Even today there exist numerous vessels withthis kind of power system. As Fig. 2a indicates, the conven-tional power system consists of two separated subsystems; onefor propulsion and one for auxiliary loads. Due to the separa-tion between the subsystems, the engines of each subsystemare only connected to their respective systems and can onlybe used within that subsystem. This configuration has beenthe leading design for ensuring maneuverability; almost 90%of the vessel’s generated power is locked into the propulsionsystem [34]. However, this separation, where the majority ofthe vessel’s power supply are limited to the propulsion system,can be a disadvantage as the propulsion power is not availablefor other mission specific systems.

To tackle the disadvantage with the conventional powersystem, the IPS was introduced as a solution. Instead ofseparating power generating units into stand-alone subsystems,the IPS shares all generated power from all the generators onan integrated power grid, which distributes the power to allindividual consumer systems located throughout the grid in autility fashion. The IPS’s ability to share the generated powerbetween all (online) consumers is also an important propertyfor easing aftermarket installations of electric equipment, asnew equipment is simply connected to the distribution grid.The property of power sharing is the main advantage of IPS,and improves power flexibility (operational flexibility) andavailability. At low- and medium-speed ranges, the IPS cangenerate the same amount of power as a conventional powersystem with fewer running prime movers. This is preferableboth from an economical and an environmental point of view,as fewer running generator sets (gensets) will enhance thefuel efficiency and reduce exhaust emissions. By starting andstopping gensets relative to the vessel’s power demand, theIPS provides a stepped power generation, and by equippinga vessel with gensets of different power ratings, the powerproduction could be optimized to avoid low non-ideal loadingconditions of the prime movers. However, this is seldom thecase since all or multiple gensets in a vessel are often of samesize to make maintenance and access to spare parts easier. Inaddition, if the IPS operates with open bus-ties (see Fig. 3),both sides should have the same power generating capacity.The future shipboard power system may have an elegantsolution to the optimal prime mover loading problem involvingEnergy Storage Systems (ESS) [35], [36], that can store excesspower to achieve ideal prime mover loading conditions, which,among other scenarios, can be used to give a green approachto harbors without emissions.

1) Electrical Stability: Reliability, dependability, and sur-vivability are important properties for many shipboard powersystems. A naval vessel must be able to survive an attack


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Fig. 3. Example of a typical redundant IPS for PSVs and small-medium naval ship. Redundancy for bus-tie breakers connecting Main SwitchBoard (MSB).Redundant power supply for vital loads using Automatic Bus Transfer (ABT) [22], [34].

where parts of the power system are down, but still be able tobring the ship away from the situation and have the powerneeded for initiating defense measures. An offshore vesselconducting a station-keeping operation (DP) near offshorestructures needs to survive single faults and have the powerneeded to bring the vessel to a safe position away from thestructures. In the same way, a deep-sea drilling vessel musthave a reliable power system that survives faults and maintainsstation-keeping to avoid critical situations that can harm bothequipment and crew.

• Reliability is often explained as a fail-safe operation [34],and the term system reliability is a standard measure forthe effect of component failures and internal errors and iscalculated using component mean time to failure (MTTF)statistics and static dependency analysis [37].

• Dependability is given as the system’s ability to continueoperation despite component failures, internal errors andexogenous disruptions.

• Survivability, on the other hand, is mostly used for navalvessels and military applications and deal with continuityof vital services during major disruptions associated withbattle and damage control operations.

In many settings the terms are mixed together, and reli-ability often comprises both dependability and survivability.To achieve a reliable IPS, which cultivates both dependabilityand survivability, the most used design principle is redundancy,however, spatial separation and manual backup systems havealso been used to a great extent.

An often used redundant two-split IPS design for small andmedium size vessels is shown in Fig. 3. As can be seen, thepower generating units are split in pairs, each pair connected

to a switchboard (MSB 1 and 2), and the switchboards are con-nected through redundant bus-ties. Each switchboard suppliesone propulsion system, and both switchboards are serving theservice loads. The load center is split in two switchboards.The vital service loads have redundant power supply fromboth switchboards using an Automatic Bus Transfer (ABT)unit, while the non-vital loads are served by one of theswitchboards, one on each side of the vessel. Depending onthe vessel type and class regulation from classification entities,the IPS may include an emergency generator supplying vitalloads, and in some cases part of the propulsion loads. Thebus-tie between the load center switchboards has the ability toconnect the switchboards if, for instance, one of the servicetransformers fails. The IPS is equipped with many breakers,which may be used to isolate faults from propagating throughthe grid and causing a complete blackout. Hence, this property,reconfigurability, is important for achieving the needed systemreliability, and is closely related to the IPS’s practical designand installation, as well as fast and reliable fault detectionsystems that are able to invoke protection schemes isolatingthe faults.

2) Radial and Zonal Grid Designs: Traditionally, thepractical solution to provide redundant power distributionwas to install alternate power routes between componentsusing longitudinal cables connecting vital loads to multipleswitchboards. This solution, a radial distribution, was shownto be a bulky and heavy solution with the ever-increasingnumber of vital electrical loads. As a solution, the zonaldistribution grid was introduced in the 1990s, where theredundant power supply was realized by providing vital loadswith alternate power routes using shorter transverse feeder


Generator

Switchboard

Bus TransferLoad CenterBus Tie Point

Zonal Distribution System

Plan View

Profile View

Radial Distribution System

SWBD Load Center

Plan View

Profile View

Fig. 4. Comparison of radial and zonal power distribution systems in a marine vessel [20].

cables from port and starboard switchboards [34], [37]. Thismay be seen as stretching the switchboards along the vessel’slongitudinal axis, one switchboard for starboard side and onefor port side. Bus-ties are used to isolate faults, or segregateparts of the switchboards. With this solution, the long feedercables in a radial system are removed, with the effect ofreduced cost and weight - which again leads to lower fuelconsumptions and emissions. The zonal distribution topologyis usually adopted in the IPS design philosophy, enabling eas-ier aftermarket installations of equipment and more flexibilityregarding installation of redundant solutions for achieving adesign with the needed level of reliability and survivability atrelatively low cost. An illustration showcasing the differencesbetween radial and zonal grids is given in Fig. 4.

It is expected that tomorrow’s power system design so-lutions will be completely different from today’s solutions.Future shipboard power systems should aim for a higherquality of service (QOS), increased reliability and efficiencyas key requirements, which may be achieved by, amongother means, a completely new design strategy, advancedmonitoring of system health and state as part of new sensortechnology, and advanced and efficient stability and powerquality improvement methods and devices.

D. Power Electronics and Harmonic Pollution

Electricity enables a more flexible way to utilize energythan any other energy source. Technology such as infor-mation systems, radar and sonar systems, advanced motioncompensation, and military precision weaponry would not bepossible without electricity. Future predictions show that moreand more equipment is of an electrical type, and the marinevessel is asymptotically converting towards an All-ElectricShip (AES), where all installed equipment and systems areof an electrical type [22], [38]. The broad variety of electricalequipment and systems connected to the power system requiredifferent power conversions. Some of the equipment andsystems are powered by ac, while others are powered by dc.In addition, the needed (and rated) voltage levels may spanfrom a few volts to thousands of volts, and different systemsand equipment may require different frequency levels. Almost90% of the vessel’s generated power may at some points

go to the propulsion systems [34], flowing through powerelectronics devices. Power electronics is at the heart of powerconversion, and, because of this, the IPS includes numerousdifferent power electronics devices to be able to supply theright form and level of power to the connected systems andequipment. The shipboard electrical power demand continuesto increase, from tens of MW and in some cases even greaterthan 100MW [38]. However, such high power ratings leadto power electronic devices that are both heavy and have alarge footprint. This is a real handicap for serving high powerdemands. In general, in the given order of priority, size, losses,cost, and weight are interrelated factors that limit acceptableapplications of power electronics.

An important power electronic device is the con-verter/inverter, which is able to convert the electric powerfrom one form to another, i.e. ac/dc, ac/ac, dc/ac, dc/dc. Thenecessary power for each load, or group of loads with the samepower requirements, are in an IPS converted at point-of-use.In fact, almost all power sources and loads need a converter.Pulse-Width-Modulation (PWM)1 has been widely used formodulating small- and medium size converters. A switch-modepower electronics converter, which consists of switches that areeither on or of, uses PWM to control the time the switchesare on and off, and by this, the converter (which in fact isan array of switches) can be programmed to produce voltageand current waveforms, different power factors, and obtain adesired frequency from a range of different input waveforms.From this point of view, there is little difference betweenmotor drives, power supplies, and active power filters, and thecomposition of power electronics in such devices can be gen-eralized to form a Power Electronics Building Block (PEBB)[39], [40]. These building blocks are intended to minimizethe number of different power electronics devices in a powersystem and can be mass-produced due to their generality. Thegeneral design will also allow the power electronics to betightly packed, which will reduce weight and footprint. Theblocks may be controlled by different algorithms and softwaresolutions, through a general interface (communication proto-col), and can be changed in the field, depending on operational

1Often realized with a hysteresis control scheme.


status or mission type. The blocks can easily be installed (plugand play) with an interface which allows information sharingbetween the components. Depending on the way the blocks areconnected to each other, different algorithms may be deployedas part of a configuration scheme; and, depending on thepower system’s status and classification requirements, differentalgorithms may be enabled to perform functions such as powerconversion, harmonic mitigation (as an active filter), active orreactive power control, or inherit a simple breaker’s propertiesto isolate faults. Due to its generality, a wide range of differentmodelling and simulation tools may be developed around thisblock, which will ease power system design and realizationdramatically, thus ensuring stability, reliability and efficiency.An important part of developing PEBB is the continuationof improving power electronics in the sense of minimizingweight, size, and losses, to achieve components that can handlemore heat and have faster dynamic response with increasedpower ratings. The PEBB is seen as tomorrow’s solution foradvanced power systems. Even though a lot of research anddevelopment has been devoted to realizing such a standardizedbuilding block, a general solution has not yet become availableon the market.

A lot of research has also been conducted towards powersemiconductor devices, which consist of a variety of diodes,transistors, and thyristors. New designs have produced compo-nents with better performance and lower losses, but few of thedesigns have reached the market. Also silicon carbide (SiC)has been devoted attention due to the material’s propertieswhich leads to lower switching losses, high voltage and hightemperature capabilities. SiC devices are expensive, but havea huge impact on converter size, losses, weight, coolingrequirements and potential for high PWM frequencies [20],[38], [41].

The composition and use of different power electronicsto make a general PEBB will affect the shipboard powersystem in many ways. The transition from early solutions usingLine Commutated Converters (LCC) and Cyclo-converters totoday’s PWM Voltage Source Converters (VSC) had manyadvantages, including lower harmonic pollution, four-quadrantoperation and converter reversibility [42]. It is also expectedthat the introduction of the PEBB will lead to an increasedpower quality: The PEBB can be designed and controlled toachieve redundant and reliable solutions, with fewer buildingblocks, which minimize losses and keep the power qualityhigher than what is achieved in today’s solutions. However,power electronics in general are non-linear elements, with non-linear behavior, and are in most cases sources of harmonicpollution. In thyristor-based devices (which is often the casein motor drives) the harmonic spectrum is not dependenton impedance, thus introduces characteristic harmonic pollu-tions relative to the devices’ different designs. In a 6-pulseconverter, the characteristic harmonics are of 5th, 7th, 11th,13th, etc. order, and in a 12-pulse converter, the characteristicharmonics are of 11th, 13th, 23rd, 25th, etc. order. In avoltage source converter (VSC), which is not based on thyris-tors, these characteristic harmonics do not occur, and motordrives consisting of VSCs instead of thyristor-based drivesmay solve the problems with the characteristic harmonics.

However, the VSC introduces harmonics dependent on themodulation frequency, which may be 1kHz or higher. LCLfilters are often used to suppress the harmonics generatedby the VSC, but LCL filters are passive devices and tunedfor a given modulation frequency. If, for some reasons, theVSC changes its modulation frequency the LCL filters haveto be re-tuned. In addition, the harmonics from a VSC maycause harmonic resonances due to interaction with passivefilters [43]. Hence, harmonic pollution can, to some extent,be suppressed by design, but the ever-increasing number ofelectrical devices, which are directly or indirectly dependenton power electronics, will introduce even more non-linearelements into the power system, making harmonic mitigationand power conditioning devices a necessity.

Harmonic pollution is defined as any waveform with fre-quencies that are multiples of the fundamental frequency, andis measured as Total Harmonic Distortion (THD), which is anormalized quantity describing the relation between the am-plitudes of the harmonic frequencies and the amplitude of thefundamental frequency. Most shipboard power systems todayare affected by harmonic pollution in some or another way[43]. Harmonic pollution, which impairs the power system’spower quality, leads to higher fuel consumption and emission.Harmonics are closely connected to reactive power, and highlevels of harmonics may lead to equipment and system breakdown, and even cause catastrophic events like explosion andfire [43]. Theoretically, this can, in the worst case, cause acomplete blackout as a result of voltage collapse. A completeblackout may occur due to high levels of harmonic pollution,but is usually caused by operational mistakes. The term voltagedip ride through capability is often used to describe theconsumers’ ability to cope with faults and malfunctions wherein worst case it must be assumed that the voltage becomeszero until the faults are fixed or isolated. Examples of suchmalfunctions and failures may be short circuits and high inrushcurrents while starting large motors. The allowed voltage dropis dependent on the vessel and its operations and is set byclassification entities [44].

In DP-operations (e.g. DP2 [45]) with closed bus-ties,assessments regarding voltage dip ride through capability mustbe conducted as part of FMEA to assure continued opera-tion after faults or malfunctions occur. Many DP-operations(station-keeping operations) are performed with open bus-tie, splitting the power system in two, thus minimizing thechances for a complete blackout. This is not an economicalnor an efficient solution, as splitting the power system intwo requires an increased number of online prime moversfor power generation, and also requires multiple separatedpower management systems (PMS). Harmonic mitigation istherefore not only important for the power system’s efficiency,but also for its stability and reliability. Harmonic mitigationand power conditioning is a active research topic, and manyactive and passive filter solutions have been proposed [46].Passive filters do not have the ability to change their tunedfrequency, and due to changes in power system configura-tions (and changes in load profiles) as a result of differentoperational requirements and mission types, passive filtersare not always a good solution for harmonic mitigation as


a change in the harmonic spectrum requires a re-tuning ofthe filters. An active filter, on the other hand, has the abilityto mitigate any frequency spectrum, the only limitation beingthe bandwidth of its controller, thus increasing flexibility forchanges in the power system’s harmonic frequency spectrum.Active filters have also a smaller footprint than passive filters,which is a desired property in marine vessels. Active filtersare expensive devices, thus location of installation in a powersystem is important for maximum utilization (and mitiga-tion) of the filter’s power rating. A conceptual method usingoptimization (Model Predictive Control) to perform systemlevel harmonic mitigation has also been proposed [47]–[50].Active filters come in many forms, and can be part of e.g. apropulsion system’s motor drive, realized as controlled ActiveFront End (AFE) converters or simply stand-alone devices.Harmonic mitigation (and power conditioning) is, as earliermentioned, important for achieving an efficient and reliablepower system, and the harmonic pollution problem is alsoexpected to be an issue in future power systems, consisting ofeven more non-linear components. As of today, there are noclassification entities that require real-time THD surveillance,which would be an important measure for detecting potentialstability issues as well as performing fuel efficient operations.THD requirements are checked by classification entities duringthe vessel’s commissioning and certification using handheldmeasuring devices. The future power system, where relia-bility and efficiency are cultivated, may require real-timeTHD surveillance and power conditioning devices (possibleconsisting of PEBBs), which may be backed on optimizationfor system level harmonic mitigation, to comply with stringentair pollution regulations, as well as achieving higher reliabilityin terms of blackout-prevention due to increased power quality.

E. Energy Management Systems (EMS) and Energy StorageSystems (ESS)

Planning power generation, energy management, is impor-tant for achieving an economical and efficient power genera-tion with optimal prime mover loading conditions, thus keep-ing the fuel consumption at a minimum. In ac power systems,the prime movers are speed-controlled, mostly connected tofixed speed generators, to maintain a desired (and designed)frequency within allowable variations (deadband). As theprime movers’ speeds are more or less fixed due to frequencycontrol, the loading of each prime mover determines the fuelefficiency in terms of amount of fuel per delivered amountof useful energy - Specific Fuel Oil Consumption (SFOC)

gkWh . The prime movers often experience speed deviations asan effect of dynamically changing load profiles (active andreactive power demand), in which affects the inertia on theshafts between the prime movers and the generators. If suchprime mover speed variations result in frequency fluctuationsexceeding the allowed deadband, the prime mover needs to beisolated and shut down. Large negative frequency fluctuationscan also be an indication of the running prime movers areunable to meet the load demand, thus additional supervisorysteps should be taken to either shed non-essential loads orspin up idle prime movers, and after synchronization connect

them to the power system. Because of speed variations and al-lowed frequency fluctuations within a designed deadband, thefrequency in shipboard ac power systems cannot be assumedconstant.

In addition to frequency fluctuations, the speed variationson the motor shafts will also increase wear and tear leadingto higher maintenance costs. Controlling the prime movers totrack a constant speed greatly affects the power generationas an optimal increase or decrease in power generation isrelated to starting and stopping prime movers in a stepwise(ac) power generation [51]. As the load demand must be metat all times this means that prime movers running at low non-optimal loading conditions is often the case in shipboard acpower systems. To increase the fuel efficiency related to thepower demand, the prime mover loading could be increasedand power stored to be used in situations where the powerdemand surpasses the power generation. An example wouldbe to provide the difference between consumed and generatedpower while additional prime movers are being started andconnected to the power system to meet an increasing powerdemand.

In dc power systems, where the power distribution isconducted on dc grids, the prime movers may run at vary-ing speeds to meet the power demand. As in ac systems,the voltage level is maintained by controlling the generatorsexcitation fields. Due to the flexibility of being able to changethe prime movers’ speeds, the power generation will adopt amore stepless behaviour than in ac systems. However, primemovers running outside their optimal speed ranges are proneto wear and tear, and especially at low speeds the combustionis not optimal and will increase sooting of the prime movers,thus increasing maintenance costs. At high speeds the fuelconsumption is not in line with the produced power (non-linearrelationship between fuel consumption and produced power),thus reducing the fuel efficiency which leads to increased fuelcosts and emissions. As with ac power system, the dc powersystem could also benefit from a ESS that facilitates optimaloperation of the prime movers.

1) ESS Applications: Many suitable ESS technologies thatfacilitate a more economical and redundant operation in amarine vessel are available on the market today. The choice ofESS technology is related to area of application, energy den-sity, size, weight, and cost, expected lifetime, charge/dischargerates, and other functional requirements. Examples of ESStechnologies are Battery Energy Storage System (BESS),Compressed Air Energy Storage (CAES), flywheels, Super-conducting Magnetic Energy Storage (SMES), capacitors (in-cluding ultra-capacitors) and Pumped Hydro Storage (PHS)[52]. Depending on the power system (ac or dc) most ESStechnologies need power conversion devices that convert thepower from and to the power system for charging and dis-charging purposes. An obvious application of a ESS would beto serve as a backup power source similar to an UninterruptiblePower Supply (UPS), in which sets strict requirements tothe ESS technology’s energy density and rate of discharge.This type of application can be beneficial for many marineoperations. An example would be an offshore vessel con-ducting a DP-operation alongside an offshore structure that


experiences faults that cause power losses which may lead toa blackout. The ESS may in this case be crucial for poweringthe propulsion system for a short period of time to be able toreposition the vessel at a safe distance away from the structureto get the time-window needed for isolating the faults and tore-power the vessel.

Many power consuming systems and equipment on a vesseldo not have a flat load profile. Propulsion systems, whileconducting station-keeping, have a load profile which corre-lates with waves and ocean currents. Weapons systems aboarda naval vessel may give a pulsed load profile at irregulartime instants, which would be more or less impossible topredict. Due to the vessel’s dynamic load profile, the energymanagement is not an easy task, and, as earlier mentioned,often more prime movers are running than are actually neededto be sure of serving the load demands. One application ofthe ESS, which is a feature that is sought for in a shipboardpower system, is load shaving or more precisely peak shaving[53]. By using the ESS to flatten the vessel’s total loadprofile, energy management, in terms of starting and stoppinggenerators, would be easier, and fewer prime movers have tobe on line to meet a potential high and instant power demand.Under low non-ideal loading conditions the ESS charges,and while the load demand exceeds the power generationcapabilities the ESS discharges. Whether the power systemis dc or ac, the prime movers can be run at optimal speed formaximum fuel efficiency. This feature, peak shaving, may beseen as one of the strongest arguments for installing a suitableESS in a shipboard power system, as peak shaving may resultin a lower fuel consumption (and emission) due to the needfor fewer running gensets.

Another interesting application of ESS, dependent on ESStechnology employed, is harmonic mitigation [34], [54]. De-pending on the ESS’ speed of discharge, it may be used tosuppress harmonic pollution. The ESS may also be used tocharge a dc capacitor in an active filter, which strengthens thefilter’s capabilities, and thus enables the filter to use activepower in harmonic mitigation. Also frequency control by useof ESS has been proposed [55]. As an example, an ESS such asBESS may be installed alongside an Active Front End (AFE)(Fig. 5), which is a realistic scenario if for instance the ESS ispart of a motor drive. In this case, when the ESS is installedalongside an AFE, the ESS could attain application flexibility,thus being able to do harmonic mitigation, peak shaving, andeven act as a reactive power source or consumer to increase thepower system’s voltage stability margins. Fig. 5 showcases twodifferent locations in the grid for installing a BESS. The BESSmay be installed alongside an AFE or other power electronicdevices which supervise the BESS state of charge (SOC) andstate of health (SOH), and control charging and dischargingdependent on the load demand and the BESS’ SOC.

Even though the advantages of ESS in shipboard powersystems are many, it doesn’t come without challenges. Manyof the available and suitable ESS technologies are expensivesolutions, and are dependent on power conversion devicesrelative to ac or dc power systems. An effective solution, whichwas illustrated in Fig. 5b, is to install an ESS, such as BESS,as part of motor drives, thus eliminating the need for additional

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(a) Battery Energy Storage System (BESS) connected to the main bus(switchboard) in an IPS configuration. An Active Front End (AFE) isinstalled alongside the BESS as a solution for supervision and BESScontrol purposes.

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(b) Battery Energy Storage System (BESS) as part of a motor drive forpropulsion systems [34]. Point of Common Coupling (PCC) refers toe.g. the vessel’s main switchboard. An Active Front End (AFE) is partof the depicted motor drive and supervises charging and discharging ofthe BESS.

Fig. 5. Simplified illustrations of different installations of Battery EnergyStorage Systems.

power conversion devices, reducing weight, footprint, andcosts [34]. For BESS the available battery technology alsointroduces challenges, as the battery packs are heavy (relativeto power capacity) and in many cases have a large footprint.Despite weight and volume, the BESS may allow removalof one prime mover from a vessel, which justifies the useof BESS. Another issue is the battery packs’ lifetime. Rapidcharging and discharging of battery generates a lot of heat,which can be seen as losses, and may have critical effect onthe battery’s life. Thus a possible realistic outcome is thatthe battery pack dies before the BESS manages to pay backthe installation costs by reduced fuel consumption. In someapplications ultracapacitors or fuel cells can switch places withthe battery pack, giving the energy storage system differentproperties such as increased lifetime, charge/discharge speed,energy density relative to footprint and weight, etc. Alsohybrid energy storage systems, including different energystorage devices, may also be interesting possibilities, thusincreasing applications and system flexibility [56], [57].

When moving towards all electric-battery powered vessels, anew emerging technology -the inductive charging technology-has attracted the attention of the marine vessel communityand the old concept of Inductive Power Transfer has re-emerged for contactless battery charging of marine vessels[58]–[60]. Significant progress toward the development ofcommercial solutions for wireless charging is already on itsway for high power wireless transfer in the MW range [61].


This technology will greatly benefit coastal vessels operatingwith a tight schedule as it will significantly reduce chargingtime and improve reliability. This will bring unavoidablechallenges to the local power grid from which high powerwill be tapped in a short time to charge the vessel’s batterypacks. This impending impact on the local electrical grid willrequire grid reinforcements and new solutions that will requirecollaborative efforts between the utility and marine vesselsectors.

2) Standards and Guidelines: Many classification entitiesand interest groups impose strict regulations and set forthguidelines for redundancy for many types of marine vesselsto avoid total loss of maneuverability. This is mostly the casefor offshore vessels, like PSVs, but the requirements can alsobe found for passenger vessels and cargo vessels transportinghazardous materials. The International Marine ContractorsAssociation (IMCA) [62] states (for an offshore vessel) that ifthere is a realistic chance of the bus-ties not opening or notopening fast enough then the switchboard should be split forthe work (two-split in Fig. 3), and if so the power systemmust include an independent power system (Power/EnergyManagement System - PMS/EMS) for each individual split[63]. Furthermore IMCA states that for a diesel-electric vessela task appropriate mode could mean operating with closedbus-ties, whereas a critical activity mode of operation mayrequire open bus-tie configuration [64]. These guidelines arebased on risk assessments (Failure Modes and Effect Analysis- FMEA) and fault tolerance (isolation of faults) dependent onclassification and control system redundancy [65]. DNV-GL(earlier DNV) [66] describes that the traditional interpretationof the DP-3 requirements has been to run the power system asseparated (segregated) subsystems with open bus-tie breakers.This is backed on IMO [67] MSC/Circ.645 guidelines forvessels with dynamic positioning systems, which states thatfor equipment of class 3 the power system should be divisibleinto two or more systems such that in the event of failure ofone system, at least one other system will remain in operation[68]. However, closed bus-tie DP operations have economical,technical, operational, and environmental benefits, thus someDP operators run the power system with closed bus-ties foras large periods of operation as possible [45], [69]. ABS[70] also refers to the IMO MSC/Circ.645 guidelines, andstates that these guidelines should be followed in the designof DPS-2 (DPS - Dynamic Positioning System) and DPS-3systems where loss of position is not allowed to occur inthe event of a single fault [71]. For ships normally operatingin transit, such as tankers and cargo ships, the equivalentconcept is redundant propulsion as described in e.g. DNV-GL’s class notation RP. In short, all these regulations andguidelines state that, dependent on the vessel’s classification,one should not loose maneuverability, and due to the factthat it has been difficult to both engineer completely fail-safe power systems and prove that there is no chance forpower losses impairing the maneuverability, the trend hasbeen to operate the power systems with open bus-ties (a splitpower system). This type of operation increases the number ofneeded online prime movers, which results in lower efficiency(higher fuel consumption) and increased emissions. To be able

to close the bus-ties in all operational scenarios would be anecessity for future power systems with increased efficiencyand stringent emission requirements. To achieve this, thepower systems must be equipped with stability-improvingsystems and devices that, in a safe way, handle faults withoutharming the rest of the power system. Such systems mayinvolve harmonic mitigation, reactive power control, voltageand frequency control, peak (load) shaving, UPS systemsand advanced power system segregation and fault-isolationsystems. In order to take advantage of new technologicaldevelopments to increase operational flexibility without in-creasing risk, DNV-GL recently introduced the DYNPOS-ER(Enhanced Reliability) notation for DP class 2 and 3 vessels.

3) Emission Free Operation: In tomorrow’s shipboardpower systems the BESS (or another suitable ESS) may beessential to cultivate reliable and efficient power systems (bothac and dc), and applications such as harmonic mitigation, peakshaving, reactive power control (voltage stability), voltage andfrequency control, and backup power can simply be differentalgorithms deployed to a PEBB-based ESS. It is also expectedthat in the near future harbors may require an emission-freeapproach for vessels to load and unload, thus an ESS maybe part of a larger green system keeping the air pollution(emission) in harbors at a minimum. In addition, the EMS mustbe intuitive and easy to understand, and provide supportiveand advisory actions which are trusted by the operators. ManyEMS systems today are hard to understand, as a result theyare disregarded by the operators and kept out of the controlloop with the effect being an inefficient power system. Alot of work remains to map the operators’ behaviors andinteraction with the system to make an optimal, reliable, andtrustworthy interaction for efficient and economical control ofthe shipboard power systems.

F. Increasing Need for Measurements, Big-Data, and SoftwareComplexity

To achieve a reliable and efficient shipboard power system,many different measurements are needed. Active power mea-surements (voltage and current measurements) are importantfor the EMS to be able to meet the load demand, and an ESSneeds power measurements for conducting peak shaving. Inac distribution systems reactive power measurements (voltageand current measurements) are important for voltage stabilityassessments, and give a measure of the system’s efficiency.Frequency measurements are needed in ac distribution systemsas feedback to the prime movers’ speed controllers. Voltagemeasurements are needed for controlling the generators’ exci-tation fields, which are done by Automatic Voltage Regulators(AVR), and also for transformers and power converters con-necting equipment and subsystems (including energy storagesystems) to the power system need voltage measurements.When starting a prime mover and connecting it to the grid in asynchronization process both phase and voltage measurementsare needed. Voltage measurements with high sampling fre-quency are needed for harmonic mitigation, to assure voltagequality within boundaries set by classification entities. Theseare only a few examples of needed measurements.


Many parts of the power system have high real-time de-mands (high sampling frequency demands) for measurements.Harmonic mitigation using Active Power Filters (APF) andpower converters such as Active Front Ends (AFE) are ex-amples of systems that require (internal or external)a highrate of sampling measurements. In addition, fast hardwareand software is required to process the measurements inreal-time to be able to utilize the information for controlpurposes. Redundancy in measurement devices (sensors) isalso a requirement for achieving a reliable system. If onemeasurement device goes down another has to take over tokeep the needed information to the system flowing. Redun-dancy in measurement devices comes in many forms, and acommon approach in systems that relies on correct informationis to have a minimum of three measurement devices and usevoting algorithms to assure the correctness of the measurementinformation.

Some measurements may be contaminated by noise, andcommunication delays between taking the measurement andsending it to the subscribing system may make the informationno longer valid. Thus the use of filtering techniques forremoving noise, and estimators for estimating biases andtransport delays may in some cases be a necessary requirementfor optimal control, giving the subscribing system correct andvalid information. Advanced signal processing methods mayalso be used to detect and solve measurement drop-outs aspart of a solution to redundancy requirements for improvingsystem reliability.

With increasing system integrity that cultivates both effi-ciency and reliability of the shipboard power system, there isalso an increasing need for measurements. The present trendshows that more and more devices and subsystems are givenan IP-address and system information and measurements arebroadcast on a local network in the vessel in a cloud-basedarchitecture - The Industrial Internet of Things (IIoT). As aconsequence the future system integrity may involve consumersystems planning their power consumption, which is availableinformation for the EMS for use in power generation planning.

With the expected enormous amount of data as a conse-quence of an increase in measurement devices and broadcast-ing of system information to get more efficient and reliablecontrol, problems such as limited network throughput and dataprocessing resources may appear. Maybe the most frighteningissue is that when all the vessel’s systems ”come online”,the vessel is vulnerable to cyber-attacks. Even though anincrease in available system information, measurement data,and distributed control may be beneficial for controlling thevessel’s power system in an optimal, reliable, and efficientway, the development of the future shipboard power systemshave to address the Big-Data challenge in the design of itsarchitecture and assure cyber-security. There exists a rangeof different types of cyber-attacks, some of which are basedon gaining access to data and information, and others thatare disruptive and intended to take over or break down asystem. The latter may have catastrophic consequences if theyenable the attacks that gain control over the vessel’s powerand propulsion system. A small selection of potential externaland internal cyber-attacks will be treated separately in the

following:

• External cyber-attacks can be classified as cyber-attacksoriginating remotely from the marine vessels. There aredifferent strategies for protecting the vessel from suchattacks. A vessel’s access point to the rest of the worldand potential remote systems, which normally is a 3 layerswitch, has authentication and VPN capabilities whichprovide basic security. The switch can also limit input andoutput network ports, which restrict the communicationchannel. By enabling only output ports, the vessel datacan be encrypted and exported to e.g. onshore fleetmanagement systems without allowing any input trafficfrom a potential cyber-attack. A practical approach isdescribed by DNV-GL [72], where the main access pointto remote systems is to be powered on only when allowedby the vessel’s crew. Another form of attack is relatedto connection to other equipment or systems that areinfected. An example of such a case might be the vessel’sshore power connection while docking, where the shorepower is altered to harm the vessel’s power system andput the vessel out of operation. Another example could beinfection of onshore fleet management systems, or othervessels within the same fleet that have dedicated ship-to-ship communication equipment.

• Internal cyber-attacks can be classified as attacks orig-inating within the marine vessel. This could either be apassenger or trusted insider (crew) that gains control over,or infects, one of the vessel’s distributed control systemnodes. The cyber-attacks could be based on malewaredelivery by a USB stick or different internal access inter-faces such as an Ethernet that connects the vessel’s officenetwork to the control system network. These types ofattacks are more difficult to handle, however proceduressuch as disabling unused potential access points (such asUSB connections) and limiting input and output portson the router that connects the office network to thecontrol system network can reduce the risk of internalcyber-attacks. If one of the distributed control nodes getsinfected it is important to isolate that controller fromthe rest of the system. However, to quickly realize andidentify the attack before any harm is done might bea challenge, which puts stringent requirements on thevessel’s distributed control system’s middleware to limitpotential attacks [73]. Such requirements can be basedon each control node’s accessibility and level of securityclearance to distribute control actions to the rest of thevessel’s control nodes. If for instance the middlewaredetects that one of the control nodes tries to control partsof the system outside the controller’s security clearance,e.g. the vessel’s prime movers or propulsors, it might beconsidered as an attack, which should trigger isolationprocedures and alert the crew. In addition, it is essentialto keep operation systems and firmware up to date to bemore resistant to cyber-attacks.

There is a drive towards increased fuel optimality, reductionof emissions, increased safety, and performance and oper-ational flexibility. The technologies that are supporting this


development tend to increase system complexity, which hasconsequences for ship designers, ship builders, ship owners,crew and other stakeholders such as classification societies andauthorities. Like the automotive and aerospace industries, theelectric power plant is a highly computer controlled systemwith advanced functionality offering endless user configura-tions and options embedded in software. The control of thepower plant itself is also integrated with the control of powerconsumers, e.g. [74]. This leads to more complex processeswith new tasks, skills and training required by the crew.Due to the safety-critical nature of the ship’s power plantand electric system, the maritime industry is looking to learnfrom the automotive, aerospace, and defense industries thathave experienced the paradigm shift due the huge impact ofinformation and communication technology. This has led tonew standards, certification, and classification schemes relatedto integrated systems development and more extensive use ofsimulator-based training and verification technologies, [75],[76]. Future visions for unmanned and autonomous shipping,[77], [78], are indicators of the opportunities and challengesthat are emerging.

V. CONCLUSION

Past, present, and future challenges in the electrificationof the marine vessel have been discussed in this paper. Themilestones in the evolution of the development of marinevessels, from the earliest introduction of electricity in com-mercial vessels with the SS Columbia in 1880 to the newera of the all electric vessels marked by the Ampere ferry,have been highlighted in the historical part of the paper. Theuse of electricity in marine vessels which started far fromthe idea of an electric power system on board has, however,spurred the development of electric propulsion systems, andalso the concept of the integrated power system. The electricalsystem of today’s marine vessels can be compared to a land-based stand-alone microgrid system, with which the marinevessel power system shares many common features. Presentand future challenges include issues such as harmonics, powerquality, fault handling, and stability. These issues will be asrelevant during normal operation of the marine vessel as theyare at commissioning today. Many of the features requiredtoday to handle the modern land-based electrical system (smartgrid) will be a necessity in marine vessels as the use of elec-tricity becomes more intensive. Characterization of the marinevessel electrical grid through real-time measurements, and themonitoring of fundamental parameters such as impedance inaddition to fundamental and harmonic currents and voltages,will be essential to ensure the safety, integrity, and stability ofthe marine vessel power system. Lately, re-emerging wirelesspower transfer for battery pack charging in vessels will makethe link between the land-based power grid and the marinevessel power grid even tighter and will create a new form ofinteraction. Ultimately, as the use of all electric ships becomeswidespread, the electric vessel will become a part of theland-based power grid as a high impact electric load, thusbringing new challenges. This paper aims at anticipating thepotential new challenges and the associated research needs

for the future by stimulating the discussion and identifyingsynergies between the modern power grid and the electricalgrid of the marine vessels today.

ACKNOWLEDGMENT

This work has been carried out at the Centre for Au-tonomous Marine Operations and Systems (AMOS) whosemain sponsor is The Research Council of Norway. The workwas supported by Ulstein Power & Control AS and TheResearch Council of Norway, Project number 241205.

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Espen Skjong received his MSc degree in Engi-neering Cybernetics at NTNU in 2014, specializingin model predictive control (MPC) for autonomouscontrol of UAVs. He is currently employed in UlsteinPower & Control AS as an industrial PhD candidate.His research topic is optimization within marinevessel power systems. His industrial PhD fellowshipis with the Center of Excellence on AutonomousMarine Operations and Systems (AMOS) at NTNU.

Rune Volden received his BSc Hons. deg. at UMIST(University of Manchester) in 1989. He attendedJunior Officer School at Royal Norwegian Marine1991 (Submarine). He finished his PhD at Norwe-gian University of Science and Technology in 1995.He was Associate Professor at Østfold EngineeringCollege in 1994 - 1995. He has 20 years of industrialresearch and development experience in automaticcontrol within production machines and maritimeapplications, e.g. dynamic positioning, winch controland automation. He was chairman and member of

the board for Norwegian Society of Automatic Control (NMO of IFAC) 2009-2014. He is currently R&D manager at Ulstein Power & Control AS andAssociate Professor at NTNU Campus Alesund.

Egil Rødskar received his BSc degree in Elec-trical Power Systems from Møre og Romsdal In-geniør Høgskole (MRIH) in 1987. He is currentlya Technical Manager within Systems with UlsteinPower & Control AS, Ulsteinvik, Norway. He has25 years of experience in marine electrical systemengineering and project management for installationand commissioning of marine electrical systems.His experience also covers failure mode and effectanalysis (FMEA), failure mode, effect and criticalityanalysis (FMECA) for marine ship systems and

knowledge of ship operations.

Marta Molinas (M’94) received the Master ofEngineering degree from Ryukyu University, Japan,in 1997, and the Doctor of Engineering degree fromthe Tokyo Institute of Technology, Tokyo, Japan, in2000. She was a Guest Researcher with the Uni-versity of Padova, Padova, Italy, during 1998. From2008-2014 she has been professor at the Departmentof Electric Power Engineering at the Norwegian Uni-versity of Science and Technology (NTNU). From2008 to 2009, she was a Japan Society for thePromotion of Science (JSPS) Research Fellow with

the Energy Technology Research Institute, National Institute of AdvancedIndustrial Science and Technology, Tsukuba, Japan. In 2014, she was VisitingProfessor at Columbia University and Invited Fellow by the Kingdom ofBhutan working with renewable energy microgrids for developing regions.She is currently Professor at the Department of Engineering Cybernetics,NTNU. Her research interests include stability of power electronics systems,harmonics, oscillatory phenomena, and non-stationary signals from the humanand the machine. Dr. Molinas has been an AdCom Member of the IEEEPower Electronics Society. She is Associate Editor and Reviewer for IEEETransactions on Power Electronics and PELS Letters.

Tor Arne Johansen (M’98, SM’01) received theMSc degree in 1989 and the PhD degree in 1994,both in electrical and computer engineering, fromthe Norwegian University of Science and Technol-ogy (NTNU), Trondheim, Norway. From 1995 to1997, he worked at SINTEF ICT as a researcherbefore he was appointed Associated Professor atNTNU in Trondheim in 1997 and Professor in 2001.He has published several hundred articles in theareas of control, estimation and optimization withapplications in the marine, automotive, biomedical

and process industries. In 2002 Johansen co-founded the company MarineCybernetics AS where he was Vice President until 2008. Prof. Johansenreceived the 2006 Arch T. Colwell Merit Award of the SAE, and is currently aprincipal researcher within the Center of Excellence on Autonomous MarineOperations and Systems (AMOS) and director of the Unmanned AerialVehicle Laboratory at NTNU.

Joseph Cunningham received the B.S. degreein physics from St. Francis College of BrooklynHeights, Brooklyn, NY, USA. His interest in electricpower systems dates to his youth when his highschool science project, ”The Theory and Operationof AlternatingCurrent,” was awarded a first-placegold medal. This success led to a scholarship anda degree in physics from St. Francis College ofBrooklyn Heights, Brooklyn, NY, USA. He hasresearched and authored numerous booklets, articles,and books on topics such as industrial electrification,

electric utility power systems, and electric rail transportation. His latest book,New York Power, was published in 2013 by the IEEE History Center Press.In 1976, he coauthored a definitive three-volume History of the New YorkCity Subway System. He has also lectured and taught widely on the historyof electrotechnology and has consulted on numerous history projects andtelevision productions.

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