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vessel and distributes power through a single 1,000 V DC cir- cuit, thereby eliminating the need for main AC switchboards, distributed rectiÞ ers and converter transformers (Figure 3). All generated electric power is fed either directly or via a rectiÞ er into a common DC bus that distributes the electrical energy to the onboard consumers. Each main consumer is then fed by a separate inverter unit. When an AC distribution network is still needed, for example with a 230 V hotel load, it is fed using island converters, developed by ABB to feed clean power to these more sensitive circuits. Additional converters for energy storage in the form of batteries or super capacitors for leveling out power variations can be added to the DC grid. The system has been remodeled in such a way that most of the products used in todayÕs electric ships such as AC genera- tors, inverter modules, and AC motors can still be used. The onboard DC grid can be conÞ gured in several different ways. With a centralized approach all converter modules are located in one or multiple lineups within the same space that the main AC switchboards used to occupy. With a distributed approach, the various converters can be placed where it suits the vessel operation or design best. The AC generators can have either integrated or stand-alone rectiÞ ers installed in cabinets. As a result, the volume of components that, by regulation, must be installed in the main switchboard room is drastically reduced. This affords the vessel designer a new level of freedom in de- signing the electrical power system to increase vessel func- tionality and value. Best Practices Meet New Thinking The DC grid concept utilizes AC generators and motors, but allows for increased efÞ ciency because the system is no lon- ger locked to a speciÞ c frequency (usually 60 Hz on ships), even though any 60 Hz power source may still be used. The new freedom of controlling each power consumer indepen- dently opens up numerous ways of optimizing fuel consump- tion. When operating marine combustion engines at constant speed, the fuel consumption is lowest at a very small operating window, typically around 85 percent of rated load. With the introduction of variable-speed operation of the engine, this window of optimal efÞ ciency can be extended as far down as 50 percent, depending on the engine (Figure 4). If the engine is operated at loads below this, the engine efÞ ciency remains signiÞ cantly higher than that of the traditional Þ xed speed equivalent. The end result is that a typical offshore support vessel can achieve fuel savings of up to 20 percent. By eliminating bulky converter transformers and main ??Two longstanding and crucial principles have been carried over from the traditional AC distribution system to form the framework of the onboard DC grid philosophy: Personnel and equipment must be protected in case of failures and sound selectivity shall be ensured in such a way that safe operation is maintained after any single failure. The new system merges the various DC links around the vessel and distributes power through a single 1,000 V DC circuit. www.maritimeprofessional.com | Maritime Professional | 31