Stand-alone electric power generating systems are used in situations or locations in which electrical connection to a wide area electrical network is not possible. Such stand-alone systems are also used in ships, yachts and other sea crafts in which prime movers connected to electrical generators are employed for producing electrical energy to be consumed by the various electrical devices in the ships or the like. The generated electrical energy is used by the devices required for propulsion of the ship and for powering the various components and systems related to the operation of the ship. Further, in passenger ships, the passengers also consume vast amount of electrical energy either directly with electrical equipment or indirectly using the comforts available on board.
It is known to provide electrical energy to a ship by generating AC power with generators connected to the prime movers. The prime movers use various energy sources, such as diesel fuel and fuel oil, for producing rotational movement for the generator. The generated AC power is transformed to a suitable voltage level for different purposes. In large ships multiple generators are operated in parallel and the operation is supervised by an upper control system. The upper control system controls the power management of the generators by changing the number of active generators when the power demand requires it. The upper control system is configured such that the operation of the power generating system operates in a stabile manner regardless of the changes in the power demand.
The control of the generated power is carried out by controlling the fuel injection of the prime mover such that the prime mover keeps its rotational speed constant. When, for example, the load of the generator increases, the fuel injection is also increased so as to keep the rotational speed constant. As the generator is connected directly to the shaft of the prime mover, the output frequency of the generated AC power is also kept constant.
In another approach each prime mover-generator set is equipped with an AC to DC rectifier. The outputs of the rectifiers are connected to a common DC bus. The generated electrical power is further converted to AC power so that the power can be consumed with AC devices. In such structure the output frequency of each generator-prime mover set can be controlled to optimize the operation of the system. In a power generation system with DC distribution the amount of electrical components is large as each generator requires a separate rectifier. In large installations where the amount of installed power is high, the required DC components, such as circuit breakers, are expensive as the operation is in medium voltage level. Further, another conversion of power is needed when the DC voltage is converted back to AC voltage for the consumers.
Document EP 2682339 A1 discloses a system, in which the frequency of the AC-distribution network is variable. In the system the speed of the prime movers is kept as low as possible to maximise efficiency.