Electric power generation in ships has been traditionally carried out by using either separate auxiliary gensets or by using so called shaft generators connected to the main engine. Auxiliary gensets typically consist of a constant-speed, 4-stroke diesel engine equipped with a standard asynchronous or synchronous generator. The main benefit of this concept is that due to constant-speed operation, the generator feeds constant voltage and frequency into the ship’s electric grid and therefore, no power electronics are needed for frequency conversion. Additionally, since this concept is independent of the propulsion, power generation can continue during manoeuvring and harbouring. The main drawbacks are that auxiliary gensets require more space, more maintenance, and they cannot run with cheaper heavy fuel oil (HFO) without expensive auxiliary equipment, such as pre-heaters to reduce viscosity.
Coupling a PM shaft generator with a frequency converter gives the maximum amount of freedom in optimizing the engine and propeller efficiencies, as both the speed and propeller pitch angle in the case of a CP propeller can be independently varied using the combinatory mode. The optimal combination of propeller speed and pitch angle minimizes the specific fuel oil consumption. The lower the ship’s speed. the bigger is the benefit. Even when sailing on open seas, lowering the engine speed and simultaneously using a smaller propeller pitch in the case of the CP propeller can result in huge savings in fuel consumption. Although there are shaft generator systems that allow variable speed operation by utilizing a frequency converter, so far they all use electrically excited synchronous generators.
Such a generator works very well in traditional power generation, leading to efficiencies of up to 98-99%, but their efficiency are considerably reduced down to 93-94% when used as a shaft generator. due to the low operational speed as described above. When such a machine is coupled with a frequency converter that allows variable speed operation, the total efficiency of the electric power conversion will be only around 90% or even less. Using a PMG instead will boost the efficiency by 3-496 up to 94% including the converter losses, which will result in significant savings in fuel costs.
Reduced fuel consumption will further help in fulfilling tightening emission regulations (e.g. IMO Tier III). In addition to the benefits mentioned before, a frequency converter connected to a PM shaft generator can easily handle all the existing requirements that have been traditionally set for the shaft generator system, including for example, synchronization, parallel operation with other generators, reactive power control and generator fault diagnostics.
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enau Group, established in 2016, offers sustainable and environmentally friendly advanced engineering solutions in energy, automotive, maritime industry in Germany, Austria and Turkey.
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