Shaft generators driven by the main propulsion engine(s) to provide electrical power are in widespread use, reducing the need to run auxiliary generator sets. But there have been limitations. The ship’s electrical system normally requires a fixed frequency and this means that engine speed has to be kept constant, which often leads to inefficient engine operation.
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The situation has now changed for the good with the introduction of the HSG-Concept (patent pending). HSG stands for Hybrid Shaft Generator, and is actually an advanced power electric system for conditioning the power coming from a shaft generator so that the switchboards see a constant voltage and frequency at any engine speed.
This opens the way for much more flexible use of engine and propeller speed variations to maximise both propeller and engine efficiencies by running them at their design points. A more efficient system also helps reduce CO2 and NOx emissions.
The shaft generator can also act as a motor, feeding in power to drive the propeller, attractive where a vessel may spend extended periods cruising very slowly, or loitering for example waiting for a place at the quay. The main engine can be shut off and power from one or more generators used for propulsion.
The HSG concept is suitable for offshore, merchant and fishing vessels.
Upgrading ships in service can be cost effective and significantly reduce emissions. Where an existing shaft generator is driven by a secondary PTO, conversion can be simple.
The HSG-Concept can handle both synchronous and asynchronous electrical machines, so the existing generator may be retained, conversion involving installation of the drive cabinet and some switchboard alterations. Other types of vessel should be evaluated on a case by case basis.