Shaft generators converter
A solution for reduction of the fuel consumption for the vessel is to equip the shaft generator supply with a static power converter and thus allow the prime mover to run on various speeds, obtaining the best efficiency in every operational situation. This solution is available both for new vessels and for existing vessels. Retrofitting a static power converter to an existing installation, the existing breakers can be left in and used to achieve bypass functionality for the converter. Bypass breakers can also be used in new-build if required by class or safety requirements. Operation of bypass functions will require proper sequences for syncronization and power transfer functions.
A typical shaft generator system can be based on a standard inverter unit used as rectifier, a vessel grid tied converter and a transformer. Additionally the converter system needs to be controlled in a proper way. It is recommended to use the vessels Power Management System for controlling the shaft generator system if possible.
If a by-pass function is included, an isolation transformer is required to avoid common mode currents.
Generators can be asynchronous, synchronous with separate magnetizing windings or synchronous based on permanent magnets.
In most cases the shaft generator feeds power to the vessel grid, but in certain situations it can be beneficial to feed power from the auxiliary generators connected to the ship grid to the shaft generator using it as a motor for propulsion.
Using an asynchronous generator, an inverter unit can be connected to the generator. There are no real limitations in the speed range for the asynchronous generator, however, it is optimal to operate the generator close to synchronous speed. With large powers required it is possible to parallel power modules.
Permanent magnet synchronous generators
Permanent magnet synchronous generators enable a black start of the system without DC-voltage in the converter.
Synchronous generators (separately exited)
Synchronous generators with separate magnetizing circuits require an Active Front End unit as rectifier. In a black start situation the generator voltage needs to reach nominal voltage before the rectifier becomes operational.
Equipment connected to vessel grid
The configuration of the vessel grid equipment is mainly depending on vessel grid type, additional power sources and load types.
Shaft generator converter configuration
The recommended configuration for most shaft generator converter systems includes a converter unit with sine filter and transformer. The converter will also need to fulfill the vessel short circuit level and to be able to parallel with other power sources. The transformer is included to avoid common mode currents and to allow single phase loads. For large power configurations, typically more than one power converter is used. Each power converter also includes its own sine filter and transformer and is paralleled to increase power and redundancy.