Shaft generator with ACS880

The shaft generator can be installed in one of three ways in a vessel, depending on available space.

  1. The first installation method is direct on the shaft installation, separately from the main engine. The rotor is part of the shaft line and the stator is installed separately. With this type of installation method, the engine rpm, shaft generator rpm, and the propeller rpm are the same. This method is typically used with slow speed engines.
  2. For medium- and high speed engines with gearboxes then the shaft generator is typically installed on a PTO shaft on the gearbox. The benefit is a smaller genera-tor operating at higher speed (commonly 1200/1500 or 1800 rpm nominal).
  3. The shaft generator can also be installed at the free end of the main engine with or without gearbox. The basic solutions consist of an IGBT Supply Unit (ISU) with Optimal Grid Control (OGC) on the ship grid side and, depending on the need, either an Inverter Unit (INU), IGBT Supply Unit (ISU), or Diode Supply Unit (DSU) on the generator side. These solutions and some special solutions are introduced in detail in the Application guide in chapter ”3. System design with ACS880”

ACS880 following generator types:

  • Asynchronous
  • Permanent magnet
  • Synchronous reluctance
  • Synchronous
  • Synchronous - excitation controlled by converter
  • Individual AVR
Hybrid-vessel-shaft-generator-drive
Application guide
Hybrid vessel - shaft generator drive

Benefits of shaft generator and drive

A shaft generator drive is a power source which generates cheaper electricity under most conditions than the auxiliary generator sets since the bigger main engine has a lower specific fuel oil consumption for the same generated power. This also reduced maintenance need since you avoid running the additional genset which leads to fewer running hours and longer overhaul intervals.

Shaft generators can be part of a solution to enable peak shaving, providing the auxiliary power to cover peaks and troughs, and allowing main engines to operate within optimally efficient operating zones.

A shaft generator associated to a controllable pitch propeller and variable frequency drive installation allows the controlled use of propeller pitch to achieve the desired speed with maximum efficiency.

With a variable frequency drive, the shaft generator can be utilized at a wide range of main engine rotation speeds. This enables operational flexibility e.g., so that nominal voltage and frequency output of the shaft generator can be maintained.

The efficiency can be improved because the shaft power of the main engine can be used to produce electricity over the entire operating area. This also enables the use of shaft generators on vessels with fixed-pitch propellers such as most 2-stroke powered vessels.

 

Engine specific fuel consumption

Reducing engine rpm can often lead to better specific fuel consumption due to a more optimal operating point.

Propeller power demand

Reducing the propeller rpm typically leads to a lower total power requirement to achieve the same vessel speed.

shaft-generator-optimal-operation-point

For fixed-pitch operation, drives make shaft generators possible and for controllable-pitch operation drives enable variable rpm operation to reduce propulsion power demand and fuel consumption compared to constant rpm operation.

This chart does not represent an actual produced engine model but resembles a typical diesel engine performance. There will be variations between engine models but the general principles explained herein remain true.

Shaft generator operating modes

Power Take-Off mode (PTO)

Part of the propulsion energy is taken by the shaft generator and converted to electricity. This energy is used for the ships systems and to charge batteries.

Shaft generator can operate in:

  • Island mode (grid-forming single generator)
  • Parallel mode (with other generators or ESS)
  • Maneuvering mode (supplying power to thrusters)
The drive adjusts voltage and frequency when operating with other generators and can compensate for load unbalance in the grid. It can also compensate reactive power.
Power-take-off-(PTO)

Power Take-In mode (PTI)

In PTI shaft generator operates as an electric motor powered by vessel's main grid in tandem with the main engine. Power is supplied by generators or ESS. PTI mode can be used to boost power for propulsion (e.g.  ice breaking, higher speed to make up for delays).

This configuration allows in some cases to design for a smaller main engine optimized for typical operation and shaft generator in PTI mode covering less frequent peak power demands.

In this design both gearbox and propeller shaft line need to be dimensioned for the combined power of main engine and shaft generator.

Power-take-in-(PTI)

Power Take-Home (PTH)

Shaft generator can be used as an alternative propulsion motor powered by the vessels main grid, where the power can be taken from generators or energy storage system.

PTH can be used as an emergency system in case of a main engine failure to limp to safety. It is also efficient for low load operation (slow transit, loitering) powered by a small genset or energy storage system, instead of a large main engine running at low load. PTH also enables zero emission operation in sensitive areas.

PTH requires a clutch to the main engine, which must be taken into account in vessel design.

Power-take-home-(PTH)
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