Power and Propulsion

Managing electric drives

Malcolm Latarche
Malcolm Latarche

05 December 2018

Managing electric drives

A power management system is needed to start and stop gensets and alternators according to the network load and the online alternator capacity. The power management system is a vital part of a diesel-electric system and must monitor and anticipate changes in demand. This involves bringing in an additional alternator if the available power (the installed power of all connected alternators less the current load) drops below a preset limit.

This triggers a timer and if the available power stays below the limit for a certain time period the next genset alternator in sequence is started. It also blocks heavy consumers from starting or sheds unnecessary or low-priority consumers, if not enough power is available, in order to avoid unstable situations.

Class rules allow gensets/alternators only 45 seconds for starting, synchronising and beginning to share load. It is always a challenge for the power management system to anticipate the situation in advance and to start gensets/alternators before consumers draw from the network and overload the engines. Overloading an engine will soon decrease the speed/frequency with the danger of motoring the engine, as the flow of power will be altered from network to alternator (Reverse power). The electric protection system must therefore disconnect the affected alternator from the network. An overload situation is always a critical situation for the vessel and a blackout must be avoided.

DC systems resolve issues

Electric propulsion systems that include many variable frequency drives can provide significant benefits but by drawing current in a non-linear or non-sinusoidal manner can introduce excessive levels of both current and voltage harmonics. Harmonic distortion in diesel-electric systems is a problem mostly associated with AC distribution.

One alternative to an AC system is ABB’s Onboard DC Grid concept, which is a reworked and distributed multidrive system where distributed rectifiers are eliminated. It merges the various DC links around the vessel and distributes power through a single 1,000V DC circuit, thereby eliminating the need for main AC switchboards, distribution rectifiers and converter transformers. All electric power generated is fed either directly or via a rectifier into a common DC bus that distributes the electrical energy to the onboard consumers. Each main consumer is then fed by a separate inverter unit.

As well as the ABB system described above other similar technologies include Wärtsilä’s LLC (low loss concept), Siemen’s Bluedrive PlusC, Norwegian Electric Systems’ (NES) Quadro Drive variable frequency drive system and MAN Energy Solutions’ EPROX (electric propulsion excellence).

Hybrid systems without batteries

There are various definitions of what constitutes a hybrid propulsion system with the term now most commonly applied to one in which a battery is included alongside some other power source. A more conventional definition would be one in which various engine, drive type and fuel options are recognised. Examples of this include Combined Diesel and Gas (CODAG), Combined Diesel Electric and Diesel (CODED) along with several other acronyms.

In a CODED system, the combination of mechanical power, delivered by Diesel engines, and electrical power, provided by variable-speed electrical motors, delivers propulsion power which assures the ship a broad operational capability, providing the right amount of power and torque to the propeller in each operation mode. By contrast, a typical CODAG propulsion system will be wholly mechanical with each of the Diesel engines and gas turbines having its own gearbox and a further connecting gearbox where power from each source can be combined and transmitted to the propellers.

Magnetic drive

Over the past few years permanent magnetic (PM) drive has appeared in some areas of ship machinery, notably in pumps and in rim-driven thrusters. Rim drive thrusters are covered in the next chapter but there are other areas where PM technology can feature in the drive train. A Norwegian company, Inpower, has patented a design for a propulsion system employing permanent magnet drive, which it calls PhiDrive. The company was acquired in 2017 by another Norwegian engineering company, Bostek.

The PhiDrive is a development of diesel electric that uses a permanent magnet generator, which tends to be far smaller than conventional induction generators for the same power output. The power produced is transferred directly to the propulsion motors without the need for frequency convertors or transformers.

As well as being more compact, permanent magnet units generally have higher efficiencies than either induction motors or standard synchronous generators since the rotor windings are substituted with magnets that are able to produce the required flux without the presence of copper losses. The uses of PM technology can include propulsion motors, generators and, as already mentioned, drive systems for thrusters. While PM is a relatively new development in marine, it is a mature technology in many other industries and greater use will allow additional markets for some newcomers to the marine sector. One such is Japan-based Yasakawa Group which plans to grow its marine business through a new Finnish subsidiary called Switch.