Traditional diesel-electric propulsion consists of several generating sets and drive syste ms, each consisting of a propulsion transformer, a frequency converter for speed control, an electric motor and a thruster. The propulsion transformers are heavy and take a lot of space. Instead of a conventional propulsion transformers, the Wärtsilä LLC system uses a phaseshifting LLC transformers which filter out undesirable harmonic currents.

In LLC system, the main switchboard is constructed in four separate sections, each section connected to one genset. Thrusters are connected to the four switchboard sections in such a way that each frequency drive is fed by two gensets. If a failure occurs in one genset, electrical energy from the other aggregate can still be supplied to both propulsion motors.

Bus bars A1 and A2, as well as bus bar B1 and B2, are connected to each other via two LLC transformers. These transformers also feature secondary windings which eliminate the need for distribution transformers for other auxiliary customers.

In traditional systems, the use of low-voltage components is restricted to applications with a maximum of about 10MW installed propulsion power. By using LLC, the propulsion systems with higher installed power can be designed using low-voltage (690V) components, reducing equipment weight and saving valuable space. In some applications, weight reductions of 35- 40% can be achieved.

The LLC systems are particularly suitable for dynamic positioned offshore support vessels due to their high redundancy. As a result, a required station keeping capability can be achieved with smaller installed power that in case of conventional diesel-electric systems.

Advantages of the Wärtsilä LLC technology:

1. Reduced losses in the vessel’s electrical system (15-20%) result in fuel savings and lower levels of emissions.

2. Higher levels of availability when a major failure occurs increases thruster robustness.

3. Less-severe consequences in the worst single failure case mean that LLC solutions offers improved DP capability.

4.  Increased operational flexibility and availability through a segregated, two-section switchboard and bus connections via buslinks.

5.  Significant increase in levels of personnel safety due to the reduced likelihood of short circuits.

6.  No inrush current at thruster start-up as the transformers are always energised.

7.  Reduced weight and space requirements as the usual thruster transformers are not required.

8.  Additional flexibility in vessel design as the LLC phase-shift transformers do not need to be located close to the electric drives for which they provide power. They also feature secondary windings which can be used to supply some of the vessel’s auxiliary power requirements.

9.  More efficient power distribution in damage scenarios.

Wärtsilä first complete LLC system was delivered in 2004 and installed on the Norwegian PSV NORMAND SKIPPER. The concept has subsequently been developed. Patents have now been granted for the Quattro LLC design and for LLC in medium-voltage application.

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