Mitsubishi launches 7MW turbine

WORLDWIDE: At the European Wind Energy Association (EWEA) offshore conference last month Mitsubishi Power Systems Europe (MPSE) presented a new 7MW offshore turbine boasting a 165-metre-plus rotor diameter developed for high wind speed conditions.

In November 2010, Mitsubishi Heavy Industries (MHI), through MPSE, acquired Scotland-based Artemis Intelligent Power, a digital displacement hydraulics technology specialist.

The new turbine concept, called SeaAngel, features a full hydraulic continuously variable drive system and two conventional fixed-speed brushless synchronous generators, eliminating the need for a power electronic converter.

The hydraulic drive-system concept comprises a main rotor shaft with two bearings, a large-diameter low-speed ring-cam pump providing oil pressure and flow for two smaller hydraulic motors connected to a generator. MPSE general manager of engineering Masahide Umaya said two-bearing systems are more reliable than three-point and single bearing systems.

Regarding the hydraulic systems, Umaya explained that the low-speed pump comprises banks of large rotating ring cams driving numerous pistons around the machine's periphery.

Hydraulic systems are known for their excellent load-sharing capabilities. The SeaAngel low-speed pump can drive two hydraulic motors or switch off one motor/generator unit during low wind conditions for enhanced efficiency when the load factor drops below 50%. The same functionality can be employed as a redundancy capability in the event one of the hydraulic motors or generators suffers a failure. A second inherent built-in redundancy capability is provided by the fact that a digital-displacement transmission (DDT) system comprises a large number of common parts that work in parallel, share the burden and are individually protected against overload.

Hydraulic transmissions have faced scepticism for different reasons. One of the main critisisms is that they are inefficient. Some expert opinions on the SeaAngel in Amsterdam were in line with the lower-efficiency reputation, whereas others expressed concern at the high number of moving components.

The SeaAngel design team are familiar with such views, and highlighted figures indicating 98% efficiency for the low-speed pump and 96% for the high-speed motors, adding that the efficiency curves are very flat across the power range.

However, compared with the 97% total efficiency quoted for mechanical three-stage gearboxes in the 5-6MW range, the SeaAngel's 94% hydraulic system efficiency is lower.

Umaya acknowledges this. "It is correct, but we also like to point out that our slight disadvantage is compensated by the fact that the SeaAngel drive system eliminates a converter and its losses," he says. "The overall power transmission efficiency therefore matches that of a conventional turbine. In addition, we will employ 33kV generators, which also eliminates the need for a medium-voltage transformer and its associated loss."

The current technology validation status is that a full-scale 1.6MW hydraulic drive system has been built and is being tested at the moment in Scotland. This will be followed next year by an upgraded 2.4MW unit integrated in an MHI onshore turbine, and the installation of a 7MW prototype in the third quarter of 2013 in Scotland.

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