With the V164-7.0MW offshore turbine, Vestas has both introduced a higher power rating compared to several 6MW+ class main competitors and a record-size rotor diameter. One uncommon aspects of the machine, both for Vestas and the industry at large, is the use of a medium speed geared drive system.
When it came to designing the V164, Madsen says the decision-making process was driven by two aims beyond lowering the machine’s all-important cost of energy. These were choice of the drive system and its serviceability.
He says: "Initially we followed two parallel drive system development tracks, direct drive and geared. A key design team question was to determine the main factors causing wind turbine downtime. Our in-house studies supplemented by third party reports from 2010 show that faulty electrical components cause more failures than gearboxes."
A second main finding he referred to is that other mechanical components such as fail-safe brake and blade bearings also cause more downtime compared to gear boxes.
"Direct drive generators contain a factor four more electrical components compared to medium-speed geared," Madsen explains. "For Vestas, the main conclusion was that wind turbine reliability discussions should not focus on gear boxes. We had also decided that no technology solution should create high and unwanted dependence upon rare earth elements used for the magnets in permanent magnet generators."
Three-stage medium speed
The main outcome of the comparative drive system study favoured a three-stage medium-speed drive system, which in the specific V164-7.0MW configuration steps up 10–12 rotor revolutions to 400rpm rated generator speed. The drive train concept itself consists of a main shaft supported in two main bearings, and as an assembly incorporated into a single cast housing. It is further connected to the gearbox and generator via a low-speed coupling. All main drive system components are flanged together in a line arrangement without additional suspension support. According to Madsen the layout ensures that only rotor torque –and no harmful rotor bending movements- is transferred into the gearbox.
In order to ensure service and upkeep, all the main components can be removed and replaced as individual units. Even the main shaft can be exchanged without dismantling the rotor, says Madsen. In case of a major gearbox failure issue the solution would be to remove the complete unit and conduct all necessary repairs either onshore or on-board a service vessel.
Electrical power conversion system
The 3.3kV Vestas-design permanent magnet type generator is liquid cooled and based upon similar design principles like the smaller V112-3.0MW generator. The V164-7.0MW converter and 33 – 34kV transformer –optional 66kV- will be located into to the tower base. This measure aims at easy service access and higher reliability due to lower vibration levels and reduced temperature variations when compared to placing them in the nacelle. For the same reason the switchgear can be placed outside the tower, said Madsen.
Vestas will not provide standard towers for the V164-7.0 MW, but has stated a typical hub height of about 107m.
The new 80-metre slender blade follows a similar design strategy to that used on the 54.6 metre V112-3.0 blade while the Risø B design airfoil is identical. Vestas used carbon fibres to enhance stiffness and as a mass-optimising feature in the central blade spar. The V164-7.0MW blade mass is 35 tonnes. The combined mass of the nacelle plus separate transformer is about 290 tonnes, whereas rotor mass –hub plus blades- is approximately 210 tonnes. The nacelle is 24m long, 12m wide and 7.5 metres wide.
In conclusion, Madsen says: "A number of self-diagnosing systems and health monitoring systems will be incorporated. In addition a built-in ‘failure tolerant operation’ capability, which means that in case of certain failure modes the turbine can continue operation until the next scheduled service -potentially with reduced output. Finally for some components we will apply redundant ‘extra capacity’ enabling the turbine to continue operation in case of one component failing."