But the reality of the incident is less dramatic from a technological point of view. The turbines are characterised by a compact, fully integrated Multibrid drive system comprising a single-stage planetary gearbox and a permanent-magnet generator incorporated in a combined cast housing. The gearbox shafts turn in slow-rotating slide bearings, known for compactness and long lifetime. They are as yet a component still uncommon in wind turbine gearboxes.
Gearbox supplier Renk supplied the M5000's gearboxes, which include four onshore prototypes. A root cause analysis pointed at insufficient room for rotating parts to move axially during operation. "We found as the underlying cause that Renk had switched the sliding bearing material to an aluminium-zinc alloy due to successful experiences in other applications," says Areva Wind service director Michael Munder-Oschimek. Renk overlooked that the new material expands twice as much as the initial material when it heats up, he adds.
Hence, Munder-Oschimek insists the failure did not represent widespread technical faults, saying: "The Alpha Ventus turbines all operated with 97% availability before an overheating issue in the stationary bearings of two gearboxes occurred." According to Areva, the six Multibrid M5000 turbines have together already generated close to 90GWh since October 2009.
In early October, four nacelles were successfully exchanged and fully re-commissioned. Before November all turbines were to have been exchanged and fully operational, according to Munder-Oschimek.
As a remedying measure, Areva Wind and Renk have returned to the original alloy for the Alpha Ventus turbines and all their future installations. Areva will also build an in-house drive system test facility where all future turbines will undergo an extensive three-to-four day, full-load testing procedure.