Bending with the wind

5 November 2002

The Horns Rev offshore wind farm development was shut down on 4 November when a test wind turbine of the type being used in the project suffered damage owing to the failure of a safety system. The unit in question was a Vestas V80-2.0 MW offshore unit located at Tjæreborg, Denmark. All damage was confined to the turbine blades. But it was the second turbine failure due to overspeed in just a few days, the other occurring on a Nordex site in Norway. Both were caused by human interference in control systems, and have serious implications for how testing and service procedures are currently carried out, and how they should be.

In relatively low speed wind (10 m/s), a failure occurred in the control system causing the turbine to over-speed. The safety system that has to stop the turbine in such a situation failed. However, the turbine's secondary emergency system cut in and stopped the rotor.

The effect was to expose the rotor to very high loading, causing damage to all three blades. Thirteen commissioned turbines of the 80 strong Horns Rev array were stopped for safety reasons until the cause of failure had been identified. In the event the manufacturer, Vestas, determined that the failure occurred 'due to a human failure in connection with changes made to the turbine's controller on 24 October', that it was limited to this single turbine, and that there was no reason to expect it to happen to other units of the same type. The turbines at Horns Rev were then restarted. Vestas has decided to modify the procedure for changes to test turbines.

Despite Vestas' confidence about recurrences, the Nordex event, a remarkably similar accident - similar, that is, in cause, not outcome - had happened only a few days before, but to a turbine sited at the Arctic Wind site near Havöygavlen, Norway. It occurred on October 29 and the mechanical damage was far more extensive. It also was an overspeed accident, in 15 m/s wind, with the rotor getting up to 44 rpm (tip speed 663 km/h) before catastrohic failure occurred and the entire nacelle with its rotor was ripped from the tower. Again, though, no further damage resulted. The normal speed limit for this model is 19 rpm.

The cause was also a safety circuit failure, created by human error. The system had signalled that the batteries serving the pitch drives, the rotor brake that operates in the event of power failure (and other faults), were not sufficiently charged. Normally an automatic cut-out would have prevented operation in this condition, but it had been manually de-activated in order to start up the turbine. During operation the line power supply did fail, but being without battery power the brake could not cut in, resulting in the rotor's freewheeling at uncontrolled speed.

The pitch drives are the only effective means of stopping the turbine, so all safety routes requiring shut down lead ultimately to that end. That is why their operation cannot be interfered with except by reprogramming. Nordex are now trying to find out why, during service operations at a 16x2.5 MW energy park, an engineer de-activated a vital safety procedure at a programming level. Perhaps they will also investigate whether or not such practices are widespread; and why rechargeable batteries whose main function is to brake the rotor should be in a state of discharge.

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