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Wind turbine speed control patent win for Siemens AG

6 August 2015

Wobben Properties, the IP holding company of Germany’s Enercon – the world’s 4th largest wind turbine manufacturer, holds a European patent on Enercon’s Storm Control technology relating to a method of operating a wind turbine.

Wobben asserted that Siemen’s High Wind Ride Through (HWRT) technology installed in turbines in the UK infringed their patent, to the tune of £13million. Siemen’s denied infringement and counter-claimed that the patent, dating from 1995, was invalid.

Back in 1995 it was conventional for a wind turbine to automatically shut down when the wind speed was too high to avoid damaging the turbine.

In the patented technology, above a certain wind speed, the power output of the wind turbine as well as the rotational speed of the turbine blades (the rotor) is continuously reduced according to the rise in the wind speed. The aim being to extend the operating window of the turbine thereby increasing power generating capacity.

Siemens relied on an earlier 1982 academic paper by E.A. Bossanyi which suggests “the use of a windmill whose output can be reduced gradually to zero as the wind speed increases”. The paper modelled only a constant speed turbine, as was common in 1982, and so only refers to a reduction in power output. Wobben’s patent required a reduction in both the power output and the rotor speed because by 1995 the technology had moved on to so-called variable speed variable power (VSVP) turbines.

The question then was whether it would have been obvious to apply the teaching of Bossanyi to the newer VSVP turbines at the date of the patent in 1995.

The dispute reached the High Court in England & Wales (Wobben Properties GmbH v Siemens PLC & Ors [2015] EWHC 2114 (Pat)) where Mr Justice Birss agreed with Siemens that it would have been entirely obvious to apply Bossanyi’s 1982 teaching to a VSVP turbine in 1995, declaring the patent consequently invalid. Whilst acknowledging it would be possible to decrease power output without also decreasing the rotor speed, e.g. by using the gearbox, Birss J considered that the skilled person would have at least tried reducing the rotor speed to avoid unnecessarily stressing the blades and the gearbox.

He also disagreed that the Siemen’s HWRT would have been an infringement in any case due to the way it controlled the power and rotor speed according to wind accelerations, not wind speed.

This case highlights the importance of considering the impact of general advancements in the state of the art between the date of prior art and that of a later filed patent against which it is used; and more importantly what the skilled person would have obviously done in light of those advancements. This may be used to great effect when challenging patent validity.


Richard Worthington
Clean Technology group

If you require further information on anything covered in this briefing, please contact Richard Worthington (; +44 1179 253 030) or your usual contact at the firm.

This publication is a general summary of the law. It should not replace legal advice tailored to your specific circumstances.

© Withers & Rogers LLP, August 2015