Gone with the wind: Wobben’s patent found invalid and not infringed

Wobben hold a European patent that relates to a “Method of Operating a Wind Power Station”, with a priority date of 1 September 1995. They contend that a particular feature of Siemens’ wind turbine technology, which has been installed in the UK, infringes their patent, with an estimated value of this action being (a whopping) £13 million. Unsurprisingly, Siemens deny any infringement and contend that the patent is invalid.

Mr Justice Birss held that the patent is invalid, and even if it was valid it would not be infringed. The decision, Wobben Properties GmbH v Siemens PLC & Ors [2015] EWHC 2114 (Pat), is not available on BAILII yet but can be read here.

How did wind turbines work in 1995?

The main external features of a wind turbine are the blades (1), tower (17) and nacelle. The nacelle is pivotally mounted to the top of the tower and houses most of the turbine equipment, such as a generator, gearbox, brake and wind sensors.  The circular area that the blades cover when rotating is known as the rotor area, and in normal winds this area is maximised by turning the nacelle about the top of the tower so that blades face in to the wind.

The generators are designed to have a maximum rated power, which in 1995 was around 300-600kW (today this can be as high as 6MW). Exceeding this rated power can lead to overheating and other nasty things, so when it’s blowing a hoolie the power captured by the turbine has to be limited.

In 1995 there were three known approaches to prevent the rated power being exceeded:
  • turn the turbine so that it does not face in to the wind, reducing the effective rotor area. 
  • adjust the angle of the individual blades to reduce aerodynamic lift and so limit the power captured. 
  • use the generator controls to limit the amount of mechanical power converted in to electrical energy.
In high winds (above 25 m/s), a point can be reached where it is better to shut down the turbine, so as not to expose it to potentially damaging dynamic loading.

The Inventive Concept

This Kat welcomes that Mr Justice Birss states that “there is no need to spend time identifying an inventive concept over and above the words of the claim”. This first step (“The first step is to identify the inventive concept embodied in the patent in suit”) of the structured approach to assessing inventive step in Pozzoli Spa v BDMO SA & Anor [2007] EWCA Civ 588 has always seemed to this Kat like a distracting red herring, since the “inventive concept” ought to be the invention as defined in the claims.  But oddly Birss J then went on nevertheless to state an inventive concept:  “The heart of the invention is the concept of running a VSVP turbine in high winds so as to reduce both speed and power in dependence on the rise in wind speed.”  This has resonances of what the patent itself says in the introduction: “The object on which the invention is based is to increase the yield of a wind turbine and nevertheless limit the load on the turbine at higher wind velocities.”

A set of amended claims as applied for by Wobben were allowed, so that claim 1 recited:
Method of operating a wind turbine for the production of electricity in an electrical network with pitch control, in which the power of the wind turbine as well the operating speed of the rotor is continuously reduced when a wind velocity is reached which is in danger of overloading the wind turbine, in dependence on the rise in the wind velocity”.
The idea is to operate at higher wind speeds above the shutdown wind speed so as to extend the operating window and potentially increase power yield. This (i) extends the power characteristic at greater wind velocities and improving the power yield and (ii) improves the network compatibility of the wind turbine.

The Alleged Infringement

The feature of the Siemens turbine in question is High Wind Ride Through (HWRT); a wind turbine will gradually reduce power output when wind speeds are particularly high, rather that shutting down completely, as was conventional in 1995 when the patent was filed. HWRT is turned on when wind speed reaches a certain threshold and results in a more stable power output at high wind speeds by pitching the blades out of the wind as soon as the rated power output is reached and by limiting rotational speed in proportion to the increase in wind speed and turbulence intensity.

A Pitch Reference is a value representing the desired pitch angle of the blades so as to achieve a target rotation speed.  The pitch control system adjusts the pitch of the blades in accordance with the Pitch Reference.

For blade rotation speed, an algorithm continually calculates a value for the filtered moving average of absolute acceleration experienced by the rotor.  A speed limit is set by reference to this value.



Siemens relied upon JP 56-150999 (Shozaburo). Upon first inspection, Figure 3 of this document appears to show a continuous reduction in speed and power in dependence upon a rise in wind speed, as required by claim 1 of Wobben’s patent.

However, as argued by Wobben, if the document is read as a whole, it is found that region 24 illustrates the process of shutting down the turbine when the cut out speed 29 is reached and, although the axis refers to wind speed, it is in fact concerned with changes in speed and power over the time during shut down and not behaviour with changing wind speed. There is no mention in the description that rotational speed was increased again as wind speed dropped back to speed 29.

Mr Justice Birss concluded that Shozaburo discloses a system in which the turbine shuts down at the cut out wind speed using a time dependent process. Wobben’s patent was therefore novel over Shozaburo.

They say a picture paints a thousand words – just make sure you’ve read and properly understood those words before you go to court!

Inventive Step

Siemens relied on the article “Probabilities of sudden drop in power from a wind turbine cluster” written by E.A. Bossanyi and published in September 1982. Bossanyi models different control strategies that might mitigate the impact on the grid of fluctuations in the power output of wind farms (clusters).

In particular, Bossanyi suggests “the use of a windmill whose output can be reduced gradually to zero as the wind speed increases”, which “reduces severe ramping of the cluster output as well as being advantageous to the individual windmill”.

Bossanyi models only a constant speed turbine, as was common at the time of writing in 1982, and so only refers to a reduction in power to cause the turbine to stop. Wobben’s claim 1 however requires a reduction in both the power of the turbine and the operating speed of the rotor, because by 1995 the industry was focussed on 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.

Siemens submitted that in applying Bossanyi to a VSVP turbine in 1995, it would be obvious to at least consider reducing rotor speed as well as reducing power in the operation above the usual shut down wind speed because that would obviously reduce loads and fatigue. Wobben disagreed, claiming that Bossanyi’s data does not make a convincing case to do what is proposed, and that it was not obvious to reduce rotor speed when using a VSVP turbine.

Mr Justice Birss agreed with Siemens that it would be entirely obvious to consider Bossanyi’s teaching to a VSVP turbine. While it could be applied by reducing the torque in the generator, Birss J held that it was obvious to instead reduce rotor speed because the skilled person would be considering loading and fatigue of the blades in high winds.

Wodden’s claims were found to be obvious over Bossanyi.


Birss J conceded that there was no real dispute that the way HWRT controls power is in accordance with claim 1, because Pitch Reference is effectively a measure of wind speed, so power is reduces as wind speed rises. However, the claim required that power and rotational speed are reduced in dependence on the rise in wind speed.

Consideration was therefore given to the way HWRT controls rotational speed. The speed of the rotor is calculated from the generator speed, and is differentiated to give a rotor acceleration or deceleration. The absolute difference between each reading is then converted to give a rotor acceleration in units rpm/s. This value is then subject to a low-pass filter to give a “filtered rotor acceleration”.

Siemens claimed that this was not infringement because rotor speed is reduced in response to the acceleration or deceleration of the rotor, rather than wind speed. So, a high speed, but smooth wind will not result in acceleration of the rotor, so the rotational speed of the rotor will not be reduced. Whereas, a low speed but turbulent wind would cause rotor accelerations, so the rotational speed would be reduced.

On the balance of evidence, the judge found that the filtered rotor acceleration used by HWRT is not a measure of wind speed; it is not an increase in wind speed itself which causes the effect but an increase in the change in wind speed which causes speed reduction.


The patent is invalid.  Even if it was valid it would not be infringed.
Gone with the wind: Wobben’s patent found invalid and not infringed Gone with the wind: Wobben’s patent found invalid and not infringed Reviewed by Darren Smyth on Thursday, July 23, 2015 Rating: 5


  1. "Method of Operating a Wind Power Station"

    All embodiments would infringe my method patent claim in this area:

    Method of Operating a Wind Power Station comprising blowing.

  2. my method is not for a wind power station, merely a wind power plant. Plus my method comprises a differential in temperature causing the air to be sucked and not blown. According to Briss J. there is no infringement I presume?

  3. It is interesting to note that the EP patent had been opposed by Vestas, another contender in the field of wind turbines, but the opposition was rejected. None of the documents used in the UK procedure were in the opposition.
    Simply reducing the speed would be obvious and adding a power reduction does not mean much either, as it has to be made sure that the turbine does not disintegrate.
    It is also interesting to note that Mr Wobben is the owner of Enercon, and until recently all applications and patents were in his name, and he also was the inventor. There are no actions in the U.S. by Enercon or Wobben, and this for very good reasons.

  4. Thanks for the summary at the beginning of the post - attracts and informs those of us who have an interest in patents but no hands-on knowledge or experience. Left with a feeling of sadness - few have the finance-determination combination to fight on and on and on (almost ad infinitum ?) in such circumstances

  5. Dear Anon at 15:14, the skilled person would appreciate that 'blowing' was only a figure of speech. Therefore you infringe.

    If not, I shall be suing my attorneys for negligence.

  6. Must be a Wobben Weliant patent.


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