Enercon is arguing that the patent, now owned by Enron Wind Corp, is for technology that it says was already in the public domain -- in a textbook by a University of Minnesota academic that preceded the patent by several years. The book, "Power Electronics: Converters, Applications and Design" by Ned Mohan et al, was first published in 1989 by John Wiley & Sons of New York, a reputable publisher.
Prior art that is publicly available cannot be patented, argues counsel for Enercon. Thus the original patent should not have been issued to Kenetech Windpower Inc, continues the request for re-examination. Enron, part of the natural gas giant Enron Corp, later acquired the patent from Kenetech.
The patent has been the subject of controversy for years. In August, a US Appeals Court upheld a ban on all imports of variable speed technology into the US until the year 2011 (Windpower Monthly, September 1998). The ban came into effect just over two years ago, in August 1996. It covers all Enercon designs of variable speed turbine and probably other designs from European manufacturers.
Enron is in the process too of getting the same technology patented in most of Europe. In February it passed the most difficult hurdle when the European Patent Office in Munich issued a "Proposal for Granting." This patent can be appealed against once it is officially published. It is thus not yet final.