In granting the four US patents to Kenetech, the examiners warned of their limited validity by citing as prior art seven patents issued to Hamilton Standard. The Kenetech patents are much less basic and they go beyond the Hamilton Standard patents only by adding confusing detail claims that are of very limited value. I believe that, in some cases, the functions claimed are just not feasible.
While working for Hamilton, I was inventor or co-inventor on many of the subject patent cases. Also, I managed their preparation for submittal to the patent offices. Since transferring to Wind Energy Systems Taranto (WEST) in Italy, I have had occasion to study the Kenetech patents. In my judgement they are of little value and will not, in the long run, stand up against good prior art in the power electronics field and art such as that in the Hamilton patents.
My perspective on these matters is influenced by the fact that in the early 1980s I was active in the United States in the public promotion of variable speed concepts with SERI (Solar Energy Research Institute), NASA and later NREL (National Renewable Energy Laboratory). We employed outside consultants to explore the power electronics aspects, and organised public symposia to push the concepts. Those sessions were often attended by people who later emerged at Kenetech.
The Hamilton Standard variable speed patents, along with many others, are licensed on a worldwide basis to WEST. They have been "reduced to practice" very successfully in the development of the 60 metre Gamma wind turbines.
Because WEST has those basic and adequate patent rights, and because the patents now pursued by Zond are at best weaker and may yet be rejected, one could reason that only WEST has any important patent position.