Perplexing patent

Establishing just where and how Enercon has infringed Kenetech's patent on variable speed wind turbine technology is no easy task. The court case and patent documents comprise hundreds of pages of legal terminology, not all of it enlightening. But buried in the midst should lie the answers to two questions: what does Kenetech have a patent on and where did Enercon contravene it?

Most perplexing is the patent itself, which uses over 100 clauses to cover every conceivable aspect of variable speed technology and power control. Yet the question of whether it is possible to patent the wind energy equivalent of a fuel injected motor car has not been dealt with by either side's lawyers. Instead the case has concentrated on the fine details of power factor control and frequency matching.

The particular clause that Enercon is deemed to have infringed, though, appears to do no more than set out the general principles for conversion of variable frequency alternating current -- the electricity generated by variable speed turbines -- to usable "fixed frequency" alternating current. Written in pseudo-scientific language, the clause is framed in such a way that most variable speed machines would be caught in the net, whichever method they chose for power frequency conversion. Specifically, the use of the general term "fixed frequency" when referring to "grid frequency" is inexact.

"Fixed frequency" is a misnomer when applied to any electricity network. Nominally the frequency of alternating current is 50 cycles per second in Europe and 60 cycles in the United States. In practice, though, frequency continually varies. In Europe, for example, the variation is usually between 49.8 and 50.2 cycles as generation and demand are rarely matched exactly.

Nonetheless, any device feeding power into the grid must exactly match the grid's frequency. With fixed speed induction generators this happens automatically, but not with all variable speed designs. Some need to "form a reference waveform," as the patent puts it and "use the template waveform to define desired output currents." This is at the heart of the dispute and it is here that Enercon has been judged to infringe Kenetech's patent.

The point is that patent clause 131 on which the court bases its judgement deals with the principles of power frequency conversion, not Kenetech's specific arrangement of rectifiers and inverters. Likewise, power factor control, referred to in the next clause in the patent and again central to the dispute, has been a standard feature of most variable speed drives for years.