The jury is in on the recent breakdown of an advanced 500 kW prototype wind turbine being tested by the US Department of Energy -- and the fault has been located to the control system. The failure of the machine, built by The Wind Turbine Company (WTC), resulted in the partial loss of a blade (Windpower Monthly, July 2002). Both the National Renewable Energy Laboratory (NREL), which is overseeing the turbine's testing in southern California, and WTC staff have concluded that a data signal converter failure caused one blade to strike the turbine tower, breaking off about one-eighth of the blade. WTC president Larry Miles says the converter's failure resulted in an erroneous signal to the pitch control drive. The turbine's protection software should have shut the machine down within one second, but the "detection and response timing was set too loose" and the turbine's two blades continued to spin for about ten seconds. After four seconds it was too late, says Miles. WTC staff should finish a complete review of protection control settings this month. It will take more time to find new blades, but Miles says he expects the turbine to be back in service before the end of the year. The mishap set installation of blades specifically designed for the turbine back about three months to the spring of 2003. The addition of those blades will boost the turbine's capacity to 750 kW, the size Miles hopes to use commercially in the future. Both NREL and WTC say the prototype's weight-reducing downwind design is not at fault (by far the majority of commercially sold turbines are upwind machines) and they remain confident the design will deliver lower cost wind power. NREL should finish a report on the May incident this month.