A "force fight" between two hydraulic pitch control systems on the WTS-4 led to a build up of internal forces, says Doman. Running at about 4 MW in a moderate wind, the turbine experienced a cyclic pitch effect that made the rotors collide with the tower, according to Medicine Bow Energy Inc. Later inspection revealed that a structural element attaching hydraulic pitch actuators to the hub had broken. The force fight had been seen in factory tests and, though improvements were made, extremely precise synchronisation of the servo valves was needed.
The incident virtually closes the book on an era of huge turbines, dinosaurs that were testament to the big-scale thinking of the early 1980s and the Department of Energy's MOD programme. Although large turbines, up to 1 MW rated capacity, are now being designed again -- industry experts do not foresee a return to the mammoth sizes such as the Wyoming machine. "It's really a tragedy," says Doman, who helped design the machine at Hamilton Standard. "The failure was caused by a very, very small maladjustment." Because of the maladjustment, the rotors flew into the tower, bashing off the outer 20%. The machine stopped in six to eight revolutions, but restarted itself again the next day, flinging off the remainder of the hydraulic control system and the work room. The turbine is not salvageable.
The machine had been operating since November of 1992 after its damaged generator was repaired (Windpower Monthly, January 1992). Originally installed in 1982 for about $10.8 million as part of the Department of Energy/NASA large turbine programme, the WTS-4 was later bought for $20,000 by Doman and former Bureau of Land Reclamation engineer Bill Young.