According to experts who have seen photographs of the incident, a fibreglass blade on the Z-750 took such a powerful jolt of lightning it ignited, dropping burning debris onto the nacelle, which then started to burn as did the hub, catastrophically damaging the whole machine.
The experts, who prefer not to be identified because the industry is close-knit, say the incinerated blade cannot have contained metal that is earthed. Enron Wind, part of Enron Corp of Houston, would only say the turbine, in the huge 107.25 MW Lake Benton I project-also known as Northern States Power (NSP) II-has Zond's "standard lightning protection." It is thus not clear exactly what preventative measures against lightning damage have been taken by Enron Wind, under the gun to install hundreds of megawatts in the Midwest, or by NSP. Zond turbines are equipped with blades from several different suppliers. The huge Lake Benton I wind farm came on-line in the autumn of 1998.
Most leading wind developers and turbine and blade manufacturers now insist upon having internal metal lightning protectors in all of their blades. They say it is only sensible-financially and in terms of operator safety-given the industry's knowledge and their own assessment of the long term costing of project development and operation. Experts also point out, though, that even the best protection is not fail-safe. It prevents damage in more than 90% of cases, but not in every case. In addition, costing decisions will differ from project to project.
The Midwest is lightning-prone, especially from June to August, and has about five times as many days with lightning than Denmark, and ten times as many as California, the two areas where the wind industry cut its teeth. Other especially prone areas are central Italy, mainland Greece, southern Spain and parts of Japan.
The US National Renewable Energy Laboratory (NREL) is studying lightning at a 6.6 MW demonstration project in Texas, part of the same high risk zone in the Midwest. Those Zond 550 kW turbines, however, have ailerons, which often act as natural conductors from near the blade tip to the hub and which are not usually used in the company's turbines.
Earlier research indicates that most often damaged are a turbine's control system, followed by the electrical system or grid. Many such hits are due to lightning faults on the electrical system and are not because of the turbine design. Blades are the third most likely component to be damaged-in 10 to 20% of cases according to one study. Damage to the blades is the most costly.