Maple Ridge was completed last year, but in February 2007 a lightning strike at the 322 MW facility seriously damaged a blade, causing pieces to fall to the ground. A decision was taken by Vestas, in conjunction with the owners of Maple Ridge, Horizon Wind Energy, now owned by Portuguese national utility EDP, and PPM Energy, to retrofit all the blades. The work consists of fitting each blade tip with a copper sleeve, installed so that it makes a connection to the lightning receptor already in the blades, says Strøm Madsen. Vestas has fitted the copper sleeves to about half the turbines so far. Completion of the job depends on the schedule decided between Vestas and the wind farm's owners.
The decision to equip all blades with copper tips was taken after Vestas established that some wind farm sites in the US suffer from extremely high lightning intensity requiring that lightning protection of turbines be improved. Strøm Madsen declines to say how many turbines or sites are affected or what the work will cost. Vestas is not considering any fundamental change to its lightning protection systems as a result of the incident, he says.
Lightning strikes on wind turbines are far from a rare occurrence. "It's a known secret in the industry," says Kim Bertelsen of Electricon, a Danish company that specializes in lightning protection for the wind power industry. "With lightning there's this unknown factor where we don't know the real truth about the phenomenon. It's not only at Maple Ridge that you see blade damage." In the US Midwest, 40 strikes a kilometre during the year is not unusual. "When you erect turbines these numbers can be even higher," he says. Bertelsen, an electrical engineer and former Vestas employee, has studied lightning for more than 15 years.
Lightning protection of turbines has improved over the years, he says. "Mostly you see minor damage. You do a surface rebuild and this is not a problem. Of course, if a blade is seriously damaged, then you have to change the whole blade. There can be some special situations with one particular blade maybe not performing that well. But in general the quality is much higher than before."
Horizon's Mike Kelly agrees that turbine owners expect a certain amount of lightning damage, although at Maple Ridge it went beyond the expected. "Industry standards permit some degree of damage in the sense that, statistically, blades are going to be 99% efficient," Kelly says. "What that means is that some of the strikes that hit the turbines may cause some damage without pieces of blades coming off turbines, which is completely unacceptable," he adds.
"I've seen turbines get lit up by biblical lightning and not miss a beat," he says. "You go up the turbine and you can't find any sign of a strike anywhere. But I also woke up one morning to five turbines with their blades spread all over the ground -- although that was in the nineteen-nineties before lightning protection was a standard feature." Kelly says the copper tips at Maple Ridge are an acceptable solution. "I know that they've been applied successfully to some other turbines in the past," he says. "Vestas has responded well to the situation."