When Lightning Strikes

Posted on Tuesday, January 27, 2015 in UEA Blog

UEA has happily been providing slip ring assemblies for the wind turbine industry for the past nine years. There are many advantages to wind turbines, mainly the clean renewable energy they provide. However there is one major disadvantage: they are really tall.

We can make some pretty big slip rings, but height can be an area for concern. The height causes all types of challenges for wind turbine engineers but also creates a very serious potential issue for the slip ring and other electrical products located inside the turbine. Thinking back to physics class, a tall, pointed object creates ideal conditions for a lightning strike due to the reduced resistance path to earth in comparison to air. Factor in that many of these wind turbines are located in flat fields so that wind flow isn't obstructed and we have pristine conditions for a lighting strike. This means that a lightning bolt, or specifically the hundreds of kiloamperes associated with it, will pass through the wind turbine down to earth. This is why wind turbines have lightning protection systems to keep the valuable electrical systems working both during and after the inevitable lightning strike system.

That being said, lightning tends to go where it wants to and will find the best path to ground if an alternative exists. UEA recently performed high-voltage testing according to IEC 60060-1 testing techniques on a slip ring assembly designed for wind turbine use. The finding is that following a 1.2/50 microsecond waveform the breakdown strength from ground to any circuit is approximately 18 kV. Compare this to the electrical potential in a lightning strike and that may seem meaningless, but this value shows that even if the lightning strike tries to take a route through the slip ring assembly and the critical components throughout the electrical system it won't be able to do it very easily. Voltage of 18 kV is about 30 times the continuous voltage rating of the circuits. Without knowing the rest of the system and given the unpredictable nature of lightning strikes we can't know where it's path may divert to if it breaks the protection system but we can work to design a slip ring assembly that keeps it away from damaging critical components. Of course, it would be best to just follow the protection system in place.