Effect of a lightning strike on stainless steel bolt

[norris]

Hello, I was recently onsite in the Philippines and the equipment owner stated their wind turbine had been hit by lightning. The only obvious damage was to the bolted connections at the root of one blade. One was sheared, the other bent. Does anyone have any experience of lighting causing this type of damage? The blades appear to have no more than normal wear from 6 years of age and once we got the PMG back to our workshop it still reads a voltage.

 

[Astronuc]

The only obvious damage was to the bolted connections at the root of one blade. One was sheared, the other bent.

What was bent? The bolts? or the bolted connection at the blade root/hub?

If a large current went though the bolt, it my have significantly heated the bolt and softened the material, and possibly melted it. If there was any moisture in any void space in the connections, that could flash to steam if heated, and perhaps blow a bolt out.

Lightning damage to marine vehicles and aircraft can be significant.

See - Lightning damage mechanisms - in http://www.pas.rochester.edu/~cline/ASK lightning strike/ASK accident report.htm
read the part on Joule heating

http://www.sailingbreezes.com/sailing_breezes_current/articles/Sept03/lightning.htm

 

[Baluncore]

What are the blades made from ?
How were the bolts fitted, did they have washers that could provide a reservoir for water ?

Water trapped under a bolt may explode as steam and break the bolt. The other bolt may then bend due to the strike or the loss of its partner. The high dielectric constant of water significantly slows down the transient current on a wet metal surface. The strike current will first run outside the water, not on the metal surface of the bolt.

How do you know the lightning strike broke the bolt? Could it not be fatigue? Was it windy at the time?

 

[norris]

The blades are a fibre glass / resin. Yes, washers were fitted and water in the thru hole is a definite possibility

I don't know it was a lightning strike, I'm relying on the community who own the equipment giving me accurate information. Fatigue seemed less likely due to the age and condition of the bolts on the other blades. The damage seemed more like impact to be but having researched the effect of lightning, this now seems like a possibility.

 

[norris]

The bolts were bent .

Thank you for the links these have been informative. The point about void spaces is interesting. I will soon start to refurb this turbine, would you recommend trying to seal the void? My thought is, if zero ingress can't be guaranteed, in a future lighnting strike the almost sealed void could create even greater damage if there is not a route for the pressure to escape

 

[Baluncore]

You seem to understand the problem.
It is important that the metallic path not be painted or covered with a dielectric. Washers must be flat and only slightly greater in diameter than the nuts and bolt heads, that way the washers do not bend up and leave a gap. You need bare metal outer surfaces for conduction. A flat bronze strap can bond bolt heads to prevent current flowing under the surface of dielectric materials.

How do the currents flow from the rotating hub through to the tower lightning conductor without damaging the bearings ?

 

[Astronuc]

One was sheared, the other bent.

The sheared bolt might have been fatigued. What are the ages of the bolts, hub and shaft?

What is the composition of the bolts (stainless steel, but which one?), hub and shaft. I would guess that the bolts and shaft are metal. If so, then a lightning strike could travel through the bolts into the shaft, and into the interior housing. The more area/volume, the lower the current density, and less damage.

One might look at how aircraft and tall structure are grounded.

Also, look at how dirty the blades become overtime. Lightning will travel over a conductive surface of deposits, especially if the surface is wet. Deposits of metal chlorides from atmospheric deposition may be a concern.

Is the turbine near the ocean? If so, look for sea spray or sea mist.

Atmospheric corrosion in conjunction with fatigue can be life limiting.Pictures would be very helpful.

 

[norris]

You seem to understand the problem.How do the currents flow from the rotating hub through to the tower lightning conductor without damaging the bearings ?

Honestly, I'm not sure. Two options come to mind, 1st a copper strap with significant play to accommodate multiple rotations of the hub. 2nd some sort of slip ring but this could be prohibitively expensive in my situation. Do you have any advice?