Windmill grounding system

[mafle]

Hello expert,

I'm working on a windfarm project. We are at the stage of designing the grounding system of the windmill. Each windmill has a concrete foundation with rebar.

Assuming that
-windturbine requires a fairly low resistivity ground system
-type of soil we are working with is mostly fragmentary rock.The resistivity of that type of rock is in general pretty high.
-concrete has the ability of retainning moisture
-foundation of a windturbine requires a lot of rebar

Considering the fragmentary rock, is it more efficient to
-use the rebar (as UFER states) only for the grounding system
-build a bare copper ground system in the soil on the top and around the foundation with ground rod
-build a bare copper ground system around the base but inside the concrete foundation

I can't attach a drawing if you have question.

thanks in advance
Mafle

 

[alehman]

I'd guess the bare copper in the concrete is the most efficient, but you need to consult the wind turbine manufacturer because some of them have very specific requirements.

 

[jghrist]

The bare copper ground system in the soil around the foundation will provide better touch voltage protection for someone standing outside the tower. This should be bonded to the concrete foundation ufer ground. I doubt that copper inside the concrete foundation will add much to the effectiveness of the ufer ground that already has extensive rebar.

I would expect lightning to be a major problem with windmills. The effectiveness of the grounding system for lightning depends on the high frequency impedance of the system. A system of radials from the ground loop around the tower might be considered.

I haven't seen anything written in particular about protecting windmills from lightning (I haven't done any searches either). How would you protect the turbine from lightning that strikes the blades? Is there a mechanism to shunt the surge around the turbine to the tower?

 

[Marmite]

Wind turbine earthing systems have to operate effectively for both power frequency currents and lightning surges. At each turbine a local earth is provided by laying a ring of conductor around the foundation at a depth of about 1m (counterpoise earth) and by driving vertical rods into the ground. It is common to bond the steel reinforcing into this local earth. The turbine earths are all connected together by laying bare copper earth cable in the same trench as the power cable. Lightning strikes on the blades caused the destruction of many early wind turbines. These days a down conductor or mesh is embedded in the blades with a lightning collector at the tip.
Mafle, I recommend you get hold of the Wind Energy Handbook by Burton, Sharpe, Jenkins and Bossanyi published by Wiley. ISBN 0 471 48997 2, which provides very useful reference material on all aspects of windfarms.
Regards
Marmite

 

[mafle]

Marmite did a pretty good summary of what is done actually in the field. I'll get the Wind Energy Handbook for sure, documentation on wind turbine are pretty hard to find here it's fairly new.

Alehman, as for now I've worked with two manufacturer, GE and Enercon and both of them has a completely different design. The grounding system of Enercon is base on ufer. Rings fo bare copper are laid in the concrete and one ring is outside of the concrete. They are all connected. There are no driven ground rod.

GE use partly ufer. they recommend one ring of bare copper inside the concrete and one in the soil on top of it. Ground rod are connected to the outside ring.

As we are building the windfarm on top of mountains and the rain are easely drained, I think that using the concrete is our best bet connected to the rebar as well as driven ground rod. The thing is not to overdesign but to be sure that the design will be a safe one.

 

[mafle]

I found this IEEE paper on rebar in concrete as electrode. It confirms what i was assuming.