Monthly Archives: July 2020

Lift me up: the braced foundation

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The braced foundation is a partially precast foundation that lift the wind turbine some additional meters above the ground.

Developed and patented by Esteyco (a Spanish engineering firm) is a technical solution validated, certified and used in several wind farms worldwide.

This solution increase the hub height up to 5 meters, which usually results in a significant increase of the annual energy production.

The “braces” are elements of precast concrete – basically double beams with a rectangular section transmitting the loads from the tower and stiffening the foundation.

They are on top of a cast in situ circular concrete slab that transmit the loads to the ground. This slab has a circular edge beam below, whose function is to absorb bending moments and contribute to the overall stiffness.

In the middle there is a central ring, while the tower rest on a smaller upper slab.

The main benefit of this solution is the increase of energy production – 5 meters of additional hub height can bring an annual increase in the 1% to 2% range depending on local wind condition.

Although this could look like a small number, compounded over 25 to 30 years it can really make a difference for the economics of the investment.

You first question could be something like “why not to use an higher tower”?

Generally, towers are designed, manufactured and sold with specific heights. Each wind turbine manufacturer has a portfolio that include only some heights (e.g. 90m, 100m, 110m, etc.).

Therefore you could find yourself in a position where the project could theoretically use a different hub height not offered by the wind turbine manufacturer.

Although every now and then project specific tower are designed and built this is not the standard and it has several implication in terms of time, cost, etc. Therefore it could be better to go for an off the shelf solution that gives you those additional few meters that your project need.

According to Esteyco this solution is also quicker to execute, at least in big wind farms. I do not have real world feedback to comment on this, although my impression is that the number of precast or partially precast foundation solutions used in the market is increasing.

This solution as a certain versatility because it can be used with different soil condition, including difficult geotechnical situation that needs piles.

It also use less material due to its geometry. I do not have actual figures to comment on the final cost, however my impression is that the real benefit will come from the additional production and that the saving in materials will be offset by the increased manufacturing complexity.

This solution has already a certain track record. I see that it has been used in Italy, Mexico, India, China and Saudi Arabia (in Dumat Al Jandal, a wind farm that I tendered 8 or 9 years ago – this gives you an idea of how long it may takes for a project to materialise).

It has also been certified by DNV-GL and TUV, undoubtedly a strong plus.

All the pictures are stolen from the presentation that Esteyco has given at India Windergy 2017.

Non linear finite element design on wind turbine foundations

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Wind turbines foundation cracks calculation

The design of wind turbine foundations is currently based on the plate theory.

“Plates” are plane structural element and the theory (or “theories” - there are at least two currently used) calculate stress and deformation when the structure is loaded.

During the analysis several difficulties emerges in satisfying equilibrium, stress-strain relations, compatibility of strains and boundary conditions. Theoretical results are often less accurate than you might expect.

These difficulties increase when the classical theory (the one usually used by foundation designer) is applied to reinforced concrete slabs.

This is due to several aspects such as:

  • The non-homogeneous nature of concrete
  • The nonlinear response of the material
  • Cracking
  • Time effects

The use of classical elastic plate theory, therefore, has been limited to reinforced concrete slabs under low levels of stress.

Classical elastic theory fails to predict either the yield moment capacity or the load-deflection behavior of reinforced concrete slabs.

Basically, the problem is that the stress distribution that we consider in our foundations project may be inaccurate due to the existance of cracks in concrete.

These cracks appear when the concrete is subjected to tensile stress.

Once the concrete cracks the stiffness of the section changes (it reduces importantly) and the forces in the section redistribute to other stiffer regions without cracks.

Subsequently, these stiffer regions may also crack after receiving these “extra loads”.

Then, that section continues the redistribution until you reach convergence and equilibrium.

Almost no wind turbine foundation designer is yet considering this effect, that should be, in most of cases, beneficial as the redistribution reduces the stress in the most loaded areas.

Why is that?

Basically because it requires a more complex and time consuming analysis.

The models required to consider these type of effects need to include the reinforcement. This can only be obtained using an iterative process.

You also need to take into consideration the crack propagations, and the bond-slip behaviour of the reinforcement (the tension in concrete, the tension stiffening of the reinforcement, and many other phenomena that may modify the final results).

Furthermore, the models used for wind turbine foundation design include always a contact non-linearity because the foundation may have a gap (that is, partially “lifting” under certain load cases creating uncompressed areas below).

Adding the sectional nonlinearity to the steel - concrete contact nonlinearity already considered may increase importantly the calculation times.

Additionally it is not completely clear how to implement the fatigue verification to the steel and reinforcement considering this type of analyses.

Nevertheless, taking into account the size of the foundations we are reaching in the market, this type of analysis may reduce the quantities in the foundations, making them more efficient.

Medium voltage power cables in wind farms: an introduction

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This post is an extension of the previous short article I wrote some years ago on the characteristics of wind farms medium voltage system.

I wrote it with the help of my friend and colleague Kamran, who spent more than an hour answering my questions on the subject. Thank you Kamran!

The medium voltage network is one of the elements that compose a wind farm project, the other being foundations, earthworks, substation and high voltage line.

Some elements could be missing: I have seen several projects without substation, for instance in France where  small wind farms were connected to the grid directly at medium voltage level. However, you will never see a project without at least several hundred meters of medium voltage cables.

Wind turbines generally produce energy with a voltage around 600V – 700V. Subsequently the voltage is raised by a transformer that can be located in the nacelle, at the base of the tower or less frequently externally in a small box near the tower.

The objective is to minimize the electrical losses, and several voltage level are theoretically possible - I have seen projects with MV levels varying from 12kV to 33kV and higher.

The objective to achieve working at the design of the medium voltage system is obviously finding the sweet spot that optimize Capex (what you pay for cables and transformers cost) and Opex (mainly the electrical losses that you will have in the cables), selecting a rated voltage compliant with local regulations and cable types that are commonly used in the country where the wind farm is located.

Cables are rated by their effective cross sectional area in mm2 – the greater the section, the greater the amount of current they can transport.

Standard sections frequently used in wind farms are 70, 95, 120, 150, 185, 240, 300, 400, 500 and 630 mm. Greater sections are commercially available but already the 400 to 630mm sections are hard to use in construction due to their weight and bending radius.

The bending radius is usually expressed as a function of the diameter. For instance, “10x D” would mean that the minimum bending radius is 10 times the diameter of the cable. This parameter is significant because you will probably need some narrow bends in your cable, for instance at the bottom of the foundation if the transformer is inside the turbine. Large binding radius can make the work at the construction site very hard.

The cables are made of several layers with different functions – many technical alternatives and constructive techniques are available in the market but in general you will find (from the centre to the most external layer):

  • A conductor core made of copper or aluminium
  • An insulation layer, usually made of cross linked Polyethylene (XLPE)
  • A metallic screen to stop the electric field
  • An external sheath, protecting the cables from corrosion, humidity and mechanical stress. In some projects this most external layer is selected to have special properties such as for instance enhanced resistance to fire or protection from aggressive chemicals or even termites (I have seen this last feature in Australia)

Different medium voltage cable layers. Copyright image Yuzh cable

Cables will be delivered to the wind farm in cable drums made of wood.

The standard design strategy is trying to minimize the number of cable drums because making the joints between different sections of cables is an expensive and highly specialized task.

There are however limits to the size of drums – basically both its weight and dimension must allow safe transport and manipulation.

The amount of meters of cable that can be transported on a drums depend on the cable type and diameter – for wind farms you will usually receive some hundreds of meters in each drum.

Single core vs. three core cables

There are two main typologies of MV cables commercially available, single core and three core.

In single core cables each comes with his own screen while in three core cables the three phases share a common metallic screen. If you select the single core technology you will need to use three different cables, one for each phase.

Aluminium vs. Copper cables

The material used for the conductor of the cables for wind farms is always almost always aluminium.

Theoretically, copper cables are available and copper has several desirable characteristics - for instance it is a more efficient electrical current conductor and requires a smaller cross section to carry the same amount of power as an aluminium conductor.

However, with the current relative prices of copper and aluminium, copper cables are simply too expensive so they are never used for the reticulation of wind farms – you will probably see them inside the substations, where distances are shorter.

The cost of raw materials such as aluminium represent a relevant percentage of the final cost of the cable. For this reason I tend to see the MV cables almost as a commodity.

Overhead vs. Buried cables

In the majority of countries, the cables are directly buried in a sand bed in the bottom of the trenches (or in very rare cases, inside a duct).

Every now and then, I see a project with an overhead medium voltage line, for instance in India or South Africa. However, they tend to be more the exception then the rule.