Technical requirements and tests for wind farm roads and hardstands

Two readers asked me how to assess the quality of the civil works in a wind farm.

This is a very broad topic and it requires some previous knowledge in geotechnics and road construction.

However, I think that someone might found useful an introductory article with some of the requirements that I would recommend to include in the technical specifications and test during construction.

The objective of these requirements and tests is to confirm that the materials used are appropriate and that the works have been properly executed.

As the readers of this website are from different geographic areas, I will not suggest a specific national or international norm such as ASTM, AASHTO, UNE, etc.

Finally, I also intentionally did not specify the value that I consider appropriate for each parameter.

However, if you need support to find an adequate standard or to define a specific value, please feel free to contact me.

For the embankments, I would recommend to define:

  • Maximum grain size. This requirement will avoid the presence of rock and boulders in the wearing surface of the road.
  • Appropriate gradation by sieve analysis (that is, the “granulometry” or particle size distribution). This requirement will ensure an appropriate interlocking between the particles and it is linked to almost all other property (e.g. stiffness, fatigue resistance, permeability, etc.).
  • Atterberg limits (Liquid limit and Plasticity index). You do not want to use plastic material behaving like clay.
  • Low organic matter and soluble salts content. This requirement will insure that only appropriate materials are used in the construction of the embankment.
  • Very low swelling or collapse potential.

The material should be properly compacted. Compaction grade is defined by the percentage achieved compared to the “optimum Proctor value”.

You also want a low deformability. This is usually defined the “Strain modulus” (or “Ev1” and “Ev2”) – basically the deformation of the material under two load cycles as resulting from a Plate Load Test.

I would also recommend defining a minimum CBR (“California bearing ratio”) value for the top and the core of the embankment.

If possible it is always a good idea to build a test section of the road, for a full-scale trial of the construction procedure. It can also help to determine the most appropriate compaction moisture – the actual value could be different from the value given by the laboratory test.

Graded aggregate is the material used in the upper layers of the internal roads.

It is made of well-graded crushed stones. The typology of arid to use and its granulometry depends on the local availability of materials.

In addition to the parameters defined for the embankments I would recommend to specify:

  • Sand equivalent (because you want as little clay as possible).
  • Loss by abrasion (because you want to use for the wearing course a material that can last for several years, ideally more than 20).

It is also advisable to define the maximum difference between the actual roads finished surface compared to the theoretical surface defined in the project.

Finally, compaction must be verified. Again, the standard option is the plate bearing test.

Wind farm testing and commissioning

This is a short (and incomplete) summary of the main test which are usually performed in a wind farm.

Test can be divided in 3 categories: factory tests, site tests and performance tests.

Some test are performed before the start of the construction works, others during construction and commissioning and others when the wind farm is completed and producing power during the defect liability period.

Factory tests

These tests, usually called FAT (Factory Acceptance Tests) are performed during the manufacturing of the WTGs and the other main equipment of the wind farm (such as the substation main transformer).

On the WTGs side, the most usual one are:

  • Test on towers (dimensional inspection, coating, non-destructive reports, etc.)
  • Electrical components (generator, transformer, converter system, etc.)
  • Mechanical components (gear box, yaw and pitch systems, etc.)

For the BoP, you will test at the very least the main transformer and possibly the MV cables.

Site acceptance tests

Site  acceptance tests can be divided in test on commissioning and test on completion.

The “commissioning” of a wind turbine is a setoff activities performed to confirm that the wind turbine has been correctly installed and it’s ready for energy production. You normally need to have  the grid connection to do the commissioning – this means that the wind farm substation (or the connection to the grid) should be ready.

A very long list of items is checked at this point. Some of the key ones are run test with the WTG connected and producing power, verification of protection systems, test of power measurements, plus many mechanical tests.

Basically, you want the turbine to work and produce many hours in a row (200, 300 or more) without faults. It can lead to delays if not enough wind is available to perform the test.

There is also a separate commissioning for the main  transformer, the substation (protection systems, power measure equipment, MV switchgear) and the cables.

Test on completions are usually for the full wind farm.

The whole system has to work without failures for many hours generating power. Among other things you want to confirm that the main transformer can evacuate correctly all the power without overheating, abnormal losses, etc..

SCADA system is assessed as well.

Performance test

This group include test like availability, power curve and acoustic noise level.

“Availability” of the whole wind farm is assessed.

Availability means that the wind farm (and each and every wind turbine) is operating for a relevant percentage of time (95%, 97% or even more depending on the contract).

Power curve is the relation between the wind and the output of the wind turbine. It is critical that the WTG produce as much as expected – otherwise the basic assumptions behind the business model of the project will be wrong.