# Medium voltage power cables in wind farms: an introduction

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
• 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.

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.

# Lightning protection of wind turbine blades

Image copyright of Elsevier & X.Bian. Published on “Numerical analysis of lightning attachment to wind turbine blade”

Specifically, the blade has been damaged before commissioning.

At first sight the consequences could be less significant than usual, above all if the main crane is still on site and there is a set of spare blades available as happens frequently in big wind farms.

I would also guess that the turbine supplier will have to absorb the cost unless the risk was already transferred to the customer.

I would also like to elaborate a bit more on the topic as lightning is a frequent cause of damage to wind turbines (specifically to the blades, as they are hit in around 75% of the cases).

Lightning are created by the electric field between the bottom of the clouds (negative) and the ground (positive).

The potential difference is significant (some MV). However, due to the distance, the average electric field is weak.

As the electrical charges at the bottom of the clouds accumulate a “downward leader” (a channel permitting the flow of negative charges) start moving toward the ground.

If this stepped leader is somewhere nearby a wind turbine (or another similar structure) the second phase of the phenomenon may start: an “upward leader” from the blade connecting with the downward leader and closing the circuit.

This is the instant where the “return stroke” start and you usually see the majority of the light.

Coming back to the blades the standard technical solution consist in embedding a copper receptor connected to the ground at approximately 1 meter to the tip. This receptor has a diameter of at least 50 mm (pretty much like the copper cable used in the earthing of the foundations).

The problem is that this solution doesn't works always: sometimes the lightning hit another point of the blade. Even if the surface of the blade is supposedly non conductive it has been observed that, due to the presence of pollution and water, it can behave like a conductor.

# Looking for an electrical engineer specialized in Turnkey / EPC (Balance of Plant)

If you are reading this blog chances are that you know someone who could be interested in this position… Please spread the word!

# Itemized sourcing: everybody else is doing it, so why can't we?

Itemized sourcing is the new mantra in the business. Basically, it means that you should split the BoP (Civil and Electrical works) in as many lots as possible, in order to achieve substantial savings. This strategy is the opposite of the “single subcontractor” approach, where you give the full package to a unique contractor or at least few of them.

What are the benefits of itemized sourcing? In addition to the possibility of achieving a lower price you also have more control on the purchase of critical items (for instance the main transformer).

Additionally, our friends in Procurement (for instance my role model Ignacio) can create PowerPoint slides showing huge savings to the rest of the organization.

The untold story is that there is no free lunch. What you are achieving is simply a different risk profile for your project: if the truck driver destroy your transformer against a bridge that is a few centimeters too low or if the foundations are build in the wrong place (believe me, both example are from real projects) you will have an hard time to recover your losses - because you purchased the transformer by yourself, or because you have in your pocket many small contracts with low liquidated damages.

Let’s assume that you split a 10 ML USD contract in 5 smaller contracts worth 2 ML USD each.

You will be able for sure to achieve substantial savings. However several other things will happen as well:

• You should tender every smaller contract separately. Therefore an additional effort is needed from the Tendering Department and from Procurement.
• Some big subcontractors will decline the job, because the size of the job is too small for them.
• You will need to discuss the contract (at least) 5x times – 10x if you have more than one option on your table.
• Unless the applicable law for your contract is something like Sharia (with dispute resolution in Riyadh) you will not be able to claim more than 100% of the value of the contract (2ML) from them. However your exposure toward the customer will be 10ML plus the value of the turbines. If one of them has a problem your risk will be huge.
• In the real world is often not so easy to understand who is causing a delay. Sometimes there are concurrent delays, sometimes it was not clear who was expected to do a job, other time a finger pointing game start. The more the subcontractor, the more the risk.
• Lastly, during construction you will need (at least) 5x more effort from the Contract Manager – and for the Site Manager, Project Manager and all people on site.

Don’t get me wrong – I understand that sometime there is no real alternative to make the project fly. However, before you embrace the itemized sourcing of the BoP as the solution for all your problems, you should keep in mind the additional work (try to negotiate 10 contracts in parallel) and risk that you are taking on board.

# Constraints to medium voltage power cables in wind farms

(This post has been updated in July 2020 with the help of my friend and colleague Kamran)

I already discussed in two other posts how the wind farm cable trenches are usually built and how the medium voltage cables are made.

However, a more comprehensive explanation should include on how the medium voltage cables are dimensioned.

The power produced by the wind turbine is usually evacuated to the substation using a medium voltage (MV) cables connection.

This cables are usually buried. This solution is slightly more expensive but it offer much more protection to the cables than an overhead line (that is, a line where the cables are hanging from poles). A buried circuit is also less problematic from the environmental point of view. I do however see every now and then projects with overhead MV systems.

The reason for evacuating the power generated by the wind turbines with a medium voltage system is purely economic. A low voltage solution would have very high power losses (“Joule losses”) in the cables – basically the resistance of the conductor would create too much heat.

A higher voltage will decrease the current flowing in the cables and the related losses.

However, high voltage equipment is very expensive. Therefore, the medium voltage solution is a reasonable compromise, an optimum balance between the losses in the cables and the cost of the equipment.

So, what are the key constraints in the design of a MV system for a wind farm?

In many projects there is some freedom on the MV level used in the wind farm – that is, you can design a MV system using one of the standard voltage levels (such as 20 kV, 33 kV, 34.5 kV, etc.)

In situations where there is no specific requirement from the owner of the wind farm (or from the owner of the grid) the smartest choice is usually to select the MV level most commonly used in the country where the wind farm will be built.

Sometime the number of feeders in the substation is defined upfront: this limit the number of different circuits that can be designed to connect the wind turbines.

Another relevant limitation is the maximum allowable cable cross section: the bigger the cable diameter, the bigger the bending radius. For this reason cross section over  630 mm are very unusual.

For a given cable section you will try to maximize the amount of current transported without exceeding the thermal capacity. Under standard operation conditions such limit is usually around 90°C and the thermic behaviour of the cable will be obviously strongly influenced by the thermal resistivity of the surrounding material. Other relevant parameters are the depth of the cable, the temperature of the air and of the surrounding material, if there are other circuits nearby, etc.

Other significant project constraints that must be checked are the allowable current (how many amperes are transmitted by the cable – to be sure that the capacity of the cable is not exceeded) and the voltage drop between the 2 sides of the circuit (usually it should be less than 1.5%).

You will also try to minimize the power drop. A rule of thumb is that the losses should be less than 2%, but some wind farms have more aggressive requirements.

Last but not least, additional checks are performed to ensure the survival of the cable in emergency conditions – for instance in case of short circuits.

The short circuit currents are checked against the cable ratings under a variety of possible faults (phase to phase, phase to ground, etc.). Cables are designed to survive a higher temperature for a very short periods (seconds, or fraction of a second).

# 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.

# Cost drivers in Electrical Balance of Plant

Due to my education as a Civil Engineer there I already wrote a substantial number of posts regarding cost of the civil BoP.

However I do not want to neglect the electrical side, which as you might already know is usually accountable for approximately 50% of the total cost  of the balance of plant of a wind farm.

I went through the cost of several projects I’ve worked at in the last 6 or 7 year together with a very good friend that I’ve left in Madrid to see if it was possible to find a recurring pattern in the numbers.

Unfortunately, the Electrical Works costs are much more fragmented than the Civil Works, where few “usual suspect” such as concrete, steel and earthworks dominate the scene and are the key cost drivers.

If you are working in the wind business you will be probably thinking  that the most expensive items will be the main transformer.

This is not always the case: in project where we had to quote a long overhead line, it absorbed up to 40% of the electrical budget, quite an impressive figure. Even shorter overhead lines could easily end in the 10% to 20% range, that in a multimillion project  is obviously a big number.

The second item competing with the transformer in the Top 3 is the medium voltage cabling system.

Obviously is extremely difficult to give a number because it will depend on the layout of the wind farm (will it be a row of WTGs or a “cloud” of scattered positions?). Nevertheless, numbers in the 3 to 4 million USD are not unusual even for medium size wind farms.

Then you have the transformer, the last of the Top 3 items. This is the easiest item to quote, usually somewhere around 1 million USD.

Last but not least we have “the rest”. This include everything from the switchgears to the high voltage equipment to the capacitor banks, substation facility and other fancy equipment in the substations.

The impact of all this item can be huge, from 30% all the way up to 70%. Obviously, with such fragmentation it becomes clear that from the cost structure point of view Civil Works and Electrical Works are totally different.

# EBoP vs CBoP - where is the money?

There are several recurring questions that I normally hear at least 3 or 4 time each year.
Some are variants of things like “How much does it cost 1 Km of road in Brazil?” - this was asked by my ex colleague Pau many, many years ago but it’s still a classic for me, and a reminder of the fact that in the wind industry BoP is something ancillary to the core business and not really understood by the majority of the colleagues.

Other questions are more interesting (or at least, it is possible to try to answer them in a more elaborate and complete way).
This is the case of the question “What is more expensive, EBoP or CBoP?”
If you are reading this blog you will probably know the meaning of the acronyms:

EBoP: Electrical Balance of Plant – that is substation, medium voltage cables, step up transformers (if any) and in some cases overhead line.

CBoP: Civil Balance of Plant that is roads, WTGs foundations, crane pads, trenches and other fancy stuff that could be requested by the specific customer/project.

And the answer is… it depends.

In some project, you are requested to build 2 or more substations: one or more windfarm substation to collect the energy plus a substation to evacuate the energy to the grid. This type of layout will also need several Km of overhead line, in single or double circuit.
In situations like this, EBoP is usually more expensive – above all if you don’t need special foundations and earthworks are not particularly complicated (e.g. a flat country, like Uruguay).

The opposite case would be a situation where the EBoP is easy (maybe because there is an existing overhead line crossing the wind farm, or an even more lucky situation where you simply have to connect to an existing substation).
In this cases, if you also have expensive civil works CBoP will be clearly more expensive. This happened for instance in some project I’ve the pleasure to work at in Chile and Honduras.

You can see 2 examples in the pie chart at the beginning of the post.

By the way, if you really need to answer the question of Pau (“How much does it cost 1 Km of road in the country XYZ?”) the best answer that you can give is 100.000 euros.
If it’s a road in an expensive country, remote location, in the mountain, etc. increase the figure (150K – 200K euros), while if it’s in a cheap place it would cost around 80K.

# Is wind energy really unpredictable?

I know that I’m probably biased on this subject but I want to spend a few words on a recurrent subject that pop up often in my discussion about wind energy with people from different sectors and way of life.

Basically, a standard argument about wind energy is that it’s unreliable and unpredictable.

In my opinion the reality is different (or at least, much more complex that that).

Obviously wind power fluctuate over time, basically under the influence of meteorological conditions.

Variations occurs at several scales: seconds (e.g. gusts), hours (e.g. day and night), months (e.g. summer and winder) and so on.

The electricity demand as well is highly variable, changing not only with well-known seasonal and night/day patterns but also incorporating several other variables such as the economic cycles.

Basically, the grid operator tries to match constantly demand and offer.

They will also need to have a reserve capacity in case of errors in the prediction of the demand or unexpected problem like power plants disconnecting from the grid for whatever problem.

The key point here is that wind energy is variable, not intermittent.

Even during severe storms, the turbines will need several hours to shout down – they will not disconnect all together.

Also, the failure of a turbine has usually no effect on the system as they are modular and diffused. This is usually not the case in other type if power plants.

It is also predictable within a reasonable margin of error – is not a random event like the number you get when you throw dices.

From the point of view of the grid, variations within the seconds or minutes are not felt.

Variation within the hours are felt by the system only when wind has a great penetration level (at least 5%-10%). This currently happen in very few countries, for instance in Denmark.

Note: the main source for this post was an interesting (at least for me) chapter of the book “Powering Europe: wind energy and the electrical grid”.