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
  • 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.

 

Crane hardstands for installation of wind turbines: a handbook

Crane pads engineering and construction handbook. Copyright STOWA

Due to the pandemic this year we have been forced to go on vacation by car – the majority of flights from Germany to southern Italy have been cancelled, so we are making a 2000+ Km car trip with our three kids.

While my wife was driving I searched something to read and I have found something unexpected and extremely interesting: a 121 pages handbook only on wind farm crane pads!

The owner of the document is a bit unusual – STOWA, the Dutch knowledge centre of the regional water managers. However it makes sense if you consider that many projects in the Netherlands are on reclaimed lands and water authorities and municipalities are involved in the permitting process.

Behind the document there is a supervisory committee composed by the very best wind companies in the country, such as Fugro, ABT, H4A, Tencate and many others.

They will give you a very detailed view on the topic: after an introductory section describing the standard wind turbines and cranes now in the market the handbook describe in detail the process, from the geotechnical investigations to the design to the execution, operation and maintenance.

I have worked personally with some of the authors and I can guarantee you that they are very experienced professionals, so this is an extremely valuable document.

Once More, with Feeling: Timber Towers

Modvion laminated wood tower. Image copyright Modvion

Approximately one year ago I wrote a post about a full scale prototype of a wind turbine tower made of wood.

It has been built in 2012, but after that the idea seemed to have stalled without progress: one of the companies involved in the construction, TimberTower GmbH, disappeared from the radar shutting down their website and I suspect they went out of business.

However, I see that someone else has taken up the challenge: Modvion, a Swedish start-up.

They could be more successful in moving from the prototyping phase to the industrialization for at least two reasons: they are coming from Sweden, a country with an extensive know how and network of companies active in wood construction, and they successfully went through a round of Venture Capitalists, Business Angels and European Union founding.

They target is to be ready to market in 2022 cross laminated timber towers in the 100 – 150 meters range. This means that they want to enter in the 4 to 5 MW segment, the current standard for utility scale WTGs. The prototype that they have just installed is 30 meters high.

Such tower could have several benefits – solve the current transportation problems (steel towers with diameters over 4 meters have huge transportation challenges due to bridges, cables, etc.), lowering the carbon footprint and possibly even be cost competitive against the current technologies (steel, concrete and hybrid).

I have no idea of the behavior of this solution from the resonance point of view although I suspect that the increased diameter at the base improve the situation. I also ignore how this tower would behave in case of fire: I have personally seen a fire very near to the wind turbines some years ago in Portugal.

Unfortunately their website does not share many technical details. I understand that it is a modular solution, with the total number of modules obviously depending on the tower height being a standard solution in the 30 - 40 modules range.

The tower section is circular, unlike the TimberTower solution that was octagonal.

Among the materials used for the tower I see glued laminated timber and laminated veneer lumber - basically a mix of wood and adhesives, with superior technical properties and more uniform characteristics as it is produced in a controlled environment.

The modules are joined together using double-treaded fasteners, preassembling on site 4 or more tower sections with a bottom diameter of 6+ meters (that is, more than a standard steel tower).

This concept is similar to some concrete modular tower solution with a key difference – the modules are assembled horizontally, so I guess that the need of big crane support is limited.

I also understand that the internals of the towers would be similar to the ones currently built (with elevator, ladder, space for transformer, etc.).

Printable 3D concrete wind turbine towers

Yes, you're reading right – you can print your concrete tower.

I have discussed in many previous articles how I see some evidence that we are reaching the maximum size for steel towers, mainly because of transportation issues.

For higher towers concrete towers could help solving the problem, as they can be transported in pieces and assembled on site.

Among the different technologies available for concrete I have just discovered this exiting evolution: a Danish company specialized in 3D concrete printing, COBOD, partnered with GE Renewable and LafargeHolcim to develop a large printable tower.

COBOD already printed some years ago a full scale building, a small but beautiful 50 m2 office with curved walls.

For this interesting evolution they already made a prototype about 10 meters tall.

The concrete is extruded by the machine in a sort of ribbon, and the internal and external sides of the tower are reinforced by a “wavy” central section.

Currently the solution that they are targeting is a hybrid tower (that is, with the top section made of steel) with an on-site printed base.

Dancing in the wind

I have discussed in other post the phenomenal growth of the dimensions of wind turbines in the last 2 decades. Bigger rotors, taller towers and more MW has been the industry trend year after year.

There is some evidence that we are reaching the limit – blades of more than 50m length pose significant logistic challenges, while steel tower more than 100 meters tall can be subject to strong vibrations and dangerous oscillations under certain circumstances.

Such vibrations can be induced by several external sources such as an unbalanced rotor, an earthquake or the wind itself.

They are dangerous because they can damage the turbine due to fatigue loading (the weakening of materials due to cyclical loads). Some type of foundation can also partially lose stiffness – for instance monopile foundations.

Additionally, these vibrations can also trigger resonance phenomenons in the tower – you can follow this link to see of how “soft soft” and “stiff” tower are designed based on the blade passing frequency.

You can see a good full scale example of this problem in the video above and read here more about wind turbine vibrations.

There are several technical solutions currently being studied to dampen the tower reducing the vibrations.

Among the most interesting concept that I have seen I would mention tuned mass dampers – basically an auxiliary mass connected to the structure with spring and dashpots (viscous friction dampers), friction plates or similar energy dissipating elements.

These dumpers are called “tuned” because they have been designed keeping in mind the natural oscillation frequencies of the structure they have to protect. The two main parameters are the spring constant and the damping ratio: by varying them it is possible to damp harmonic vibrations.

I do not know if tuned mass dampers that can work with the first fundamental frequency of  industrial size wind turbines (below 1 Hz) are currently available – however I have found quite a lot of  studies on the topic.

A similar technological solution is the tuned liquid column damper. In this case a liquid inside an U shaped tank. By varying the geometry of the tank and the depth of the liquid different damping frequencies can be achieved.

The main benefits of this solution are the geometrical flexibility (you have to put the dumper somewhere inside the tower or the nacelle – I can assure you that the space there is very reduces) and low cost.

Another variant is the pendulum damper. In this solution, the length of the pendulum is calculated to match the fundamental frequency of the WTG.

Mass Damper (a) and Pendulum Damper (b)
Copyright O. Altay, C. Butenweg, S. Klinkel, F. Taddei
Vibration Mitigation of Wind Turbine Towers by Tuned Liquid Column Dampers
Proceedings of the 9th International Conference on Structural Dynamics, EURODYN 2014

Blades repair – how to fix it

I have discussed in other posts why wind turbines blades are prone to different type of damages and how to detect them.

But what happen when a problem is identified?

Changing the blade is usually the “last recourse” option: in addition to the cost of the blade itself there are the transportation costs plus the need to fine a main crane for the job (and it is not easy to find a free slot for a crane: due to their cost owners try to keep them busy 100% of the time).

Therefore efforts will be made to repair the damaged blade with a short downtime - ideally one or two days.

The technique used to fix the blade will depend on numerous parameters such as the entity of the damage (cosmetic, structural or affecting the efficiency of the blade), the region of the blades that suffered the damage (some areas such as the leading edge are more critical) are and the type of problem (cracks, debonding, impact damage, etc.).

Among the repair techniques currently used it is worth mentioning:

  • Filling and sealing (also called "dill & fill"). With this solution small superficial cracks, delamination and other similar non-structural problems can be repaired injecting the appropriate material (usually resin, or special fillers or gel). To do it injection holes going to the depth of the defect are created. Often the resin is pre-heated before being injected with manual guns or pneaumatic tools using compressed air. The material injected can have a curing time of several hours or even a day. Curing can be at ambient temperature or at a higher temperature depending on the chemical properties of the substances used. In case higher temperatures are needed heating blankets or similar tools such as ultraviolet lamps (UV) are used.

 

  • Coatings, tapes or shields. These solutions are especially designed for leading edge erosion repair and protection – a classic and frequent problem. Basically the idea is to use an additional layer to protect the leading edge.

 

  • Plug/patch and scarf repair for major damages. This solution involve removing the damaged region, leaving a straight, stepped or, if possible, tapered hole. Subsequently the patch is applied to close the hole. There are different alternatives for the patch: it can for instance be formed from a pre-impregnated composite fibre tape cut to shape, applied in layers using intermediate layers of adhesive or preformed to the correct shape and subsequently bonded.

Wind turbne blades repair patch. Image from Report on Repair Techniques for composite parts of Wind Turbine blades
(D. Lekou)

The adhesive used in blades repair is usually especially designed for this type of application. It has to be resistant to fatigue and cracks and have a short curing time (although for wind turbines located in extremely hot environments a slow curing adhesive can sometimes be a better choice). Two-component adhesive are frequent (basically the blade repair technician mix the two different components of the adhesive, activating the reaction).

Different repair works may be needed if the damage has be caused by lightning (a frequent occurrence) or if the problem is with the ancillary elements of the blade (like the vortex generators).

Where have all the wind turbine gone? Foldable towers

Perima foldable wind turbine tower - folded. Copyright Pantano et al., Springer

In previous post some years ago I have described two alternative solution for the wind turbine tower that should help solving the problem of the huge cranes that are currently needed for the erection of the wind turbines components.

One is the self-lifting precast tower developed by Esteyco, a Spanish engineering company that developed several interesting technical solution.

The other is the Nabrawind solution – again, a Spanish company that developed a self-erecting tower. They also have another interesting product, modular blades that can be assembled.

Some days ago, I have discovered another technical solution that share some similarities with these two concept but with an interesting twist: a group of Italian engineers has developed a “retractable” tower, basically a telescopic mechanism that can be folded bringing the blades down to the ground without using cranes or other equipment.

Theoretically it could be operated from a remote location, even if I guess that some kind of supervision during the operation is advisable.

Why should you want to make your wind turbine disappear?

The authors mention several reasons, for instance minimization of the visual impact (you can make your WTG almost invisible during the day and having it work at night).

I can also think at other uses – minimization of bird impact (folding the tower during the migration period) or increased safety during extreme wind (for instance during the monsoon season in south east Asia).

The idea is not only a concept –a working prototype has already been built in southern Italy.

Perima foldable wind turbine tower - erected. Copyright Pantano et al., Springer

It is a small wind turbine (55 kW), at least for what is today the standard in utility scale projects (3 to 5 MW). Additionally it has only 2 blades, which I think can help when you retract the tower.

However the hub height is 30 meters, quite a reasonable figure.

It is interesting to observe that this technical solution needs a deep foundation, basically with a depth equivalent to the hub height.

It is mentioned the possibility to modify the concept to use the foundation hole as a well to extract water. Quite an interesting side benefit I would say.

The authors are not sharing the cost of the tower and the ancillary elements, although I suspect they could be several time the cost of the standard, non-retractile  tubular steel tower.

Finally, it would be interesting to know the applicability of this solution to WTGs in the MW class.

The authors mention a dimensioning bending moment of around 300 kNm. Such value is two orders of magnitude lower of the values that are common in industrial size turbines, so it is not immediately evident that the idea can be scaled without major modification.

An additional problem would be the length of the foundation pit.  Reaching depths of 50 meters and below, although not impossible, introduce new issues – for instance the need of very specialized drilling equipment.

Perima foldable wind turbine tower - technical details. Copyright Pantano et al., Springer

Wind turbine blade damage detection systems

Wind turbine blades damages. B.F. Sørensen, E. Jørgensen, C.P. Debel, H.M. Jensen, T.K. Jacobsen, K. Halling, et al., Improved design of large wind turbine blade of fibre composites based on studies of scale effects (phase 1). summary report.

Wind turbine blades are rarely subject to catastrophic failures, although you can find a bunch of videos on YouTube with blades flying away or falling into pieces.

They are however subject to several problems, such as cracks, debonding of the various layers, internal delamination, etc.

As an order of magnitude blade failures are accountable for approximately one fifth of the problems usually seen during the operational life of the wind farm.

Such problems have a direct impact on the profitability of the plant and can be extremely expensive to fix.

Some issues can be fixed leaving the blade attached to the rotor. However not all problems can be solved without dismantling the blade, and the fee associated with the cranes needed for the dismantling can be very high.

In the worst case scenario a blade substitution will be needed, and the transportation cost will add up.

A certain number of failure is attributable to design error (insufficient material strength under fatigue or extreme loads) or manufacturing problems.

However the overwhelming majority of issues seem to be attributable to extreme weather – basically strong winds, thunderstorm (including lightning strikes), ice, pollutants, etc.

The effect of ice accumulation is especially dangerous in case of asymmetrical accumulation, because it can create unbalanced loads in the rotor.

Pollutants, usually in the form of airborne particles (e.g. sand) can shorten the life of the blades accelerating the erosion of the superficial layer. A gel coat usually cover the outside of the blade to protect it from ultraviolet degradation, water, etc.

Therefore the erosion of the external layer may have two impacts: lower the efficiency of the blade (this will have a direct impact on energy production, and ultimately on money) and accelerating the degradation of the blade.

Some areas of the blades are more prone to damage: tip, root and joints accumulate the majority of reported damages. The distribution of failures has however a typical pattern: erosion is more frequent in the tip side (due to the higher local speed) while cracks are more frequent toward the root.

Blades crack and erosion regions. Zhang, H., 2016. Reducing Uncertainty in Wind Turbine Blade Health Inspection with Image Processing Techniques. Iowa State University, Ames, Iowa

A variety of methods has been developed to detect and monitor damage. There is a consensus on the fact that early detection of a problem mean usually lower maintenance costs.

The “perfect”, ideal method should be able to assess the full scale of the blade without contact, allowing for continuous remote monitoring.

Unfortunately such methods does not exist yet. Among the most promising techniques it is worth mentioning:

The use of strain sensors, either embedded in the blade or on the surface. It is the same type of fibre-optic called “fibre Bragg gratings” (FBGs) that I have described in another post. As each sensor measure only direct strain and shear strain only in one point of the blades many sensors will be needed to have the full picture. Another problem is that the sensor can give a "false positive" result if it fails due to creep, fatigue, etc.

Measuring the acoustic emissions of the blade is another extremely interesting method to find damages. Unfortunately it is a complex system that need many sensors and physical contact with the blade of many sensors – something that could be difficult to achieve in operational wind farms. Sensors operate on an extended frequency range (50 kHz to 1 MHz) and as in the case of the strain sensor are used not only for the blades but also for the other components (gearbox, bearings, etc.)

Ultrasound is an extremely common non-destructive technique, that basically analyse the ultrasound waves reflected by the different layers of materials and by discontinuities in the materials. The main problem of ultrasound detection is that usually physical surface contact is needed. On top of that, there is also usually time consuming (and it can be hard to process the signal). It can however be extremely useful for an in depth analysis of the situation of the blade.

Thermography is a technique that use infrared cameras to assess the temperature distribution. If there is a defect in the structure the flow of the heat will be disrupted, causing temperature gradients. Thermography comes in two types, passive (made from the ground using the “natural” temperature) and active (heating the surface).

The last method, and possibly one of the most promising giving the quick evolution of computer computational power, is Machine Vision. One or more cameras are used to capture images of the blades subsequently analysed with image processing algorithms.

There are several interesting studies on machine learning – basically using two set of image, one of blades with known failures and the other of blades without problems, to “train” the algorithm to detect issues.

 

 

Taking over – it really matters

One of most the relevant milestones in the life of a wind farm is the taking over.

It happens when the contractual requirements for the wind farm are considered fulfilled by the subcontractor, except for smaller items that are noted in a “punch list” and have to be fixed as soon as possible.

The requirements for taking over are defined in the contract. They usually include a (very, very long) list of documents to be provided and all the tests to be performed.

The taking over is officialised by a taking over certificate. From this point delay liquidated damage stop accruing, and usually there is a reorganization of the bond structure (for instance the performance bond can be replaced by a warranty bond).

Additionally the clock for the defects liability period start ticking. Subcontractors have the obligation to replace defective items or equipment (for instance, a transformer) and this usually “reset” the clock for that specific equipment.

The obligations of the subcontractor are usually guaranteed by retention of payments for the punch list items and by the warranty bonds for the defect liability period.

Under FIDIC and FIDIC-like contract the subcontractor can make a claim if he consider that the employer is avoiding to issue the taking over certificate without a justified reason. This is particularly relevant if delay liquidated damages are accruing – in this case an independent third party expert is usually involved to solve the dispute.

“Sectional taking over” is another relevant concept – it means that the wind farm is not taken over as a whole but in smaller sections. Usually those sections match the wind farm internal circuits, but in theory even a single wind turbine (or even a foundation) can be taken over.

“Deemed taking over” means that if certain events happen (for instance, the wind farm start its operation) or a number of months elapse for when the takeover certificate is requested by the subcontractor the taking over is consider to have happened

How good is the wind farm you are working at? Some indicators

So, how good is the wind farm you are working at?

There are several parameters that can be used to assess a renewable energy project and to compare different projects.

Among the most used, it is worth mentioning the Capacity Factor, NPV, IRR and LCOE.

Capacity Factor is the ratio between the actual energy production of the wind farm (that is, GWh/year) compared with the theoretical production.

Expressed as a percentage it is usually a number somewhere between 20% and 50%. Wind farm with a capacity factor above 50% are usually regarded as quite exceptional.

It is basically function of two parameters, wind variability and wind turbine selected for the project. On top of that you will have several losses - for instance electrical losses, noise curtailment, wake losses, etc.

To calculate it you will simply divide the energy produced by the wind farm by the nameplate capacity by the number of hours. Due to the seasonal variability of the wind it makes sense to make an yearly calculation.

What is interesting is that Capacity Factor is fundamentally and economical decision. At the end of the day you want to improve your business case, so it could make sense to install wind turbines giving a lower capacity factor (but with an even lower total cost).

The Net Present Value (NPV) is today’s value of a future cash flow.

In the formula C is the cash flow (-C0 is the initial investment, C1 is the cash flow of the first year and so on until the last year, n) and r is the discount rate.

This metrics give priority to the absolute return of the investment. Basically it is useful if you have only one shot: if you put all your money in a single project you will prefer (ceteris paribus) the one bringing more money.

The discount rate reflect the fact that money in the future is worth less than money today – for inflation, cost of opportunity, etc.

Internal Rate of Return (IRR) is the discount rate that makes NPV = 0.

This metrics give priority to the percentage return. It could be useful for instance if you can pick several projects among many.

Levelized Cost of Energy (LCOE) is defined as (CAPEX + OPEX ) / AEP

CAPEX (Capital Expenditure) is the money that the wind farm developer will have to put in all the assets – not only the wind turbine itself but also the infrastructure (roads, foundations, substation, etc.), and the development costs (everything from land lease agreements to the engineering studies).

OPEX (Operational Expenditure) is what the wind farm owner will spend to have the wind farm up and running.

This include basically the maintenance of the wind turbines (they need new oil every now and then, pretty much like your car) and of the substation equipment. As the lifetime of such project is increasing from what used to be industry standard (20 years) to 25, 30 years and more.

Additionally the more the wind turbine gets older the more is likely that it would need major maintenance (for instance a new gear box).

LCOE makes a lot of sense when you are trying to compare energy produced by different technologies, for instance wind and solar photovoltaic.

Lazard (a huge private investment bank and financial advisory firm) distribute periodically a study on the evolution of LCOE. Currently available in version 13 it gives you some visibility on how much different forms of energy cost without subsidies.

I still remember when I started working in the renewables sectors about 10 years ago. Comparing the cost of solar and wind I believed that my colleagues who decided to work in solar were crazy as the cost per MWh of Solar PV was huge.

Well, it looks like I was wrong.