Monthly Archives: May 2012

Wind turbine tower

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Towers are an important element in the structure of a WTG, not only for structural reasons (they transmit the loads from the nacelle to the foundation) but also for economical reasons: they highest the tower, the biggest the amount of energy produced.

For instance, with an increase of 20 meters from the standard 80 meters height an additional energy production around 5% can be obtained. In the future higher towers are expected, also because in mature market the most productive locations have already been used.

The average height of tower installed in Europe is around 80 meters. Modern towers have a lift inside, a ladder and several intermediate platforms. Another item that can be hosted in the tower, normally at the bottom, is the MW transformer.

Towers, above all standard steel towers, aren’t a high technology product and there are several companies with the expertise and capabilities to do them. For this reason they are often outsourced.

Towers can be made of several material following different design concepts:

  • Lattice
  • Steel (tubular or segmented)
  • Concrete
  • Hybrid

Lattice tower were common in the past when turbine where smaller (lass than a MW), but are seldom used today. Their biggest problems are a notable visual impact, and higher construction and maintenance costs. They have several advantages: they use less materials (about 50% of a standard steel tower with the same stiffness) and they produce less shadow.

Tubular steel towers are the most widely used solution. They normally have a conical shape and a diameter varying from approximately 4.5 meters at the base to 2 meters at the top, divided in 3 or 4 sections assembled at the wind farm (they are bolted together). The length of a section can vary from 20 to 30 meters. Basically they are manufactured with steel sheets cut, rolled and welded.

The new steel towers of more than 100 meters have a base section diameter over 5 meters: this can be a problem, because in many countries the maximum transportable size by road is less than 4.9 meters.

Siemens is currently working with Andresen Towers to a longitudinally, on site bolted steel-shell tower.

Concrete towers are a solution in countries were steel price is unusually high (for instance in Brazil, where steel production is almost a monopoly). They are made of several smaller precast pieces assembled on site. This solution allows an easier transportation due to the smaller dimension of the components and a good control of the quality of the materials. They biggest problem is the weight (unless they are designed in a biggest number of pieces, they can weight more than the nacelle)

Hybrid towers are another solution used by several manufacturer to reduce the exposition to the steel price volatility, the main drawbacks is that they are quite complicated to assembly, so they have higher installation costs.

Other solutions are available (for instance guy-wired pole tower), but they are used only in very small turbines.

 

Windfarm SCADA system characteristics

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SCADA is an acronym that stands for "Supervisory Control And Data Acquisition".
It is a fundamental tool to monitor and control several parameters of the WTGs, and it’s usual to sell it together with the turbines.
It allows combining, in a single point of control, all the info about WTGs, meteorological mast and substation, serving as the primary interface between the wind plant operator and the wind farm equipment, sending control signals from the wind plant operator to the wind park controller, retrieving and storing operational data from the WTGs and interrogating alarm signals.

The fundamentals characteristics required to a wind farm SCADA system are:

  • To integrate in a single system WTGs, substation and meteorological tower info.
  • To allow an interrupted access to the data from the local PC (normally located in the substation building) and from remote computers.
  • To allow to modify several parameters of the control system of the turbines.
  • Each communication protocol used by a part of the system must be compatible with the others.
  • A clear hierarchy of all the users must be defined (basically to guarantee the safety of people working inside the machine)
  • Several parameters for each component must be shown. Normally, at least the following are required:
  1.  WTGs: status (working, ready, paused, stopped), power (kW), cos j, speed (rotor, generator, wind), temperatures, voltage and current of each phase, active alarms.
  2.  Met Mast: Wind speed and direction, temperature, pressure, battery status.
  3.  Substation: Line voltage and current, active and reactive power delivered, status of alarms and protections.
  • The user must be able to change at any time various parameters:
  1. WTGs: start and stop of the WTG, use of the orientation system, transfer of production data.
  2. Substation: opening and closing the main switch.

Several reports can be produced with the data provided by the SCADA system, such as determination of the power curve, generated power, availability of the turbine, failures statistics, wind data (speed and turbulence), active and reactive power and cos φ at the substation.

SCADA systems retrieve, store and exports huge amount of data to a variety of stakeholders, everyone with different needs:

Remote operation center: they must be able to use the alarm condition in a quick and efficient manner, discerning the root cause of a fault without being submerged by cascading alarms.

Remote monitoring and diagnostic: after the first intervention from the remote operating center they must be able to interpret data quickly to solve the problem. Additionally historical SCADA data can be used to validate computational models or develop new models.

Asset owners: they will use SCADA output for power revenue calculation, calculation of lost energy, etc.

Wind Farm SCADA Architecture

Good practice during windfarm construction

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“Good practice during windfarm construction” is a document produced by several Scottish agencies (Scottish Renewables, Scottish Natural Heritage, and the local Environmental Protection Agency and Forestry Commission).

It provides several useful advices on wind farm construction, based on the experience they gained with the development of several pretty big projects (for instance Whitelee W.F., with 140 WTGs, 86 Km of roads and 940 Km of cables).

The document is based in a northern Europe environment, where many roads are on peat and heavy precipitations are expected. Here the main points of the documents:

 

  • They suggest designing the drain for a 1:200 year event and the use of pre-earthworks drainage (a solution seldom used in dry countries).
  • Regarding the shape of the ditches, they explain the differences between “V” shaped ditches (they maintain more the vegetation, but they have more erosion) and “U”shaped ditches (they allow easier access and egress to wildlife).
  • Considering that many times internal road are built without camber to allow the use a narrow truck crane, they propose the use of a series of cross-drain to divert the flow to the side ditches.
  • Several techniques are suggested as protection measures: silt traps, silt fencing, straw bales, settlement lagoons or even the use of flocculant.
  • They explain that cable trenches can act as a water drainage route, so in case of strong gradients clay plugs and impermeable barriers should be used to limit water flow.
  • They recommend not to install MV cables in soft peat, because they sink adding tension and potentially breaking.

 

Last but not least, there is a detailed explanation of floating tracks (road construction on peat using geotextile or geogrid). The suggestions are:

 

  • To build them only in flat areas, to avoid the risk of a slip or circular failure.
  • To leave vegetation and tree roots in place, and lay the geogrid directly on the existing terrain.
  • To avoid leaving open trenches or drainage ditches in the proximity, because the cyclic loads due to truck passage can lead to a soft materials migration and collapse of the road.

 

If you are interested you can download the entire document here: Good practice during windfarm construction

WTG earthing system

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A wind farm earthing system is designed for two different functions: to protect human beings and the components in case of power systems fault, and to protect them in case of a lightning hitting the structure.

Wind turbines have a considerably high possibility of being struck by a lightning during their lifetime. For this reason it is essential to give to the electrical current a low impedance path.

The main characteristics of an earthing system are:

  • Ensure that living beings in the vicinity of the earthing system are not exposed to dangerous potentials
  • Retain system voltages within reasonable limits in case of faults
  • Provide a sufficiently low impedance

The main factors to be considered designing a normal earthing system are soil resistivity, level of lightning protection required and power system earth fault level. In the case of wind turbines the earthing system is designed to be independent from the soil condition.

In modern wind farms all the WTGs are interconnected with a copper cable buried directly in the ground. It is called a multi-earthed system, and it reduces step voltage to a tolerable level in order to protect living beings from injuries.

Equipotentialisation of turbines and surrounding soil by means of a meshed earthing system reduces step voltage to a tolerable level in order to protect living beings from injury. This is obtained by connecting the entire interconnected foundation earthing (all the steel reinforcement bars in the foundation) to the earth interconnection wires between the wind turbines.

Basically the copper cable coming from the nearest turbine through the trench is tied to the reinforcement bars of the foundation. It goes all around the lower and upper reinforcement, and than is bond to the same earthing bar of all the electrical components of the WTG, such as the MV transformer and the switchgears.

Using this solution the soil conditions around the wind turbines and the resistance to remote earth of the earthing system are not a concern because all fault currents are handled by direct connections of known dimensions and lengths.

Tensar Triax geogrid use in wind farms

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Here you have a real world example of geogrid use.

We are in a wind farm in southern Spain, and thanks to previous experience with this technology the client decided to use a Tensar Triax geogrid TX160.

This is a triangular geogrid (the “old school” version was square). It seems that the triangular geometry guarantee a better distribution of the loads

The same geotextile  has been used in many wind farms (more than 100) around the world, with several big project in Germany and UK where in some cases more than half a million square meters have been used. Often the soil was peat (turf), with a very low CBR (even less than 1).

The saving in crushed stone using a geogrid can be around 30% to 40%, so with a price around 2-3 €/sqm it is normally a cost effective solution.

When we opt for this solution we’ve had helpful feedback and hints from Tensar, as they can study the existing info and provide a design based on the soil conditions and the available materials. There are several models available with a different triangular size, so it is not easy to choose the right model.

We haven't had any problem during the first half of the civil works circulating with machinery and trucks, but when we started moving the narrow track crane on the internal roads between the pads significant damages appeared:

Nooteboom special trailers PDF and AutoCAD blocks

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Royal Nooteboom Trailers is a Dutch company dedicated to special transport: flatbed trailers, low loaders and other amazing wind related vehicles.

They have developed several solutions for the transport of nacelles, tower sections and blades. Among their products, the MEGA wind mill transporter (for towers and nacelles) and the several families of blades trailers.

Nooteboom Teletrailers have an extendable load floor, single, double or triple extendible. They can go from 13,6 closed to 42 meters totally extended, or from16,6 closed to 48 meters totally extended,

Special tailored solutions can be developed for transport of blades over 50 meters.

They allow remote controlled, independent rear wheels steering, a really useful functionality to choose the best trajectory in complicated environments.

One of they last invention is the Tele-Step, specially designed for very long blades (50+ meters), with a peculiar turn table mechanism below the blade that permits movements otherwise impossible.

Another interesting product, the MEGA wind mill transporter, can load and unload the tower or nacelle without the help of a crane. It consists of 2 hydraulically adjustable lift-adapters that can be universally employed on various vehicle types. The lift-adapters are connected to the semi-trailers by means of a turntable, and the load can be rotated up to 80 degrees. The outstanding maneuverability is due to the fact that the swept path covered is determined by the size of the load only and not by the steering behavior of the vehicle. Pretty amazing.

It’s not easy to find technical drawings of their products online, so I’ve decided to share with the rest of the world several PDF / DWG from my private collection.

PDF

AutoCAD blocks

AutoTURN special transport simulation: pros and cons

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Working in Vestas at dozen of wind farms worldwide I’ve had the opportunity to use frequently AutoTURN to simulate bends and other complicated maneuvers (for instance in towns or near existing structures) with special transports (mainly wind blades and tower section trailers).

Although it is not cheap (the price was almost 1800 € plus VAT: a lot of money, considering that it’s an AutoCAD plug in) I am satisfied with the software.

The main advantage is that you can check very quickly if you’re going to have a problem somewhere, even if you don’t have a topography (sometimes we work with a blurry Google Earth picture). It is very easy to use and intuitive at the beginning, and it includes the rear wheels remote control used nowadays.

Main drawbacks: there are only a few special vehicles for wind components transport in the library, so you have to make your own. I’ve replicated several Nooteboom trailers, as we often works with them, using the “personalized vehicle” feature.

Moreover it is not possible to simulate reverse gear. This is a big, big problem as several times we use it in real word situations. Workaround: I’ve modified a normal trailer (not a wind blade tow), and I use it when I desperately need reverse gear.

Third problem, many times it would be useful to simulate problems in 3D, as sometimes the tow get stuck somewhere in a vertical transition. There is an upgraded version who allows you to works in 3 dimensions, but it doesn’t include the vehicles we use in the wind industry, so it’s not useful. The only solution is to build a 3D model of the terrain as I explain in another post and see what happen below the truck. Unfortunately it takes many hours.

Wind blades train transport

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Wind blades are normally carried by ship and truck.

By the way every now and than I see cases of train transportation: for instance Siemens moved 141 set of wind turbine blades (for the amazing total of 423 blades) to Portland General Electric's Biglow Canyon Wind Farm. Siemens is also transporting towers and nacelles via rail to various project locations throughout the U.S., so it seems that sometimes this is a reasonable option.

Vestas too is experimenting train transport: in the video embedded you can see how blades are transported from the factory in Germany to the wind farm in Denmark.

And the list is not ended: Enercon took a Marco Polo grant to move his blades by train.

Marco Polo is a UE found for modal-shift or traffic avoidance projects and projects providing supporting services which enable freight to switch from road to other modes efficiently and profitably.

The Grant (€1 268 577) for the ENERCON Tri-Modal project involves using rail and ship to move components and parts from Germany to Viana do Castelo in Portugal, as well as to installation sites throughout Europe.
Discover more about Tri Modal here

Maximum wind farm internal road gradient

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This is another standard problem I found in the wind farm I’m working with: mountainous areas, with difficult access and very strong inclination.

The standard maximum slope imposed by several manufacturers (for instance Repower and Gamesa) for safe transport on gravel roads is about 6% to 7%. Above 7% other technical solution may be necessary, depending on the trailer used to pull the T1 and the nacelle.

With an average quality track surface a 6x6 tractor unit can pull the nacelle approximately up to a 9%-10% slopes.

Than it is necessary to pull the truck with a bulldozer, using a steel bar or a steel cable (see pictures below). In this case the inclination was around 15%, and we used a D8R Caterpillar and a steel cable), without particular problems.

It must be noticed that in steep sections long horizontal alignment are preferable respect to closed bends.

What we normally do in extreme cases (above 15%) is to build a ramp using concrete slabs or an asphalted road: this solution is not only more expensive, but can also introduce additional problems, such as the need for environmental authorizations or from other authorities.

Here you have a 17% asphalted curve:

Concrete slabs are normally cast on site on a layer of aggregate stone, and they have dowel bars for load transfer and sealed contraction joints. An initial texturing is made with a burlap drag or a broom device, while final texturing is made with a spring steel tine.

You can find more information on the subject in the JPCP Design and Construction Guide

Jointed Plain Concrete Pavements Design Construction Guide

 

Peck's Cone (Cono de Peck)

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Every month I have the opportunity to read several geotechnical survey made for wind farms all around the world.

There are enormous differences between countries, not only in the formal presentation but also in the type of tests used, that often depends on tradition and historical reasons.

For instance reading a geotechnical study made in Peru I discovered the Peck’s Cone (“Cono de Peck”): it is a variation of the standard SPT, introduced in the ’70 in the country.

The most used correlation is:

 

Nspt = 0,5 c

 

Where:

Nspt = SPT value (number of blows for a 30 cm penetration)

c =  Number of blows for a 30 cm penetration using Peck’s cone

It is used instead of the SPT because it is quicker and cheaper: there is no need to extract the point and store the soil, as the perforating cone is closed. Actually it uses the same machine of the SPT, but is more similar to a DPSH test (but among the other differences, the cone of the DPSH has a 90º angle while the Peck’s Cone has a 60º angle).

On the downside, it has several problems: it is a “blind” test, because you don’t know the materials you are perforating, and there are many available proposed correlations (in addition to Nspt=0,5 c I have found Nspt=1 c and others).

I haven’t been able to find an international standard for this test, so I wouldn’t recommend it.
You can read more about the test (in Spanish) in this study.