August 2012

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Almost in every wind farm a step-up substation is built to collect all the energy generated by the turbines and received through the MV cables. The exceptions are new wind farms or existing wind farms extensions built near a substation that can be upgraded to absorb the additional energy produced: in these cases, only a control center with the SCADA and the medium voltage system is realized.

Although there are different possible technical solutions, normally a substation will be composed by the following elements:

  • Medium voltage system
  • High voltage system
  • Capacitors banks
  • Auxiliary services
  • Control, protection and metering system
  • Communication system
  • Fire protection and intruders protection systems

Medium voltage system is composed by the general busbar, disconnectors, circuit breakers and current transformer.

High voltage system is made of one or more transformers, together with earthing reactance, surge arresters, current transformer, voltage transformer, circuit breaker and disconnector with earthing switch.

Capacitors banks are installed to comply with the grid requirements regarding active and reactive power.

The auxiliary services supply energy both in AC/DC current, and count with a group of battery that can generate energy for several hours to operate the substation in case of emergency, a rectifier and often a backup diesel generator, with a tank big enough to provide energy for 3 days..

Control, protection and metering system allows the correct operation of the wind farm according to local regulations and grid requirements. Basically they are protection relays for the switchgear and power transformers

The communication system, must guarantee the correct communication with the adjacent substations and with the grid owner control center, in order to make possible the correct operation of the wind farm substation. Normally communications are through optical fiber/carried wire.

Fire protection system is normally composed by optical or infrared detectors, fire extinguisher, external bells or siren, while te intruders protection system are normally a fence plus a closed circuit TV.

Also the SCADA server is normally located inside the substation, together with a parabolic antenna to grant broad band connection.

There are several available solutions to connect with the existing high voltage distribution network: normally a dedicated line is used, while in other situations a tap off (or “T” connection) is used. A third solution is to open   the existing line between 2 substations.

The 3 solution are shown on the following diagrams:

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A power curve is a relationship between free wind speed at the WT location and the associated expected power being produced. Power curve warranties are often included in contracts, to assure the wind turbine performs according to specification.

Here you have an example of how it looks like:

It is measured following international standards to demonstrate compliance with warranted values.

Following a standard not only reduces variance due to data analysis, but it also provides solutions to different problems, for example:

  • How to judge if an anemometer is good enough.
  • How to deal with complex terrain and obstacles.
  • How to correct for low air pressure.
  • How to measure the production and with which devices.

Basically, it provides an accepted way of estimating the result’s uncertainty. Any deviations from the standard (in procedure or interpretation) will cause severe deviations and uncertainties that will make conclusions very hard to take.

The standards in use to measure the power curves are made by the International Electromechanical Commission (IEC):

IEC 61400-12, in use since 1998, has been replaced by IEC 61400-12-1 (December 2005), which includes MEASNET requirements and has additional requirements for anemometer.

The following version is IEC 61400-12-2, addressing nacelle anemometry.

IEC 61400-12-1 without site calibration is the method used on flat terrain.

Basically, a meteorological tower (met mast) is positioned near a turbine to measure the wind, and a uniform flow at mast and turbine is assumed. The normal uncertainty is 5-8 %, depending on annual wind.

The best position for the mast is in the middle of sector, and it has to be between 2 and 4 diameters in front of the turbine (for instance for a V90 it would be between 180 and 360 meters). It is often erected at a 2.5 diameters distance.

The met mast can be erected before or after turbines are on site.

 

IEC 61400-12-1 with site calibration is the method used on complex terrain.

In this case, a temporary met mast is placed in the future position of the selected turbine, and at the same time the wind farm definitive met mast is erected.

The 2 masts works together for a certain amount of time, until a relationship between the wind measured by the definitive met mast and the temporary met mast can be found.

After, the temporary met mast is disassembled, the WTG is erected in his position and it can be defined if the energy produced is in line with the expected generation with the wind speed (extrapolated using the data of the wind farm met mast and the previously defined correlation).

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Other AutoCAD blocks of cranes that you may find useful.

Manitowoc cranes are not very common in europe but they are widely used in the states.

Manitowoc 16000 Crane AutoCAD Block

Manitowoc 18000 Crane AutoCAD Block

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Horizontal directional drilling is a technique frequently used in wind farms when we have the need to cross the pipes below roads, rivers or even the sea.

Here you have the picture of 2 slightly different solutions, one using directional drilling and the other with horizontal drilling.

The installation of a pipe using directional drilling (also known as HDD, slant drilling, or deviated drilling) consists of three stages:

 

1.            A first drilling, with which you will realize a “pilot hole” with a smaller diameter.

2.            Hole enlargment, using one or more reamer with a diameter bigger of the cable to be installed (normally 1.7x), achieving the width required for pipes installation. This is done coming back from the exit hole.

3.            Pipe laying service, pulling them out from the exit.

 

To guide the drilling head, an electromagnetic wave transmitter is used, so that its position is known exactly in every moment. Several data are transmitted, such as position, inclination, temperature, etc. The signal can be read up to a depth of 15 meters or more.

The fluid used in the perforation may vary. Normally bentonite is used, but due to environmental issues (for instance if it's necessary to cross a water body) low pressure compressed air is often used as alternative.

There is also an "office" work developed before, where a stratigraphic map as detailed as possible is prepared using the info collected with several boreholes to define what material you should cross in every position.

Of course this technique is more expensive than traditional trenching. Depending on the country and length of the perforation it can be between 5 and 10 times more expensive.

In the “standard” direct horizontal drilling the solution is easier: we’ve made 2 holes, one on both side of an existing local asphalted road, and then we’ve realized a simple horizontal perforation. Here the distance is around 12 meters, while horizontal directional drilling goes often to bigger length (hundreds of meters, up to our record that is almost 1 Km).

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Here you have my collection of Terex Demag cranes:

Terex Demag CC2800 narrow track crane AutoCAD block

Terex Demag CC 6800 AutoCAD DWG block

More Liebherr cranes here.

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Grout is an extra high properties material that is normally used in wind farm construction just below the tower flange, above the foundation.

It is a transition material, designed to have a very high fatigue resistance. All the dynamic loads need to be transferred and absorbed by the grout connecting the tower to the foundation structure.

Grout is volume stable and his resistance grow very quickly (it is faster than normal concrete in achieving a relevant percentage of his final value, and it has a very low porosity and water absorption.

Thanks to the rapid strength build-up, it allows earlier pre-stressing of anchors at all temperature ranges (it is normal to work between 2 and 30 degrees): a short hardening time permit a short overall installation times and earlier operation of the wind farm.

In the picture you can see how it is prepared. We are working by night, because the temperature was real high during the day (more than 38 degree). In cold wind farm we need to use thermal blanket to keep the grout warm.

First of all pipe are lubricated with standard Portland cement slurry. Then, grouting is prepared in the mixing machines. We are using 2 machines in parallel because mixing time (minimum 7 minutes) is about 2 times the time needed to empty the mixer. Water consumption was 2.25 liters for every 25 Kg grout bag.

After the grout is tested: a “pizza” is made to see if the viscosity is right (not too dense, not too liquid). The technical name of the test is ASTM C230 ring.

When it’s considered ready, the small channel between the tower and the foundation is quickly cleaned and the grout is pumped from the highest point of the circumference (it should be leveled, but there are always small difference in elevation.

It can’t be vibrated, so a steel rebar is used to shake it a little. We needed around 2 hours to fill the channel. At the end of the work, pipes are cleaned pushing a sponge inside them to remove the rests of grout.

The grout slowly fill the empty space below the tower flange:

Meanwhile several samples are taken to test it: only 24 hours after we had the astonishing result of 62 N/mm2. Here you have the expected hardening curves:

Testing is normally made with 12 75 mm cubes, although cylinders are considered acceptable.

There are numerous producer of this material. In this wind farm we used chemical giant BAST “Masterflow 9200” grout designed specifically for Vestas. We employed around 96 x 25 Kg bags (2400 Kg).

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