Posts Tagged ‘Theory’

Sodar: Operation and Limitations (1/2)

February 18, 2012

On last post I explained briefly some features related to this Remote Sensing (RS) apparatus, but I consider that it should be explained further. Therefore, I am going to detail other considerations related to Sodar’s technology through two posts based on the academic paper of S. Bradley, I. Antoniou et al. (2005). By now, physical principles and uncertainties will be described. Next post will focus on its calibration methods.

Sodar is a Remote Sensing apparatus that measures 3D wind speeds at high altitudes. By emitting vertical sound beams of sound, it is possible to interpreter the backscattering frequencies due to the Doppler Effect and thus, wind components can be decoded (See Fig. 1). Usually, three or five beams are necessary to obtain reliable raw data measurements. Each of them is usually tilted 15-20º (ϕ) to the vertical (See Fig. 2). Though the emitted signal produces a continuous backscattering echo after crossing the infinite turbulent layers in the atmosphere, the echo generated at the studied altitude (Z) can be recognized according to the following formula. This means that among the continuous echo signal received, the specific signal generated at the Scattering Volume at Z height is generated at t (time) moment. The same principle is used by pulsed lidars.

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Power Quality Standards.

July 13, 2010

Voltage provided by Wind turbines must comply some requirements specified in the IEC 61400-21 standard. Here we are some power quality parameters that power energy from wind turbines must be under control:

Rated Data

  • Rated Power (Pn) (or Active Power): Maximum continuous electric output power for a wind turbine under operating conditions.
  • Rated Reactive Power (Q­­­­­n): Reactive power under rated power, nominal voltage and frequency.
  • Rate Apparent Power (Sn): Apparent power from the wind turbine while operating at rated power and nominal voltage and frequency.
  • Rated Current (In): Current from the wind turbine while operating at rated power and nominal voltage a frequency.

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Brake Systems in Wind Turbines

July 12, 2010


According to quality parameters, DS472 (Danish Standard) and GL rules, wind turbines must have two independent braking systems. However, the IEC 61400-1 requirement does not specify what kind of two braking systems must have, but requires the protection system to remain effective even after the failure of any non-safe-life protection system component.

Usually, it makes sense to supply both aerodynamic and mechanical braking. Aerodynamic brake is more benign than mechanical braking, so it is always used in normal shut-downs. On the other side, although a braking system must be used during shut-downs, sometimes technicians allow rotor free turns (idle) within low winds. This strategy reduces the frequency of imposition of braking loads on the gear train.

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Power Electronic Concepts in Wind Turbines.

July 10, 2010

In recently years, we have lived big discoveries on the Power Electronic field that unveil new opportunities to improve the performance in the electric circuits and therefore, in Wind Power industry. Power Electronic can be defined as the brand of the electrical engineering which studies the control and conversion of electric power. Therefore, the power conversion systems can be classified according to the type of the imput and output power:

Here we are the basic components in the Wind Power industry.

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Generators in Wind Turbines.

July 9, 2010

Basically, any type of three-phase generator could work in a wind turbine but they must fulfill some requirements. We have just classified them according to their generator type and the coming prototypes that nowadays we are studying. This post, as the last one, is a summary from the book Wind Power in Power Systems (Chapter 4) by Thomas Ackermann. However, for any further information, this link can be interesting.

Asynchronous (Induction) Generator.

It is the most numerous in wind turbines. Advantages: Robustness, mechanical simplicity, low price. Disadvantages: It has to receive an exciting current from another source and consumes reactive power (so, reactive power is supplied by the grid or by a power electronic system).

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Active and Reactive Power.

July 6, 2010

At this moment, the book I most admire on the Wind Power field, it is this one written by Ackermann. He can transmit dept concepts of power systems by using a clear vocabulary and illustrative examples. In this book, for example, there is an appendix in the chapter three, that explains in a very easy way how the Power System works by using a long bike as an equivalent.

Imagine a long bike with much cyclists running on a flat and straight road. We are the manager of this team and we want to keep the bike at a constant speed (the same frequency) and in an upright position (the nominal voltage equals to 1). How can we achieve this?

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Types of Wind Turbines.

July 3, 2010

Wind Turbines can be classified in two big groups: HAWT (Horizontal Axis Wind Turbines) which its rotor is connected to a horizontal shaft, and VAWT (Vertical Axis Wind Turbines) which have a vertical shaft.

HAWT

Since 1980’s, most of the commercial wind turbines are included in this group. The HAWT consist of a tower and a nacelle that is mounted on the top of the tower. The nacelle contains the generator, gearbox and the rotor.

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Book: “The Crash of 2008” by George Soros.

July 1, 2010

George Soros, “the man who broke the Bank of England”, gives us his personal perspective about the Financial Crisis. I have been reading this book for several months and I couldn’t finish it until now. So, let’s write something about his live and what we can learn from his experiences.

George Soros was born in Hungary in 1930. After the Nazy occupation, he emigrated to study Economics in London by his father’s recommendation. When he accomplished his studies, he moved to New York City where he started working as an arbitrage trader. Then, he had a considerable success as a financial investor by putting in action his famous “Reflexivity Theory”.  At this moment, he holds the 28th position of the World richest people in 2009 according to the Forbes magazine.

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The Wind.

June 29, 2010

Earth is surrounded by a fluid that we call air. When air is heated creates High pressures in the atmosphere, and wind flows from High (H) to Low (L) pressure points. But, why does our planet have different temperatures? And even the most important, how can we take advantage of this natural power? In order to explain these questions, on this post we will see how wind acts and the latest researches on the aerodynamic field to convert the wind power in a useful and ecological energy.

There are some reasons to explain why the atmosphere has temperature gradients. Since Christopher Columbus discovered that Earth is a sphere (or almost) five hundred years ago, now we can understand easily that Sun’s radiations impinge on the surface with different power depending on the latitudes. Also, we have the rotation. When the Earth’s face is in front of the Sun, it means that the another face is in the dark, and as we know, days are usually warmer than nights. Finally, our planet’s axis is inclined and when it is summer in the North means it is winter in the South. Furthermore, our planet has other peculiarities (clouds, oceans which keep better the heat than land, white glaciers, so on) that provoke High and Low pressures in the atmosphere.

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