Wind Turbines & Weather Radar: A Review of the Problem

Recent Advances in Finite Differences and Applied & Computational Mathematics

Wind Turbines & Weather Radar: A Review of the Problem T.G. KOSTIS1,2, A. K. GOUDOSIS2, I. DAGKINIS3, Ch.K.VOLOS1, N.V. NIKITAKOS3 1

2

Faculty of Mathematics and Engineering Studies Department of Military Science Hellenic Army Academy Vari, GR-16673 GREECE

Hellenic Quality Assurance Agency for Higher Education Leof. Syggrou 44, Athens 11742, GREECE 3

School of Business Administration Department of Shipping, Trade and Transport University of the Aegean 2a Korai Street, 82100 Chios GREECE. {tkostis@iee.org, a.goudosis@hqa.gr , idag@aegean.gr, chvolos@gmail.com, nnik@aegean.gr} Abstract: - Doppler Radars and Wind Turbines have an asymbiotic relationship. Both wish to exist in the same environment of space and time but when doing so wind turbines pose great problems to Doppler radars. This is important because Doppler radars are found in weather stations and air traffic control systems. In this paper we will describe the operation of a Doppler weather radar and particularly explain its visibility potentials. Then we will explain why this visibility potential can be interfered by the modulations produced by the wind turbines and pose such great problems to the Doppler radar’s operation. During this process we will make a state of the art review by referring to characteristic current bibliography and references about this issue. We finally pitch our theoretical opinion about the solution of the wind turbine interference problem.

Key-Words: - Pulse Doppler Weather Radar, Wind Turbine, Interference, Renewable Energy Issues.

1 Introduction Doppler Radars and Wind Turbines have an asymbiotic relationship. Both wish to exist in the same environment of space and time but when doing so wind turbines pose great problems to Doppler radars. And this is important because Doppler radars are found in weather stations and air traffic control systems. Analytically wind turbines induce modulations in the Doppler radar that constitute false targets. Or these modulations can even make aircraft appear and reappear on the radar screen thus confusing the radar operator about the validity of a target.

This paper will be of use to an audience that wishes to understand the technical foundations about the antagonistic relationship between Doppler

ISBN: 978-1-61804-184-5

radars and wind turbines. It is divided in six parts with this being the introduction. The next part reveals the intricacies of Pulse Doppler radars.

There we will describe the operation of a Doppler weather radar and particularly explain its visibility potentials. Then in part three we will give an introduction to the wind turbine as a renewable energy form. In the following part four we will explain why this visibility potential of the pulse Doppler radar can be interfered by the modulations produced by the wind turbines and pose such great problems to the Doppler radar’s operation. In part five we will summarize the state-of-the-art solutions to this problem. And the last part provides the conclusions of this effort.

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accuracy of the Doppler resolution. But these two characteristics are competitive with each other and we have the radar uncertainty principle. The radar uncertainty principle may sound familiar with the uncertainty principle of quantum mechanics but it is not. In the quantum mechanics field the uncertainty principle is a mathematical inequality that restricts to a limit the precision with which the position x and momentum p of a particle can be known simultaneously and with certainty. In radar engineering field this principle has another notion that low Pulse Repetition Frequency (PRF) eliminates range ambiguities but has velocity ambiguities and high PRF avoids velocity ambiguities but presents ambiguities in range.

Therefore the logical upcoming question would be whether medium PRF multiple waveforms in the form of a schedule could provide a golden rule, in other words procure acceptable ambiguities for useful operational use.

Actually medium PRF radars are the cornerstone of pulse Doppler radars. They are found in all related civilian (air traffic control) and military (surveillance and tracking) applications.

2 Pulse Doppler Radars Pulse-Doppler is an advanced signal processing 4D radar system capable of detecting a target’s spatial coordinates (longitude, latitude, altitude) plus its radial velocity (range-rate) while being able to track a target through the fifth dimension which is time.

Its development was spawned by the need to track and fire upon low flying Mikoyan MiG aircraft during the Vietnam War. For at this low altitude flight over the jungle detection was impossible and a firing solution could not be locked because the jungle clutter was overwhelming the MiG’s backscattering. Actually, MiG-17 and MiG-21 aircraft camouflaged in jungle colours used to ambush American aircraft from below as they passed by them. Therefore a new type of radar that could look down in the jungle and discern a target by virtue of its speed was required in order to obtain the corresponding firing solution. Consequently, pulse Doppler radar was devised which expectantly first flew on-board a McDonnell Douglas Phantom F4-B during the 60’s during the era of the Vietnam War. First let us see the need for pulse Doppler radar in technical terms. Simple continuous wave radar can measure speed but not range. Adding a frequency modulation can provide range measurements but still antenna isolation problems persist. Moreover it has serious issues with target tracking functions, power consumption and more importantly weight reduction for compact airborne systems especially for military applications. On the other hand a radar that uses a simple pulsed configuration can give range information from measuring the round-trip time but cannot estimate the speed of the associated target. Doppler information is inherent in the pulse’s frequency spectrum but it is not processed with this simple pulsed radar system. Since the use of short pulses instead of a continuous wave avoids the risk of overloading antennas as well as reduces a radar’s power consumption and weight, then a pulsed radar that could at the same time extract Doppler information about a target is the desirable solution for many civilian and military applications.

Poignant examples are airborne weather radar systems that help pilots avoid stormy weather for civilian use and advanced radar systems for the military that can provide not only detection but classification and in some cases identification services.

Now remember that the pulse’s duration determines the accuracy of the range resolution and the reciprocal which is its frequency determines the

ISBN: 978-1-61804-184-5

3 Renewable Energy Systems The search for economical, renewable and sustainable solutions for electricity production, due to climate changes from fossil fuel emissions, has been focusing on solar, wind, biomass energy and is extracting energy from ocean waves and related water movements. Hence, if economical and technical solutions exist, the area of renewable energy exploitation would have a vast impact on the electricity production in the world.

Wind energy, as a renewable energy source, is the kinetic energy of air in motion. Winds are produced by the movement of air that is caused as the sun warms the Earth's surface. This movement happens because some places of the Earth accept direct sun rays, while others get indirect sun rays, so different temperatures are created in the atmosphere.

These differences arise the warm air, which weighs less. Then cold air moves and fills in the space that warm air used to occupy. This movement of air is what makes the wind blow. The terms "wind energy" or "wind power" describe the process by which the wind is used to generate mechanical power or electricity.

How wind turbines exploit the wind energy? The Wind turbines convert the kinetic energy of the wind into mechanical power. This mechanical power can be used for specific tasks (such as

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easily classified and rejected. Air traffic control and military personnel monitoring Doppler radar screens at installations important for civilian transport and homeland security may mistake a wind turbine for an aircraft or report nonexistent weather conditions.

Moreover, this interference (clutter) may also create blind spots to the Doppler radar so that real or a small aircraft will not be seen, or disappear and reappear with all the grave consequences of this case.

grinding grain or pumping water) or a generator can convert this mechanical power into electricity used at homes, or integrate it to the electrical grid. Wind turbines, working like aircraft propeller blades, are rotate from the moving air and power an electric generator that supplies an electric current.

Modern wind turbines are divided into two basic groups: the horizontal-axis variety, and the verticalaxis design, like the eggbeater-style Darrieus model, named after its French inventor. Most large modern wind turbines are horizontal-axis turbines and their components at a glance include:   

Aging radar systems unable to tell the difference. [http://www.scientificamerican.com/article.cfm?id= wind-farm-radar-clutter]

blade or rotor, which converts the energy of the wind to rotational shaft energy, a drive train, usually including a gearbox and a generator, a tower that supports the rotor and drive train,

Furthermore, wind farm installations relatively near and at line of sight to radar systems cause clutter returns which disproportionally affect the normal operation of these radars. For example a wind turbine farm that occupies a total of 2.5% of the terrain visible to a Doppler radar creates around 10 to 20 percent of the clutter induced to the radar system.

Moving turbine blades can be indistinguishable from airplanes on many radar systems, and they can even cause blackout zones in which planes disappear from radar entirely. For example a wind turbine which can reach as high as 400 feet looks very similar to storm activity on weather radar [9].

Therefore wind turbines can add Blind Zones to a pulse Doppler radar that produces missed aircraft detections.

Also it can act as an electromagnetic honeypot and seduce the tracking function of a radar away from the original track which was a valid aircraft.

Moreover the constant modulation will eventually saturate the radar receiver and result in a high false alarm rate [7].

In a final note the interference problem becomes bigger when adverse weather conditions, like rain, are convoluted with the wind turbine clutter producing wider and more unpredictable clutter maps.

and other equipment, including controls, electrical cables, ground support equipment, and interconnection equipment.

Today to achieve the economies of scale, the wind turbines group together into a single wind power plant, known as a wind farm, and generate bulk electrical power. These wind turbine farms are installed on mountain peaks or they are founded offshore as on floating or bottom supported platforms where potential wind energy is exploited.

Electricity from these wind turbines is fed into a utility grid and is distributed to customers, just as with conventional power plants.

Wind turbines are available in a variety of sizes, and therefore have power ratings. The largest machine has blades that span more than the length of a football field, stands on 20 stores high building, and produces enough electricity to power 1,400 homes. A small home-sized wind machine has rotors between 3 and 7 meters in diameter and stands at a height of 9 meters and can supply the power to all home electrics or a small business need.

4 Problem Formulation

5 Problem Solution

Therefore many wind turbines are grouped together in power farms, are placed in high elevations and have considerably big blade dimensions in order to produce a low cost kilowatt per hour utility.

Unfortunately this configuration produces increased radar clutter to Doppler radar systems. Both the moving blades and the towers they sit on create false readings with a random nature that cannot be

ISBN: 978-1-61804-184-5

First of all older radar transmitters and receivers cannot mitigate the wind turbine problem. Therefore only modern solid-state components with specially designed waveforms and complementary reception processing abilities have the potential to further reduce false targets injected by renewable source energy moving part generators [11].

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Concurrent Beam Processing, Improved Constant False Alarm Rate (CFAR), Clutter Map per Doppler Filter Optimization and Enhanced Tracking techniques.

Prospective solutions will tend to extend the capability to deal with the clutter problem, for example [7]: Polarimetrics (Dual Polarization), by looking at the same target with multiple polarizations can deduce a conclusion about its validity.

Pulse-burst (High PRF), high prf schedules induce more energy on the target and therefore can get stronger responses about the velocity of the contact.

Blind Spot / Gap-Filler maybe with advanced medium PRF schedules that adaptively eliminate pulse Doppler radar blind areas of range and/or velocity.

Now the ideal solution would transform the air turbine into a stealth target that would produce as little interference as possible while ideally some time it would produce no interference at all. In other words the effort is to reduce the radar cross section of the wind turbine [12]. And blade modulations with low radar cross section (RCS) would be masked by the velocity filter on the radar size. In technical terms this would mean that the radar returns of the air turbine would assimilate the overall noise conditions of the surrounding area.

Therefore a simple adaptive noise routine at nearby radars would compensate for this problem in a great degree. Unfortunately air turbines are big targets and their shape cannot radically change to produce less interference. Thus this solution is theoretically infeasible. On the other hand a practically feasible solution would be to coat the rotating blades with anti reflective material just like it is conducted in stealth planes [13].

In the open literature there are many proposals towards the solution to this highly competitive relationship between Wind Turbine Clutter (WTC) and pulse Doppler Radars.

Most suggest that addition of extra radar processors, parallel usage of many frequencies that are set on the same area, adaptive radar receiver calibrations and more modern, adaptive clutter map that can edit out the false targets so that they are not passed downstream to the radar display systems is the most promising technological solution [1-6, 8, 10]. Another solution would be to place adaptive beamforming cancellators close and opposite to the air turbines to try to cancel the returns at the source.

Usually this method is implemented by adaptive beamforming and Space-Time Adaptive Processing (STAP) technologies. At the moment of this publication we have not found any references that support or implement this approach, therefore we will claim this as our own theoretical approach to this problem.

Generally modern solutions which are already implemented suffer from one major drawback, which is loss operational range of detection [7].

These are Sensitivity Time Control (STC), Range Azimuth Gating (RAG), Track Initiation Inhibit, Velocity Editing and Plot Amplitude Thresholding (PAT) Therefore advanced signal processing can mitigate the problem but reduces the detection range. Thus these solutions are dubious for military purposes. Currently proposed military acceptable efforts which are considered to be under development include [7]:

ISBN: 978-1-61804-184-5

6 Conclusion The benefits of wind energy are valuable. It is a renewable resource that can provide energy both to the main electricity grid and to remote areas. On the other hand there is at least one hazard that although less obvious is of great importance; the injection of interference or clutter into Doppler radar systems.

Considering that Doppler radars are used in civilian and military applications this is a problem that needs to be mitigated. There are numerous efforts that try to solve this problem. The main line of approach is the use of technology and the research of advanced pulse Doppler radars that will try to eliminate this interference using an adaptive clutter map methodology. In conclusion an air turbine is a radar target that we need it to be as stealthy as possible. In conclusion there are legal, operational, environmental and political issues that arise when wind turbines and operational pulse Doppler radars collide.

References: [1] Massachusetts Institute of Technology Lincoln Laboratory, Wind Turbine Impact Mitigation for QVN ARSR-3 Radar, Executive Summary, June 21, 2010, http://www.acq.osd.mil/ie/download/green_ene rgy/exec_sum_windturbinestudy.pdf.

[2] B.M. Isom et al, Detailed Observations of Wind Turbine Clutter with Scanning Weather Radar, J. of Atmospheric & Oceanic, http://journals.ametsoc.org/doi/pdf/10.1175/20 08JTECHA1136.1

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[3] Air Traffic Control Wind Farm Interference Mitigation, Raytheon Company, http://www.raytheon.com/newsroom/technolog y_today/2012_i2/airtraffic.htm [4] M. Brenner, Wind Farms and Radar, The MITRE Corporation, 2008.

[5] Radar Cross Section Reduction of Wind Turbines, http://prod.sandia.gov/techlib/accesscontrol.cgi/2012/120480.pdf [6] Losco, Felix A.; Collick, Thomas F, When Wind Turbines and Radar Mix: A Case Study: Air Force Law Review, December 22, 2012 |.

[7] P. Drake, Overview of Raytheon Wind Farm Mitigation Techniques and Test Results, , In. CNS/ATM Conference, 15 June 2011, http://www.afceaboston.com/documents/events /cnsatm2011/Briefs/03-Wednesday/Wednesday -PM%20Track-3/01-Drake-Wind%20Farm%20 Presentation-Wednesday%20Track3.pdf.

[8] M. Kaveh et al, Wind Turbine Radar Interference, http://environment.umn.edu/e3/archive/archive _2010/E3_Kaveh.pdf [9] [http://www.nytimes.com/2010/08/27/business/ energy-environment/27radar.html?_r=0].

[10] [http://www.scientificamerican.com/article.cfm ?id=wind-farm-radar-clutter].

[11] [http://www.acq.osd.mil/ie/download/green_en ergy/exec_sum_windturbinestudy.pdf] [12] [http://prod.sandia.gov/techlib/accesscontrol.cgi/2012/120480.pdf].

[13] [http://www.raytheon.com/newsroom/technolo gy_today/2012_i2/airtraffic.html].

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