Technical Information and Guidelines on the Assessment of the Potential Impact of Wind Turbines on Radiocommunication, Radar and Seismoacoustic Systems

Radio Advisory Board of Canada (RABC) and Canadian Wind Energy Association (CanWEA) April 2007

These guidelines serve as a risk management tool that helps wind project proponents and radar, radio and seismic system operators avoid any potential conflicts at an early stage in wind farm development. In essence, the Guidelines provide a series of analytical methodologies and thresholds that help to indicate where a potential interference may occur, thereby acting as a voluntary (but highly recommended) trigger for the proponent to notify the applicable authority. The Guidelines are not intended as a regulatory document, nor should they be used as the basis for any regulatory decision.

It is important to point out that the Guidelines themselves are not able to determine if unacceptable interference actually will occur. The determination of whether or not a proposed turbine or wind farm may create an unacceptable level of interference with existing radio, telecommunications, radar and seismoacoustic systems is very complex and it is not possible to categorically determine if unacceptable interference will occur unless a site-specific analysis is undertaken. The scope of that site-specific analysis and any potential mitigation measures undertaken (Step 4 of the above process), are not addressed in the present Guidelines.

This document has been written by a wide range of stakeholders, and it represents a general consensus in terms of analytical approach and acceptable thresholds for Canada. To the extent possible, it is consistent with documentation either existing or under development in other countries.

Sustainable Energy Australia Pty Ltd

This document is briefing paper discussing the electromagnetic compatibility (EMC) and electromagnetic fields (EMF) implications of wind farming in Australia.  This paper was prepared as background information for the preparation of a fact sheet for dissemination to the general public.

Prepared May 2004.

A. Calo, M. Calvo, L. de Haro y Ariet, Pedro Blanco-Gonzalez

International Renewable Energy Congress November 5-7, 2010 - Sousse, Tunisia

One of the environmental effects of wind farms is the electromagnetic interference due to the scattering produced by the wind turbines on the electromagnetic waves of different radio communication services propagating through them. A previous work is updated here and the scattering models for the nacelle and the wind turbine are shown and validated. Radio wave propagation losses are estimated more precisely through a parabolic equation approach. Finally, a comparison between theoretical and measured values for the Power Delay Profile (PDP) of the multipath channel through a wind farm is showed.

Dipak Sengupta

Prepared for presentation at The Working Committee on EMI, International Energy Association 21 August 1984.  Copenhagen, Denmark

Any static or time-varying multipath source in the vicinity of a receiver (or transmitter) of signals from a given electromagnetic system may act as a potential source of interference to the performance of that system.  When the installation of large wind turbines was proposed in the 1970s it was therefore speculated that the rotating blades of a wind turbine could produce interference to television reception in its neighbourhood.  Our studies carried out in 1976, and subsequently, confirmed the fact that such television interference does occur.  To some extent, all electromagnetic systems are affected, and by analysis, simulation and where practicable, experiments, the effects on a variety of systems have been quantified.

During 1976-1983 the University of Michigan Radiation Laboratory, under the sponsorship of US Department of Energy, studied the electromagnetic interference effects of a variety of wind turbines on the performance of a number of electromagnetic systems.  We have performed wide-ranging theoretical and experimental investigations of the EMI effects of horizontal and vertical axis wind turbines on the performance of:

• FM Broadcast
• Television reception
• Very High Frequency OmniRange (VOR) and LORAN-C
• Microwave Link
• Earth station communicating with satellite.

The present paper gives an overview of our studies, highlighting some of the most recent results.

David A Spera and Dipak L Sengupta

NASA Contractor Report 194468

During the late 1970's and early 1980's, concerns about the potential interference of wind turbine generators with electromagnetic communication signals led to a series of research studies, both in the laboratory and in the field, conducted by the staff of the University of Michigan Radiation Laboratory. These studies were sponsored by organizations such as the U.S. Department of Energy, the Solar Energy Research Institute and private developers of wind power stations. Research objectives were to identify the mechanisms by which wind turbines might adversely affect communication signals, estimate the severity of these effects for different types of signals (e.g. television, radio, microwave, and navigation), and formulate mathematical models with which to predict the sizes of potential interference zones around wind turbines and wind power plants. This work formed the basis for preliminary standards on assessing electromagnetic interference) by wind turbines.

With the current renewal of interest in wind energy projects, it is appropriate that the many experimental and analytical aspects of this pioneering work be reviewed and correlated. The purpose of this study is to combine test data and theory from previously published and unpublished research reports into a unified and consistent set of equations which are useful for estimating potential levels of television interference from wind turbines. To be comprehensive, these equations will include both horizontal-axis and vertical-axis wind turbines, blade configuration parameters (e.g. number, size, material, twist, and coning), signal frequency and power, and directional characteristics of the receiving antenna.

The approach that is followed in this report is as follows: First, some basic equations that describe electromagneticsignals with interference are presented without detailed derivations, since the latter are available in the references. Minor changes in terminology are made for purposes of consistency. Next, the concept of a signal scatter ratio is introduced, which defines the fraction of the signal impinging on a wind turbine that is scattered by its blades onto a nearby receiver. Equations from references are modified for the calculation of experimental scatter ratios (from measured signals containing interference) and idealized scatter ratios (from rotor characteristics and relative locations of the transmitter, the turbine, and the receiver). Experimental and idealized scatter ratios are then calculated and compared for 75 cases from the literature, in which TVI measurements were made around a variety of wind turbines. An empirical equation is then defined.for estimating the probability that an actual scatter ratio will differ from an idealized ratio by a given amount. Finally, a sample calculation of the size of a potential TV interference zone around a hypothetical wind power station is presented.

Carlos Salema, Carlos Fernandes, Luca Fauro

Abstract

Wind turbines may be a source of disturbance in the radiation fields of TV broadcast transmitters. The situation is particularly serious when the direct path from the transmitter to the receiver antenna is obstructed while both transmitter and receiver antennas have a unblocked path to the wind turbine. Starting from an analysis of the diffracted field by the pylon we proceed to implement a simple rule derived from ITU Recommendation 805 to define a minimum clearance distance from an isolated wind tur- bine and a TV transmitter antenna. Measurements using a scaled model confirm the existence of the floor level in the scatter model used in ITU Recommendation BT.805.

Comment

The paper deals exclusively with impairment of analogue signals.  ITU-Rec BT.805 is discussed in Report ITU-R BT.2142-1 (10/2010), where a draft replacement recommendation is described.

NB The publishing source of this paper has not been identified.

Report ITU-R BT.2142-1 (10/2010)

This Report is on the topic of performance of television reception in the presence of reflected signals, specifically those from wind turbines as identified in the draft new Recommendation ITU-R BT.[WINTURB] – Assessment of impairment caused to digital television reception by a wind turbine (Document 6/297(Rev.1)).  It results from studies in Australia and Spain.

Background

Wind turbine farms are proving to be a popular energy source. Due to this growth many administrations are now experiencing interest from developers in constructing wind farms. This has raised concerns about the potential impact of wind farms on the reception of broadcasting services.
In considering the planning for digital television services within the VHF and UHF broadcasting bands traditionally used for analogue television services, some administrations have sought to reference Recommendation ITU-R BT.805 – Assessment of impairment caused to television reception by a wind turbine. Recommendation ITU-R BT.805 – was approved in 1992 in response to Question ITU-R 6/11. Draft new Recommendation ITU-R BT.[WINTURB] – Assessment of impairment caused to digital television reception by a wind turbine – has been developed to address the emergence of digital television and it retains the reference to the Question ITU-R 6/11. However, a current Question ITU-R 69/6 – Conditions for a satisfactory television service in the presence of reflected signals – deals with reflections affecting analogue television systems.

In 2004 Australia proposed a draft modification to Question ITU-R 69/6 that extended studies to include digital television. The purpose was to encourage eventual modifications to Recommendation ITU-R BT.805 as a result of further study as to whether impairment is caused by wind turbines to digital television, as well as the development of further Recommendations, should they be required, relating to reflections from other objects.

In 2006 Australia proposed that a working document towards a modification of Recommendation ITU-R BT.805 be developed based on studies conducted in Australia. The study based on theoretical modelling identified that Recommendation ITU-R BT.805 is not adequate for predicting interference from wind farms for analogue and digital TV signals.

In 2007 a further study indicated that the methods to assist in quality assessment of the coverage and service area for digital television broadcasting in System B in Recommendation ITU-R BT.1735 are not satisfactory for the type of dynamic signal variations from rotating wind turbine blades.
From studies undertaken by Australia to date, Recommendation ITU-R BT.805 does not currently provide adequate advice for predicting interference from wind farms for analogue and digital TV signals. Subsequently Working Party 6A proposed a Preliminary draft new Recommendation ITU-R BT.[WINTURB] – Assessment of impairment caused to digital television reception by a wind turbine – be developed.

Australia also observed that further study is required to review the relationship between the mean MER, slow MER variations, short deep MER notches and the receiver performance.

The annexes to this Report contain the results of studies to date:

Annex 1 – Scattering model calculations.

Annex 2 – The effect of the scattering of digital television signals from wind turbines.

Annex 3 – Summary of measurements and analysis of the results.

D F Bacon

Wind Engineering, Vol. 16, No 6 1992. 8.

A proposed method for establishing an exclusion zone around a terrestrial fixed radio link outside of which a wind turbine will cause negligible degradation of the radio link performance.

Obstruction or reflection of radio waves by a wind turbine can degrade the performance of a fixed radio link due to the effect of large blades rotating at approximately 32 rpm. Typically there are 2 or 3 blades. Thus any significant interfering signal, such as a delayed multipath component, will fluctuate in signal level around 1.0 to 1.5 Hz. This is particularly problematic to a digital link where the performance is assessed on a second-by-second basis.
Thus a special criterion for the proximity of wind turbines to radio links is considered necessary. This document proposed a practical method for establishing an exclusion zone around the path of a fixed radio link within which it would be inadvisable to install a wind turbine.

Emmanuel Van Lil, Dave Trappeniers, Antoine Van de Capelle

Abstract

Previous studies have been focusing on the influence of moving objects like wind turbines on aeronautical and maritime radars, working usually respectively in the L/S band and in the X-band. Here, those results will be summarized for a special kind of radars, usually working in the C band, namely meteorological radars. We have used both UTD methods as well as simplified methods to quantize the effects. Indeed, not many moment method solvers have the possibility to deal with moving objects (creating Doppler shifts), but with UTD it is within easy reach of the modern computers. However, for some particular applications, the procedures can be simplified to quantize efficiently the near-fields around moving objects. After a description of the general phenomena, two simplified methods will be compared. The first consists in a Physical Optics based method, and the second does make use of the flat wedge approximation.

Gavin J Poupart

UK DTI Report Number: W/14/00614/00/REP

This study has focused on the development and validation of a computer model that can be used to predict the radar reflection characteristics (Radar Cross Section, which is measured in square metres and is normally presented on a logarithmic scale) of wind turbines and understand the complex interaction between radar energy and turbines. The scope of the model includes:

• The affects of the radar propagation over the terrain between the radar and the wind farm;
• The dynamic radar scattering from the wind turbines;
• The signal processing in the radar;
• Display of results via a simulated radar display.

The model was validated through a full–scale trial, using a QinetiQ mobile radar system to collect data for a single operational wind turbine at Swaffham in Norfolk. The model was then used to perform a detailed sensitivity analysis and to compile a list of the key factors influencing the radar signature of wind turbines.

The following are some of the results generated by the project:

• The design of the tower and nacelle should have the smallest Radar Cross Section (RCS) as possible. The RCS of these components can be effectively reduced though careful shaping and choice of construction materials;
• Large turbines do not necessarily lead to large RCS (i.e. tower height does not greatly affect RCS);
• Blade RCS returns can only be effectively controlled though the use of absorbing materials;
• Spacing of wind turbines within a wind farm needs to be considered in the context of the radar cross range/down range resolutions.
• Spacing the turbines such that only one turbine can appear in any range cell has advantages in identifying the wind farm, filtering out the turbines and in tracking aircraft over the farm area;
• Single wind turbines do not create a significant ‘radar shadow’. Any shadow region is only dark to a distance of a few hundred metres behind the turbine. Beyond this there is some reduction of the radar power, and a time-variation, but these will not prevent detection except possibly for very small targets.

Rashid, Laith; Brown, Anthony

Antennas and Propagation (EuCAP), 2010 Proceedings of the Fourth European Conference on , vol., no., pp.1-5, 12-16 April 2010

Abstract

The application of radar absorbing materials (RAM) in order to reduce the interference of wind farms with radar systems is considered as a possible mitigation solution. This paper will address the key challenges when trying to efficiently apply RAM to certain parts of the wind turbine blades to significantly reduce the scattering of radar signals. Modeling of the radar cross-section (RCS) is presented for a generic 40 meter blade wind turbine before and after applying RAM to different parts of the blade. The results of modeling possible solutions of partially RAM treated blades are compared.