Wind Turbine Clutter

Abstract

The use of wind farms to generate electricity is growing due to the importance of being a renewable energy source. These installations can have over a hundred turbines of up to 120 m height each. Wind farm installations relatively near to radar systems cause clutter returns that can affect the normal operation of these radars. Wind turbines provoke clutter reflectivity returns with unpredictable Doppler shifts. Wind turbines exhibit high radar cross sections (RCS), up to 1000m2 in some instances, and then, they are easily detected by radars. A typical wind turbine is made up of three main components, the tower, the nacelle and the rotor. The tower is a constant zero velocity return that can be suppressed by stationary clutter filtering. Unlike the tower, the turbine nacelle RCS is a function of the turbine yaw angle, and then, the radar signature will depend on this factor. Moreover, most wind turbines present curved surface nacelles which will scatter the energy in all directions and so the variability of the RCS is prominent. In addition, the rotor makes the blades move fast enough to be unsuppressed by conventional clutter filtering. In this chapter, we will examine the characteristics of wind turbine clutter in great detail. For this purpose, we will use examples derived from real experimental data. After describing the experimental data gathered, we will perform several studies. First of all, a complete statistical characterization of the experimental data will be accomplished. This statistical study will show the distinctive properties of this variety of clutter and then, it will give us clues for its correct detection, as every detection theory must rely on statistics. In this case we will study isolated turbines, so that the obtained characteristics will be accurate. After that, we will make an extensive frequency analysis. Different configurations will be studied, with variations such us the number of turbines, the yaw angle or the radar dwell time. This will show various wind turbine clutter situations that most affected radar systems have to deal with. Finally, some mitigation techniques that have been published to date will be reviewed. Their main purposes, techniques and results will be analyzed and illustrated with descriptive examples.