92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Tuesday, 24 January 2012: 4:15 PM
Development of Simple Radar Signature Models of Wind Turbine for Knowledge-Aided Wind Turbine Radar Interference Analysis
Room 345 (New Orleans Convention Center )
Ying Bai, Univ. of Oklahoma, Norman, OK; and F. Kong, Y. Zhang, and R. D. Palmer
Manuscript (441.2 kB)

As a growing renewable energy source, wind farm installations can have over a hundred turbines of up to 120 m height each. Moreover, turbines are expected to be larger in size in a few years, with blade tip heights reaching 200 m. Wind farm installations close to weather radars may block the normal propagation of the radar signals and cause interference. In order to mitigate the contamination ofwind turbine clutter (WTC) in radar sensors with tractable computational load, it is very important to understand the radar signature of WTC and be able generate a simple signal model to capture its features.

Generally, radar returns from a wind turbine include backscattering from the tower, the rotor blades and the nacelle. The tower and the nacelle can be considered approximately stationary. Therefore, in this research, the rotating blades are the main targets of interest, which have unique radar spectral signatures. It is easy to set various kinds of conditions with electromagnetic solvers such as the Physical Optics (PO) method. However, for different types of wind turbine structures, they must be remodeled each time, and the computational time is too long for large size models. Thus, a simplified mathematical model is created to capture the features of radar backscattering signatures from rotating blades. The basic idea is when an object rotates, the wave scattering from the target is modulated in both amplitude and phase. As such, both the amplitude and phase information need to be studied.

A new simplified geometrical model of a “generic” blade structure is proposed. The blade structure is simplified as a half elliptical cylindertogether with two elliptical flat plates on both ends. The rectangular flat face is referred to as the leading edge, and the cylinder surface as the trailing edge. The Radar Cross-Section (RCS) of a simple geometric shape can be expressed by closed-form formulas.The RCS of the entire structure can be computed by coherently combining the scattering waves of the elliptical plates, rectangular flat plates and cylinder surface components.

The “flashes” in the WTC show differences when a blade has a specular reflection at the advancing points and the receding points. The interval between flashes is related to the rotation rate of the rotor blade. Based on the initial rotation angle, radar carrier frequency and observation geometry, the flash behaviors from each individual blade can be accurately predicted from the simple model. The change in the property of the flashes is controlled by the parameters in analytical formulas which “modulate” the phase of rotating blades over time. Another usage of this simple model approach is to simulate the Doppler signatures induced by wind turbines.

Although as a tradeoff for simplified geometry and tremendously reduced computational load, outputs from the analytical model cannot perfectly agree with the prediction results from conventional EM solvers, it provides an additional tool for wind turbine recognition and provides the potential solution to alleviate the effect of WTC for weather radar systems. Continued work is being done to study the effectiveness of such models in knowledge-aided mitigation and cancellation processing.

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