The octocopter carries the measurement technology and can be precisely guided through wind farms.© Physikalisch-Technische Bundesanstalt – PTB

Wind energy 21.03.2018

Measurement electronics with HF front-end, precision navigation and the FPGA-based digitisation and memory card in a shielded housing© Physikalisch-Technische Bundesanstalt – PTB

Measurement of a radar signal behind a wind farm – section of an on-board video of the octocopter © Physikalisch-Technische Bundesanstalt – PTB

Effect of wind farms on flight safetyThe approval of new wind turbines in the vicinity of radar or navigation systems in air traffic often causes conflicts, which sometimes end in court. It is undisputed that wind turbines can influence the signals emitted by radar systems and omnidirectional radio range beacons. So far, there has been a lack of scientifically valid, reproducible investigations on the actual extent. Together with their partners, the Physikalisch-Technische Bundesanstalt has developed a new measurement method and numerical simulation methods and has now published first results.

Civil and military aircraft depend on reliable navigation systems for safe air traffic. Even in the age of satellite navigation, omnidirectional radio range beacons (CVOR, DVOR) are needed as back-up systems for satellite signals. Airspace surveillance is carried out with the aid of radar systems. Furthermore, meteorological institutions also operate radar systems, e.g. to identify rain fronts or to measure wind as a function of altitude (wind profilers). If wind turbines and wind farms are installed too near to these navigation or radar systems, they influence their signals and thus the accuracy of the displayed information. Until now, there has been insufficient data on the influencing factors and the actual extent of the interaction.

Together with partners, the Physikalisch-Technische Bundesanstalt (PTB) has developed new measurement methods and has measured the electromagnetic fields in the vicinity of wind turbines and navigation and radar systems. The work is carried out within the framework of the energy research project “Verbundvorhaben WERAN: Wechselwirkung Windenergieanlagen und Radar/Navigation” (Joint project WERAN: interaction between wind turbines and radar/navigation) and is expected to be completed in the summer of 2018. Dr. Thorsten Schrader, project manager at PTB, explained that, “We have achieved our goal of creating the metrological basis for detecting electromagnetic signal influences.”

Recorded signals of a DVOR (normalised phases of the AM and FM signals) in space at an altitude of approx. 140 m during operation, shutdown and stoppage of the four wind turbines. The modulation of the receiver level as a function of the rotor speed of the wind turbine is notable. The bearing is the radial calculated from the geometry that the FM signal must follow (the radial is transmitted by the FM signal of the DVOR).© Physikalisch-Technische Bundesanstalt – PTB

Change of the current Doppler frequency (purple) and the signal strength (green) under the influence of multipath propagation© Physikalisch-Technische Bundesanstalt – PTB

Multipath propagation caused by wind turbines changes the pulse shape of the radar signal at the target location© Physikalisch-Technische Bundesanstalt – PTB

Together with partners, the Physikalisch-Technische Bundesanstalt (PTB) has developed new measurement methods and has measured the electromagnetic fields in the vicinity of wind turbines and navigation and radar systems. The work is carried out within the framework of the energy research project “Verbundvorhaben WERAN: Wechselwirkung Windenergieanlagen und Radar/Navigation” (Joint project WERAN: interaction between wind turbines and radar/navigation) and is expected to be completed in the summer of 2018. Dr. Thorsten Schrader, project manager at PTB, explained that, “We have achieved our goal of creating the metrological basis for detecting electromagnetic signal influences.” The resolution of high-end high-frequency measurement technology is at least two orders of magnitude better than, for example, the maximum angular error target value to be considered for the omnidirectional radio range beacon DVOR”.

In addition to measurement technology, the research partners have developed a powerful numerical simulation analysis. A comparative study of measured data and simulation results is an indispensable prerequisite for quantifying the interactions between wind turbines and air traffic control systems. The electromagnetic fields in the airspace around the wind farms are measured. In order to assess the interactions, only those variables that can be obtained in measurements and simulations may be taken into account. The Physikalisch-Technische Bundesanstalt and its partners have developed a method that can measure the signal content available as a signal-in-space, on-site for actual signals from terrestrial navigation and radar systems. Dr. Thorsten Schrader explains that, “This makes it possible to quantify the interaction using measurement technology for the first time in the world.”

Measuring DVOR signals in and behind wind farmsThe measurements were carried out in various wind farms in the vicinity of omnidirectional radio range beacons (CVOR, DVOR). For example, it was possible to measure the signals from a DVOR eight kilometres away situated behind four large wind turbines (140 m hub height, 112 m rotor diameter). The signals were recorded during operation, shutdown, standstill and when the wind turbines were restarted. In addition, the researchers also investigated the influence of nacelle orientation and rotor blade position on the signal contents of the electromagnetic fields in space. A second series of investigations at a wind farm with smaller turbines 2.5 km away from a DVOR showed that the measured and simulated values for the angular error of the DVOR coincide qualitatively and quantitatively to a few tenths of a degree. It was also possible to prove

that the measurement results can be easily reproduced. For the first time in the world, the angular error caused by wind turbines for DVOR could be determined by measuring technology. In the future, conclusions are to be drawn for the harmonised simulation models that will be used by evaluating the obtained measurement results.

Further investigations focused on the influence on radar signals of modern pulse compression radars. The focus was on changes that a radar signal experiences on its way to the target while passing through wind turbines.

On-site measurement with dronesHigh-frequency measurement technology has been miniaturised far enough that it can be carried by an octocopter. This allows investigations to be carried out in the vicinity of the wind farms, where it actually matters. The ground-controlled drone is equipped with a precise navigation system. The measurement technology acquires, synchronises and stores bandpass signals and information on location and time at a high data rate. This way, all movements and measuring points of the octocopter can be precisely defined and traced at a later time. At the same time, a selection of the current measurement data is transferred in real time to a tablet PC on the ground, where the measurements can be monitored and actively controlled.

The measurement concept with the drone has great advantages over earlier measurement methods, by which the measurement instruments were installed on planes and flown through these test tracks. Due to the miniaturisation of measurement technology, the measurement system itself distorts the data to a far lesser extent than an aircraft does. The drone can hover at a point in space and record measurement data over longer periods of time. It can

The measurement concept with the drone has great advantages over earlier measurement methods, by which the measurement instruments were installed on planes and flown through these test tracks. Due to the miniaturisation of measurement technology, the measurement system itself distorts the data to a far lesser extent than an aircraft does. The drone can hover at a point in space and record measurement data over longer periods of time. It can also fly through a wind farm at low speed and altitude, e.g. at the height of the rotor hub of a wind turbine. In addition, the bandpass signal is recorded, which contains all information without any preprocessing. For DVOR, for example, both signal components (reference and directional round-trip signal) can be determined separately and then evaluated. The entire measurement system, consisting of octocopter, antenna and measuring unit, is calibrated based on feedback.

Effects on the wind energy industryUntil now, wind turbines have had to maintain a safe distance from radio beacons and radar systems. In practice, this often leads to conflicts that end up in court. The German Wind Energy Association surveyed its members on this topic in 2015. The survey revealed that wind projects amounting to more than 4,120 MW are being blocked by existing aviation safety and radar issues. With over 2,333 megawatts of prevented wind power, the protection areas with a radius of 15 kilometres around the omnidirectional radio range beacons of civil aviation (CVOR, DVOR) are currently the biggest problem.

The measurement methods now developed by PTB will contribute to a spatially more precise and a scientifically valid specification of the areas requiring protection, in terms of these approval procedures for wind turbines in the vicinity of navigation equipment. To what extent the transfer of these scientific findings into other applicable technology is possible will be the subject of future investigations.

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