_Applied_  
_sciences_  
_Article_  
**Variable Dampers to Mitigate Structural Demand to Wind Turbines: The Role of the Monitoring System Features for the Effectiveness of the Control Strategy \({}^{\ddagger}\)**  
**[PERSON] \({}^{1,2,*}\)\({}^{\copyright}\)**, [PERSON] \({}^{1}\)\({}^{\copyright}\)**, [PERSON] \({}^{3}\)\({}^{\copyright}\) and  
**[PERSON] \({}^{1}\)\({}^{\copyright}\)**  
1 Department of Engineering, University of Naples 'Parthenope', Centro Direzionale di Napoli, Isola C4, 80143 Napoli, Italy; [EMAIL_ADDRESS] (G.P.); [EMAIL_ADDRESS] (U.R.)  
2 Institute of Technologies for Construction, CNR, San G. Milanese, 20098 Milan, Italy  
3 Department of Structures for Engineering and Architecture, University of Naples Federico II, via Claudio n. 21, 80125 Napoli, Italy; [EMAIL_ADDRESS]  
* Correspondence: [EMAIL_ADDRESS]  
1 This paper is an extended version of the conference paper presented by authors at the XV Conference of the Italian Association for Wind Engineering held in 2018.  
Received: 11 March 2020; Accepted: 1 April 2020; Published: 5 April 2020  
**Featured Application: This study investigates how the performance of the real-time monitoring system may affect the capability of a semi-active control strategy in mitigating the structural demand to wind turbines. It provides practical indications on how to select measuring devices and systems with given characteristics that help to improve, rather than reduce, the ability of the semi-active control system to reduce the stress and displacement demand to turbines against wind gusts. The results achieved may be of interest to professionals and companies engaged in the design of new wind farms or the repowering of existing plants with innovative systems.**  
**Abstract: In the last decade, some researchers and professionals have been engaged in the study of methods and techniques that can build high wind turbines while containing construction costs within the limits of economic convenience. Among the most promising solutions is that of using innovative devices to mitigate the structural demand for the towers. The reduction in the stress demand in the foundation makes the strategy particularly interesting for the repowering of existing plants, where it is convenient not to demolish and rebuild the foundation, but rather to reuse the existing one for the new plant. A semi-active vibration control strategy, based on the adoption of controllable dissipative devices, is presented herein. The proposed technique requires the tower to be equipped with a measurement system suitable for the real time monitoring of structural response. Performing reliable high-frequency measurements of the horizontal displacement of points located at heights of tens of meters is not simple. With the purpose of assessing the efficiency and feasibility of Global Navigation Satellite System (GNSS)-based systems for the control of wind turbine structures, the proposed paper tries to investigate the characteristics and data processing techniques that are able to make the GNSS useful for such applications. Several numerical simulations were carried out with reference to a case-study wind turbine to quantitatively assess how the performance of the control system changes as the features of the monitoring system worsen, and finally to draw conclusions and suggestions for the minimum performance that monitoring devices must have for an effective reduction in structural demand.**  
**Keywords: structural monitoring; semi-active control; magnetorheological damper; wind turbine**