Variable dampers to mitigate structural demand to wind turbines: The role of the monitoring system features for the effectiveness of the control strategy

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

Experimental Issues in Testing a Semiactive Technique to Control Earthquake Induced Vibration

This work focuses on the issues to deal with when approaching experimental testing of structures equipped with semiactive control (SA) systems. It starts from practical experience authors gained in a recent wide campaign on a large scale steel frame structure provided with a control system based on magnetorheological dampers. The latter are special devices able to achieve a wide range of physical behaviours using low-power electrical currents. Experimental activities involving the use of controllable devices require special attention in solving specific aspects that characterize each of the three phases of the SA control loop: acquisition, processing, and command. Most of them are uncommon to any other type of structural testing. This paper emphasizes the importance of the experimental assessment of SA systems and shows how many problematic issues likely to happen in real applications are also present when testing these systems experimentally. This paper highlights several problematic aspects and illustrates how they can be addressed in order to achieve a more realistic evaluation of the effectiveness of SA control solutions. Undesired and unavoidable effects like delays and control malfunction are also remarked. A discussion on the way to reduce their incidence is also offered

Semi-Active Control of Precast RC Columns under Seismic Action

This work is inspired by the idea of dissipating seismic energy at the base of prefabricated RC columns via semi-active (SA) variable dampers exploiting the base rocking. It was performed a wide numerical campaign to investigate the seismic behaviour of a pre-cast RC column with a variable base restraint. The latter is based on the combined use of a hinge, elastic springs, and magnetorheological (MR) dampers remotely controlled according to the instantaneous response of the structural component. The MR devices are driven by a SA control algorithm purposely written to modulate the dissipative capability so as to reduce base bending moment without causing excessive displacement at the top. The proposed strategy results to be really promising, since the base restraint relaxation, that favours the base moment demand reduction, is accompanied by a high enhancement of the dissipated energy due to rocking that can be even able to reduce top displacement in respect to the "fixed base rotation" conditions

Structural Control of a Wind Turbine Accounting for Second Order Effects

The negative impact of the use of fossil fuels on the environment has lead to a boom in the production of wind turbines. The progressively increasing turbines' height, decided to take ad-vantage of the smoother winds at higher altitude, has led to an increased demand to control tower forces. The proposed work is focused on the application of a semi-active (SA) control system to limit bending moment demand at the base of a wind turbine by relaxing the base restraint of the turbine's tower, without increasing the top displacement. The proposed SA control system reproduces a variable restraint at the base that changes in real time its mechanical properties according to the instantaneous response of the turbine's tower. This smart restraint is made of a central smooth hinge, elastic springs and SA magnetorheological dampers driven by a properly designed control algorithm. A commercial 105 m tall wind turbine has been considered as a case study. Several numerical simulations have been performed with reference to two extreme loads, different one each other for intensity, duration, frequency content, so as to understand if a unique optimal configuration of the controller can be defined for both of them. The proposed study is also focused on understanding whether and how to reduce the residual top displacement due to the possible incremental base rotation that may happen during a wind load history, especially when it is long lasting

Seismic isolation of lightweight structures through Wire Rope devices: Preliminary experimental results and simulation

The work presents some preliminary results of experimental tests performed on a Helical Wire Rope Isolator (HWRI), to the aim of describing the dynamic behavior of the metal isolation device, without pre-load and under the effect of a vertical pre-load, for different displacement amplitudes. During the dynamic tests, the testing machine allows to apply a vertical displacement or load history to the tested device, subjected to a constant vertical pre-load. The dynamic behavior of the tested device depends on the applied displacement amplitude: the HWRI displays an asymmetric behaviour, i.e. a different response when it is subjected to compression or tension; in the latter case, it shows an hardening effect for larger displacements. Moreover, the experimental tests make clear that the application of a vertical pre-load determines a different behaviour in Compression vs. Tension, with slightly variable damping properties. In order to simulate the dynamic behavior of the HWRIs in Compression-Tension direction, a nonlinear model is proposed, that is validated by comparing the experimental hysteresis loops obtained during cyclic tests with those predicted analytically

Semi-Active Control of Precast RC Columns under Seismic Action

This work is inspired by the idea of dissipating seismic energy at the base of prefabricated RC columns via semi-active (SA) variable dampers exploiting the base rocking. It was performed a wide numerical campaign to investigate the seismic behaviour of a pre-cast RC column with a variable base restraint. The latter is based on the combined use of a hinge, elastic springs, and magnetorheological (MR) dampers remotely controlled according to the instantaneous response of the structural component. The MR devices are driven by a SA control algorithm purposely written to modulate the dissipative capability so as to reduce base bending moment without causing excessive displacement at the top. The proposed strategy results to be really promising, since the base restraint re-laxation, that favours the base moment demand reduction, is accompanied by a high enhancement of the dissipated energy due to rocking that can be even able to reduce top displacement in respect to the "fixed base rotation" conditions

Experimental tests and numerical modeling of magnetorheological dampers: the influence of model parameters

Among the different semi-active control devices proposed in literature, in the last years those one based on the properties of the MR fluids have been attracting the interest of a large number of researchers. The structural control strategies need of a clear and effective modelling of the mechanical and dynamical properties of magnetorheological (MR) dissipaters. In the framework of a research project financed by the European Commission within the 5th FP (1998-2002), MR dampers have been manufactured with European technology, and subsequently they have been tested during a wide experimental campaign. The experimental behaviour of such devices turned out to be different from the one found in literature for similar devices, and allowed the adoption of a more simple numerical model with respect to the most applied model in literature. In the present paper, the semi-active MR devices tested in laboratory are characterized by using either a properly defined model or the dominant model of literature, showing that the 2 model, even if characterized by a different complexity, yield the same accuracy in fitting the experimental data