Dynamics and Control of Lateral Tower Vibrations in Offshore Wind Turbines by Means of Active Generator Torque

Abstract

Lateral tower vibrations of offshore wind turbines are normally lightly damped, and large amplitude vibrations induced by wind and wave loads in this direction may significantly shorten the fatigue life of the tower. This paper proposes the modeling and control of lateral tower vibrations in offshore wind turbines using active generator torque. To implement the active control algorithm, both the mechanical and power electronic aspects have been taken into consideration. A 13-degrees-of-freedom aeroelastic wind turbine model with generator and pitch controllers is derived using the Euler-Lagrangian approach. The model displays important features of wind turbines, such as mixed moving frame and fixed frame-defined degrees-of-freedom, couplings of the tower-blade-drivetrain vibrations, as well as aerodynamic damping present in different modes of motions. The load transfer mechanisms from the drivetrain and the generator to the nacelle are derived, and the interaction between the generator torque and the lateral tower vibration are presented in a generalized manner. A three-dimensional rotational sampled turbulence field is generated and applied to the rotor, and the tower is excited by a first order wave load in the lateral direction. Next, a simple active control algorithm is proposed based on active generator torques with feedback from the measured lateral tower vibrations. A full-scale power converter configuration with a cascaded loop control structure is also introduced to produce the feedback control torque in real time. Numerical simulations have been carried out using Energies 2014, 7 7747 data calibrated to the referential 5-MW NREL (National Renewable Energy Laboratory) offshore wind turbine. Cases of drivetrains with a gearbox and direct drive to the generator are considered using the same time series for the wave and turbulence loadings. Results show that by using active generator torque control, lateral tower vibrations can be significantly mitigated for both gear-driven and direct-driven wind turbines, with modest influence on the smoothness of the power output from the generator.

Dynamics and control of spar‐type floating offshore wind turbines with tuned liquid column dampers

“… is calculated using the blade element momentum method with Prandtl's tip loss factor and Glauert's correction, and nonlinear quasi‐static aeroelasticity is considered by introducing the local deformation velocities of the blade into the calculation of the flow angle and the angle of attack. The rotational sampled turbulence is simulated using an auto‐regressive (AR) model following the procedure in Zhang et al with mean wind speed and the turbulence intensity as inputs. For calculating , we have established different mooring line models, including linear spring, quasi‐static, lumped mass, and finite element (FE) models.…”

“…The structural vibrations in the blades, support structure, and other components of FOWT are generally larger than those of onshore or fixed‐foundation offshore wind turbines. It is well known that for fixed‐foundation wind turbines, tower side–side vibration and blade edgewise vibrations are very lightly damped due to the low aerodynamic damping under normal operational conditions, while the tower fore‐aft vibration and blade flap‐wise vibrations are much more damped due to the strong aerodynamic damping. This is also true for the FOWTs.…”

Real‐time hybrid aeroelastic simulation of wind turbines with various types of full‐scale tuned liquid dampers

“…To obtain the aerodynamic loads, rotational sampled turbulence has been generated on the basis of Taylor's hypothesis of frozen turbulence together with a first‐order autoregressive (AR) model . Next, the blade element momentum (BEM) method with Prandtl's tip loss factor and Glauert's correction for large axial induction coefficients is adopted to calculate aerodynamic forces along the blade .…”

“…In different vibrational modes of WTs, blade flap‐wise vibrations and tower fore‐aft vibrations are normally characterized by strong aerodynamic damping at normal operational conditions . In contrast, blade edgewise vibrations and lateral tower vibrations (tower side‐side vibrations) are lightly damped because of negligible or even negative aerodynamic damping . At present, most offshore WTs are installed at shallow water area.…”

Impact of the Converter Control Strategies on the Drive Train of Wind Turbine during Voltage Dips

“…Torsional oscillation is closely coupled with the tower side-side mode, and the equations of motion can be found in [23]. The impact due to the voltage dip can be transmitted to the tower and reflected on the acceleration which is harmful for the yaw bearing.…”