Tower Modes Research Articles

Analysis of Guyed Transmission Tower Dynamic Characteristic

In the paper aiming at lots of 87(LM21) towers in Dongchang Hasong Transmission Line held under Changchun Extra-high voltage Bureau jurisdiction, by using ANSYS software, the nonlinear finite element model for towers is built. And numerical analysis methods is adopted. It is considered that the combined effect of stay wire and transmission tower, geometric non-linear effect and the influence of stay wire initial stress. The Dynamic Properties research about Guyed Transmission is conducted. It concluded that stay wire and tower's mode of vibrationare are relatively independent ,coordinated and the interaction of each other. Natural frequency of vibration and mode of vibration appears regularly and alternately changes with the increase of the formation order .Stay wire initial stress is main factor affecting the natural frequency of the structure and the frequency increases with the increase of stay wire initial stress.

Seismic Analysis of Wind Turbines

An analytical model of an operating wind turbine to obtain the seismic response due to three base accelerations is presented. The model considers the flexibility of the blades in the flapping direction and the flexibility of the tower in bending and twisting. Blade aerodynamics and gyroscopic moments are included. A Vestas-V82 turbine is selected to demonstrate the methodology. The results show that only the first two tower modes in each direction (fore-aft and lateral) are mostly excited. It is found that the lateral motion of the wind turbine is more susceptible to experiencing large displacements since the aerodynamic effects are negligible in this direction. The stresses due to combined operational and seismic loads and due to extreme wind loads are less than the allowable stresses. The stresses calculated at the tower top section due to combined operational and seismic loads are larger than those due to extreme wind loads.

Damping of Offshore Wind Turbine Tower Vibrations by a Stroke Amplifying Brace

The potential of installing dampers inside the tower of an offshore wind turbine is investigated through simulations. Dampers are installed at the bottom to act on the curvature of the tower, and it is shown that dampers installed in suitable braces have the potential to increase the critical damping ratio of the two lowest tower modes by 1 percent. By using a toggle-brace system, damper stroke is increased, while the damper force is reduced. Finally, by installing the dampers in a symmetric configuration, tuning for maximum damping is approximately independent of the orientation of the rotor, thereby making this installation of dampers feasible.

Vibration Characteristics Analysis of Wind Turbine Towers under Foundation Conditions

Based on a kind of 1.5MW large-scale horizontal axis wind turbine tower, the mechanical modeling of a wind turbine tower-foundation is established, the static and dynamic analysis of the model is carried out by ANSYS software. The top displacement of the system is calculated by the static analysis to meet the design requirements in engineering. In dynamic analysis, each pile foundation is equivalent to a group of springs for the simulation of horizontal and vertical rigidity of the pile. The influence of top mass and foundation elasticity on wind turbine tower modes is analyzed, and calculated the natural frequency of the tower within a certain scope of rigidity in different directions about the piles foundation. The results show that the natural frequency of the wind turbine tower is influenced significantly by the mass on the tower top and foundation rigidity. The study provides a theoretical basis for optimal design of the wind turbine.

Investigation of a Speed Exclusion Zone to Prevent Tower Resonance in Variable-Speed Wind Turbines

A speed exclusion zone algorithm to prevent the rotor rotational frequency from exciting the side-side tower mode in a variable-speed wind turbine was designed. The algorithm was first tested on a simple wind turbine model developed in Simulink and then implemented and tested on an experimental test rig. Different speed exclusion zone widths were tested to observe how the performance of the wind turbine is affected. Results obtained from both simulations and experiments showed the effectiveness of the speed exclusion zone algorithm in preventing excitation of tower resonance. Results from different speed exclusion zone widths showed that with a wider exclusion zone, the tower mode is excited less at the expense of a larger power loss.

The Practical-Analytical Solutions of the Wind Turbine Tower System’s Natural Vibration Properties

Wind turbine tower structure system’s natural vibration properties have important effects on the system of wind power tower wind vibration and seismic response, especially the low order modes often plays a control function. How to calculate tower system vibration characteristics correctly and effectively in the design phase has important practical value. In this paper, some practical analytic solutions formulas were deduced on considering the simplified model using Rayleigh quotient method. Moreover, the results of a tower frequency and mode which were calculated by these formulas were compared with the continuous distribution parameters system’ theoretical solution and FEM method’s solutions. Analysis show that: these natural vibration properties practical analytic solutions formulas of a wind turbine tower about simplified model are useful and precision in a tower design, especially suitable for cone cube tower.

Effects of extreme wind shear on aeroelastic modal damping of wind turbines

ABSTRACTWind shear is an important contributor to fatigue loads on wind turbines. Because it causes an azimuthal variation in angle of attack, it can also affect aerodynamic damping. In this paper, a linearized model of a wind turbine, based on the non‐linear aeroelastic code BHawC, is used to investigate the effect of wind shear on the modal damping of the turbine. In isotropic conditions with a uniform wind field, the modal properties can be extracted from the system matrix transformed into the inertial frame using the Coleman transformation. In shear conditions, an implicit Floquet analysis, which reduces the computational burden associated with classical Floquet analysis, is used for modal analysis.The methods are applied to a 2.3 MW three‐bladed pitch‐regulated wind turbine showing a difference in damping between isotropic and extreme shear conditions at rated wind speed when the turbine is operating closest to stall. The first longitudinal tower mode decreases slightly in damping, whereas the first flapwise backward whirling and symmetric modes increase in damping. This change in damping is attributed to an interaction between the periodic blade mode shapes and the azimuth‐dependent local aerodynamic damping in the shear condition caused by a beginning separation of the flow. Copyright © 2012 John Wiley & Sons, Ltd.

Influence of lateral motion of cable stays on cable-stayed bridges

The aim of this paper concerns with the nonlinear analysis of cable-stayed bridges including the vibration effect of cable stays. Two models for the cable stay system are built up in the study. One is the OECS (one element cable system) model in which one single element per cable stay is used and the other is MECS (multi-elements cable system) model, where multi-elements per cable stay are used. A finite element computation procedure has been set up for the nonlinear analysis of such kind of structures. For shape finding of the cable-stayed bridge with MECS model, an efficient computation procedure is presented by using the two-loop iteration method (equilibrium iteration and shape iteration) with help of the catenary function method to discretize each single cable stay. After the convergent initial shape of the bridge is found, further analysis can then be performed. The structural behaviors of cable-stayed bridges influenced by the cable lateral motion will be examined here detailedly, such as the static deflection, the natural frequencies and modes, and the dynamic responses induced by seismic loading. The results show that the MECS model offers the real shape of cable stays in the initial shape, and all the natural frequencies and modes of the bridge including global modes and local modes. The global mode of the bridge consists of coupled girder, tower and cable stays motion and is a coupled mode, while the local mode exhibits only the motion of cable stays and is uncoupled with girder and tower. The OECS model can only offers global mode of tower and girder without any motion of cable stays, because each cable stay is represented by a single straight cable (or truss) element. In the nonlinear seismic analysis, only the MECS model can offer the lateral displacement response of cable stays and the axial force variation in cable stays. The responses of towers and girders of the bridge determined by both OECS- and MECSmodels have no great difference.

Dynamic characteristics of a curved cable-stayed bridge identified from strong motion records

An assessment of dynamic characteristics of the 455 m Katsushika–Harp curved cable-stayed bridge is presented. Dynamics characteristics such as natural frequencies, mode shapes and modal damping ratios are obtained from seismic response of the bridge by employing a time-domain multi-input multi-output (MIMO) system identification (SI) technique. The technique makes use of base motions and superstructure accelerations as pairs of inputs–outputs to realize the coefficients of state-space system matrices. The SI results indicate the occurrence of many closely spaced modal frequencies with spatially complicated mode shapes. Fourteen global modes in the ranges of 0.45–2.5 Hz were identified, in which the girder motion dominated most of the modes. The tower modes were associated with girder modes and were characterized by the lowly-damped motion. Using identification results from six earthquakes, the effects of earthquake amplitude on modal damping ratios were observed.

L i → l jγ decays within a model involving a universal extra dimension

Flavor-changing radiative lepton decays of the l{sub i} {sup {yields}} l{sub j{gamma}} type are studied within a model involving a universal extra dimension. Loop amplitudes where intermediate fermions and scalar bosons have commensurate masses may be quite large since the quadratic suppression factor reduces to a linear one in this case. The situation where fermions and scalar bosons have commensurate masses in a loop is realized in a model featuring universal extra dimensions. The contribution of Kaluza-Klein towers to the l{sub i} {sup {yields}} l{sub j{gamma}} decay amplitude is explicitly calculated. However, the enhancement expected in the amplitude on the basis of general considerations disappears because of the maximal degeneracy of massive-neutrino tower modes associated with different generations.

Ambient vibration measurements on a cable‐stayed bridge

AbstractAn extensive programme of full‐scale ambient vibration tests has been conducted to measure the dynamic response of a 542 m (centre span of 274 m) cable‐stayed bridge—the Quincy Bayview Bridge in Illinois. A microcomputer‐based system was used to collect and analyse the ambient vibration data. A total of 25 modal frequencies and associated mode shapes were identified for the deck structure within the frequency range of 0–2 Hz. Also, estimations were made for damping ratios. The experimental data clearly indicated the occurrence of many closely spaced modal frequencies and spatially complicated mode shapes. Most tower modes were found to be associated with the deck modes, implying a considerable interaction between the deck and tower structure. No detectable levels of motion were evident at the foundation support of the pier. The results of the ambient vibration survey were compared to modal frequencies and mode shapes computed using a three‐dimensional finite element model of the bridge. For most modes, the analytic and experimental modal frequencies and mode shapes compare quite well, especially for the vertical modes. Based on the findings of this study, a linear elastic finite element model appears to be capable of capturing much of the complex dynamic behaviour of the bridge with very good accuracy, when compared to the low‐level dynamic responses induced by ambient wind and traffic excitations.

Signal Separation Method for Tower Mode Shape Measurement

Vibration shapes of the fundamental modes of eight-legged, K-braced, offshore towers undergo significant changes when important braces are damaged. These changes, measurable at deck-level, provide valuable clues for detecting damage. Because of the small difference between vibration frequencies of the two fundamental translational modes in these towers, the mode shapes are difficult to measure. A simple method is presented which solves this difficulty. The method uses transient acceleration response records that can be produced by different excitation procedures, and no knowledge of the input that causes the excitation is necessary. The finite Fourier transform of such records is examined, and its important properties are pointed out for low damping. Based on these properties, procedures are set forth by which mode shape data can be retrieved from closely spaced peaks in the spectra. The retrieval is done by separating these peaks from each other. Applications of the method in a numerical experiment, and in a physical experiment on a 1/50th scale offshore platform model, are presented. Application of the method to full-scale towers is discussed.

Ambient Vibration of Two Suspension Bridges

The dynamic characteristics (i.e., normal mode natural frequencies, damping, and shapes) of two suspension bridges are estimated from experimentally measured ambient motion time histories. These characteristics were estimated using random vibration theory and the power spectral densities of ambient time histories. Twenty normal modes of vibration were observed in the 0 cycle per min-60 cycle per min frequency range for the Newport Bridge in Rhode Island. These modes include nine vertical, six lateral, and two torsional deck modes, plus three longitudinal tower modes. For each mode, estimates are obtained of natural frequency of vibration, damping, and shape. For the other suspension bridge, the William Preston Lane Memorial Bridge, 13 normal modes of vibration were observed. These modes are six vertical, six lateral, and one torsional deck mode. No tower modes were observed in the 0 cycle per min-60 cycle per min frequency range.