Modeling,simulation and control of a wind turbine with a hydraulic transmission system

A complete mathematical model of a hydraulic transmission concept for use in wind turbines is presented. The hydraulic system transfers the power from the nacelle to ground level. The main focus has been to develop a model that takes into account the most important dynamics affecting the wind turbine and the hydraulic transmission system involved, such that the model can be used to analyze the dynamic feasibility of a hydraulic transmission concept. Further, dynamic analysis of a hydraulic transmission system for wind turbines is investigated. The nonlinear dynamic model is developed in MATLAB Simulink. Analytical calculation of natural periods of a linearized model corresponds well with simulations of the overall system. A valve control system is proposed to reduce pressure and power fluctuations at operation both below and above the rated wind speed for the wind turbine. Further, a blade pitch control system based on an aerodynamic power estimator is proposed for operation above the rated wind speed. System simulations for one case below and one case above the rated wind speed show that the dynamic response of the overall system is stable and that the wind turbine variables are within typical ranges for conventional variable speed wind turbines with mechanical transmission. Copyright © 2012 John Wiley & Sons, Ltd.     

载荷优化的海上漂浮式风电机组二阶滑模变桨控制

针对强非线性、强耦合的海上漂浮式风电机组动力学系统,提出一种基于二阶滑模的统一变桨控制策略,解决受海浪风速等随机干扰引起浮式支撑平台运动而产生的疲劳结构载荷及功率波动问题。构建漂浮式风电机组的不确定仿射非线性模型,基于风电机组“额定转速”设计积分滑模面,此“额定转速”不再是恒定值,而是取决于平台纵摇速度的变量,基于超螺旋算法实现二阶滑模变桨控制律。采用FAST和Matlab/Simulink联合仿真,所提出的方案与传统PI控制相比,对稳定高风速时风力发电机功率,抑制浮式支撑平台运动及减少叶根载荷具有更好的控制作用,对塔基也有较好的减载作用。     

组合弹性系泊对漂浮式风力机Spar平台影响的研究

研究新型组合弹性系泊对漂浮式风力机Spar平台动态响应及系泊张力特性的影响。建立基于OC3-Hywind Spar Buoy平台的NREL5 MW漂浮式风力机整机模型,综合考虑叶片空气动力载荷、平台的水动力载荷及平台与系泊系统的耦合。首先验证水动力学模型的可靠性,然后分析组合弹性系泊对动态响应及系泊张力的影响。结果表明:组合弹性系泊会使平台纵荡增大25.5%,但系泊张力、垂荡和纵摇却会分别减小14.0%、20%和18.1%;弹性索位置对动态响应及系泊张力影响不大;随弹性索长度的增加,平台纵荡响应增加34.0%,平台垂荡和系泊张力分别减小25.0%和7.3%。     

Numerical modeling of the hydraulic blade pitch actuator in a spar‐type floating wind turbine considering fault conditions and their effects on global dynamic responses

This paper deals with numerical modeling of the hydraulic blade pitch actuator and its effect on the dynamic responses of a floating spar‐type wind turbine under valve fault conditions. A spar‐type floating wind turbine concept is modeled and simulated using an aero‐hydro‐servo‐elastic simulation tool (Simo‐Riflex [SR]). Because the blade pitch system has the highest failure rate, a numerical model of the hydraulic blade pitch actuator with/without valve faults is developed and linked to SR to study the effects of faults on global responses of the spar‐type floating wind turbine for different faults, fault magnitudes, and environmental conditions. The consequence of valve faults in the pitch actuator is that the blade cannot be pitched to the desired angle, so there may be a delay in the response due to excessive friction and the wrong voltage, or slit lock may cause runaway blade pitch. A short circuit may cause the blade to get stuck at a particular pitch angle. These faults contribute to rotor imbalance, which result in different effects on the turbine structure and the platform motions. The proposed method for combining global and hydraulic actuator models is demonstrated in case studies with stochastic wind and wave conditions and different types of valve faults.     

Effect of platform disturbance on the performance of offshore wind turbine under pitch control

The aerodynamic performance of offshore floating wind turbines (OFWTs) is more complicated than onshore wind turbines due to 6‐degree of freedom (DOF) motion of the floating platform. In the current study, the aerodynamic analysis of a horizontal‐axis floating offshore wind turbine is performed with the aim of studying the effects of floating platform movement on the aerodynamic characteristics of the turbine in the presence of a pitch angle control system. The National Renewable Energy Laboratory (NREL) 5‐MW offshore wind turbine is selected as the baseline wind turbine. For this sake, the unsteady blade element momentum method with dynamic stall and dynamic inflow models have been employed to obtain the unsteady aerodynamic loads. The baseline pitch angle control system is assumed to be coupled with the aerodynamic model to maintain the rated condition of the wind turbine and also to approach a closer model of wind turbine. In case of pitching motion input, the reduction of mean power coefficient for tip speed ratios (TSRs) less that 7 is expected by an amount of 16% to 20% at pitch amplitude of 2° and frequency of 0.1 Hz. For high TSRs, the trend is reverse with respect to fixed‐platform case. The mean thrust coefficient is reduced for almost all range of TSRs with maximum loss of 37%. Moreover, the mean control pitch angle that is an index of control system effort is increased. The results also represent the importance of considering the pitch control system for aerodynamic analysis of disturbed OFWT.     

不同海况及伺服系统作用下10 MW单桩式风力机地震易损性研究

为探究不同海况及伺服系统下单桩式近海风力机的地震易损性,以DTU 10 MW风力机为研究对象,建立风浪相关的地震-湍流风-波浪多物理场模型,研究其在变速变桨伺服系统下的动力特性,基于增量动力分析方法评估其地震易损性。结果表明：变速变桨伺服系统可有效缓解风力机高风速下无地震作用时的塔顶振动;当风轮在大推力下,较小的波浪载荷一定程度上可降低风力机塔顶振动及塔底弯矩;随地震动强度增加,风力机各临界损伤状态失效概率逐渐增加;风力机地震易损性主要由地震动强度决定,波浪载荷与湍流风载荷对风力机地震易损性影响较小。     

考虑黏性效应的半潜浮式风力机动力特性研究

对半潜浮式风力机动力特性进行研究，推导考虑黏性阻尼的动力学方程及传递函数。对黏性效应的影响及其计算方法进行探讨，对比附加阻尼矩阵法、Morison单元法的优缺点，并提出考虑黏性阻尼效应水动力计算的混合法，在此基础上对半潜浮式风力机气动-水动-锚泊全耦合动力响应进行分析。结果表明：黏性效应主要影响共振周期附近的响应值，在数值分析时不可忽略；附加阻尼矩阵法在考虑水平面内运动黏性阻尼时有所不足，且无法考虑黏性效应对共振周期的影响，Morison单元法在考虑垂荡、转动黏性阻尼时有所不足，混合法是考虑黏性阻尼水动力计算的更有效方法；该半潜浮式风力机垂荡和纵摇响应主要受波浪控制，而水平面内运动受风、浪、流联合作用的影响；浮式风力机运动和加速度的最危险工况可发生在发电工况时，最大锚链张力发生在极端环境条件时。     

Impact of aerodynamic modeling on seakeeping performance of a floating horizontal axis wind turbine

Over the last decade, several coupled simulation tools have been developed in order to design and optimize floating wind turbines (FWTs). In most of these tools, the aerodynamic modeling is based on quasi‐steady aerodynamic models such as the blade element momentum (BEM). It may not be accurate enough for FWTs as the motion of the platform induces highly unsteady phenomena around the rotor. To address this issue, a new design tool has been developed coupling a seakeeping solver with an unsteady aerodynamic solver based on the free vortex wake (FVW) theory. This tool is here compared with the reference code FAST, which is based on the BEM theory in order to characterize the impact of the aerodynamic model on the seakeeping of a floating horizontal axis wind turbine (HAWT). Aerodynamic solvers are compared for the case of the free floating NREL 5MW HAWT supported by the OC3Hywind SPAR. Differences obtained between the models have been analyzed through a study of the aerodynamic loads acting on the same turbine in imposed harmonic surge and pitch motions. This provides a better understanding of the intrinsic differences between the quasi‐steady and unsteady aerodynamic solvers. The study shows that differences can be observed between the three aerodynamic solvers, especially at high tip speed ratio (TSR) for which unsteady aerodynamic phenomena and complex wake dynamics occur. Observed discrepancies in the predictions of the FWT dynamic response can raise issues when designing such a system with a state‐of‐the‐art design tool.     

Dynamic response analysis of wind turbines under blade pitch system fault,grid loss,and shutdown events

This study focuses on the dynamic responses of land‐based and floating wind turbines under blade pitch system fault and emergency shutdown conditions. The NREL 5 MW turbine is studied. A hydraulic pitch system is considered, and the faults under study are events with a seized blade or a blade running out of control. Emergency shutdown is defined as a fast pitch‐to‐feather maneuver of the blades. Load cases with power production and grid fault with ensuing shutdown are also analysed for comparison. The fault scenarios and the blades' fast pitching activity are simulated using HAWC2 through external Dynamic Link Libraries. On the basis of the time‐domain simulations, the response characteristics of the land‐based and the floating turbines in the four design load cases are compared. The load effects from the fault conditions are compared with the operational cases. Strong system dynamics and resonant responses, such as the tower elastic mode and the yaw resonant response, are elicited during shutdown. If the pitch system has a fault and one blade is hindered from normal pitching, the uneven load distribution of the blades leads to large structural and motion responses. For both turbines, the response maxima vary cyclically with the instantaneous azimuth when the blades start pitching to feather. For the floating wind turbine, the interaction of waves and wind also affects the results. The effect of the pitch rate during shutdown is analysed. The responses of the land‐based turbine in grid loss and shutdown conditions are proportional to the pitch rate, whereas decreased sensitivity is found in the cases with pitch system faults. For the floating turbine, the effect of the pitch rate is small, and reduced pitch and yaw motion extremes are observed as the pitch rate increases. Copyright © 2013 John Wiley & Sons, Ltd.     

基于多失效模式的海上浮式风电机组结构可靠性研究

为准确评估浮式海上风电机组结构服役安全性,提出一种基于多失效模式的可靠性评估方法。以美国国家可再生能源实验室（NREL）研制的5 MW浮式风力发电机组OC3 Hywind为目标模型,根据不同海况下浮式风电机组结构耦合动力响应分析结果,分析关键结构的不同失效模式,最后基于多失效模式的可靠性评估方法计算整体系统可靠性。结果表明,考虑串联系统,基于多失效模式可靠性分析方法得到的海上浮式风电机组结构整体的失效概率远高于单一模式失效概率,采用此方式评估结构可靠度更加安全准确。     

Impact of aerodynamic modelling on seakeeping performance of a floating vertical axis wind turbine

This study focuses on the impact of the aerodynamic model on the dynamic response of a floating vertical axis wind turbine (VAWT). It compares a state‐of‐the‐art quasi‐steady double multiple streamtube (DMS) solver, a prescribed vortex wake (PVW), and a free vortex wake (FVW) solver. The aerodynamic loads acting on a bottom‐fixed VAWT and computed with the three aerodynamic solvers are compared, then the dynamic responses of the floating turbine in irregular waves and turbulent wind with the different aerodynamic solvers are compared. Differences are observed, particularly in the mean motions of the platform. Eventually, the aerodynamic damping computed by the solvers are estimated with aerodynamic simulations on the turbine with imposed surge and pitch motions. The estimated damping can then be correlated with the dynamic response amplitude of the VAWT. Substantial discrepancies are observed between the three solvers at high tip speed ratio, when the rotor is highly loaded. It is shown that the quasi‐steady DMS solver seems to give greater amplitude of motions for the floating VAWT because of strong rotor/wake interaction that are not correctly accounted for.     

Identification of support structure damping of a full scale offshore wind turbine in normal operation

The support structure damping of a 3.6 MW pitch controlled variable speed offshore wind turbine on a monopile foundation is estimated both in standstill conditions and in normal operation. The net substructure damping is identified from the parameters of an exponential curve fitted to the relative maxima of an impulse response caused by a boat impact. The result is used in the verification of the non aerodynamic damping in normal operation for low wind speeds. The auto-correlation function technique for damping estimation of a structure under ambient excitation was validated against the identified damping from the decaying time series. The Enhanced Frequency Domain Decomposition (EFDD) method was applied to the wind turbine response under ambient excitation, for estimation of the damping in normal operation. The aero-servo-hydro-elastic tool HAWC2 is validated with offshore foundation load measurements. The model was tuned to the damping values obtained from the boat impact to match the measured loads. Wind turbulence intensity and wave characteristics used in the simulations are based on site measurements. A flexible soil model is included in the analysis. The importance of the correctly simulated damping in the model is stressed for accurate load prediction. Differences in the identified damping between the model and the wind turbine are detailed and explained. Discrepancies between simulated and measured loads are discussed.     

新型张力腿平台漂浮式风力机动态响应研究

未来海上风电场的建设需要大量动态响应低且相互干扰小的漂浮式风力机,为此研究一种尺寸更小,能减少或防止相互间干扰及安装费用更低的新型张力腿平台漂浮式风力机(TLP风力机),基于辐射/绕射理论并结合有限元方法,运用水动力学软件AQWA对其在不同海况下及不同系泊系统下的动态响应进行模拟研究,得到了频域和时域的动态响应数据。结果表明:频域分析中,新型张力腿平台漂浮式风力机的运动响应主要集中在低频区域,其中横荡、垂荡及纵摇的峰值频率分别为0.1、0.35和0.19 rad/s;TLP风力机垂荡和纵摇方向上的运动响应都小于Spar风力机运动响应;横荡、垂荡及纵摇方向上,TLP风力机附加质量均远大于辐射阻尼;时域分析中,4根张紧系泊设计优于单根张紧系泊设计,能有效降低横荡和纵摇运动响应和延长系泊绳的寿命;随着海况恶劣程度的加剧,TLP风力机的动态响应也随之增大。     

Response analysis and comparison of a spar‐type floating offshore wind turbine and an onshore wind turbine under blade pitch controller faults

This paper analyses the effects of three pitch system faults on two classes of wind turbines, one is an onshore type and the other a floating offshore spar‐type wind turbine. A stuck blade pitch actuator, a fixed value fault and a bias fault in the blade pitch sensor are considered. The effects of these faults are investigated using short‐term extreme response analysis with the HAWC2 simulation tool. The main objectives of the paper are to investigate how the different faults affect the performance of wind turbines and which differences exist in the structural responses between onshore and floating offshore wind turbines. Several load cases are covered in a statistical analysis to show the effects of faults at different wind speeds and fault amplitudes. The severity of individual faults is categorized by the extreme values the faults have on structural loads. A pitch sensor stuck is determined as being the most severe case. Comparison between the effects on floating offshore and onshore wind turbines show that in the onshore case the tower, the yaw bearing and the shaft are subjected to the highest risk, whereas in the offshore case, the shaft is in this position. Copyright © 2014 John Wiley & Sons, Ltd.     

Modeling aspects of a floating wind turbine for coupled wave–wind-induced dynamic analyses

This paper deals with the numerical modeling of a catenary moored spar-type wind turbine in the integrated coupled analyses. The current spar-type wind turbine is inspired by the Hywind concept. In this paper, different hydrodynamic models based on the Morison formula, Pressure integration method and Panel method considering the mean drift, first and second order forces are studied. A floating wind turbine in deep water depth supporting a 5-MW turbine system is considered. Simo-Riflex (DeepC), HAWC2 and FAST codes are used to carry out the coupled wave–wind-induced analyses. The results show that the damping and inertia forces of the mooring lines are important for the tension responses; especially, the damping of the mooring lines can help to damp-out the high-frequency elastic-deformations of the mooring system. However, the motion responses are not significantly affected by the mooring line damping-effects. The drift and second order forces do not significantly affect the motion and tension responses. However, the heave motion is more affected by the drift and second order forces. The results indicate that either the Morison formula considering the instantaneous position of the structure or first order hydrodynamic forces based on the Panel method and considering the quadratic viscous forces can provide accurate results for the slender spar-type wind turbines. Considering the second order forces is found to be 10–15 times more time consuming while the responses are not significantly affected for the present floating wind turbine. The coupled aero-hydro-servo-elastic code-to-code comparison of HAWC2 and FAST codes shows that the dynamic motion responses, structural responses at the tower–spar interface and at the blade root as well as the power production are in good agreement.     

The dynamics of an offshore wind turbine in parked conditions: a comparison between simulations and measurements

Offshore wind turbines are complex structures, and their dynamics can vary significantly because of changes in operating conditions, e.g., rotor‐speed, pitch angle or changes in the ambient conditions, e.g., wind speed, wave height or wave period. Especially in parked conditions, with reduced aerodynamic damping forces, the response due to wave actions with wave frequencies close to the first structural resonance frequencies can be high. Therefore, this paper will present numerical simulations using the HAWC2 code to study an offshore wind turbine in parked conditions. The model has been created according to best practice and current standards based on the design of an existing Vestas V90 offshore wind turbine on a monopile foundation in the Belgian North Sea. The damping value of the model's first fore‐aft mode has been tuned on the basis of measurements obtained from a long‐term ambient monitoring campaign on the same wind turbine. Using the updated model of the offshore wind turbine, the paper will present some of the effects of the different design parameters and the different ambient conditions on the dynamics of an offshore wind turbine. The results from the simulations will be compared with the processed data obtained from the real measurements. The accuracy of the model will be discussed in terms of resonance frequencies, mode shapes, damping value and acceleration levels, and the limitations of the simulations in modeling of an offshore wind turbine will be addressed. Copyright © 2014 John Wiley & Sons, Ltd.     

An investigation of variable power collective pitch control for load mitigation of floating offshore wind turbines

This paper investigates the loads on offshore floating wind turbines and a new control method that can be used to reduce these loads. In this variable power collective pitch control method, the rated generator speed, which is the set point that the collective pitch control attempts to drive the actual generator speed towards, is no longer a constant value but instead is a variable that depends on the platform pitch velocity. At a basic physical level, this controller achieves the following: as the rotor of a floating turbine pitches upwind, the controller adjusts so as to extract more energy from the wind by increasing the rated generator speed and thus damps the motion; as the rotor pitches downwind, less energy is extracted because the controller reduces the rated generator speed and again damps the motion. This method is applied to the NREL 5 MW wind turbine model, in above rated conditions where the platform motion is most problematic. The results indicate significant load reductions on key structural components, at the expense of minor increases in power and speed variability. The loads on the blades and tower are investigated more generally, and simple dynamic models are used to gain insight into the behavior of floating wind turbine systems. It is clear that for this particular design, aerodynamic methods for reducing platform motion and tower loads are likely inadequate to allow for a viable design, so new designs or possibly new control degrees of freedom are needed. Copyright © 2012 John Wiley & Sons, Ltd.     

Dynamic response and viscous effect analysis of a TLP-type floating wind turbine using a coupled aero-hydro-mooring dynamic code

This paper presents a coupled dynamic motion response analysis of a floating wind turbine using an in-house code, CRAFT (Coupled Response Analysis of Floating wind Turbine). Viscous drag forces on horizontal pontoons are carefully calculated, and a nonlinear spectral method is applied to efficiently solve the coupled tendon dynamics. Viscous drag forces and tendon dynamics are two important factors when assessing a tension-leg platform (TLP)-type floating wind turbine in a time-domain simulator. The analysis object is the NREL 5 MW Wind Turbine, which is supported by a three-leg mini-TLP platform. Simulations of the free decay and response amplitude operator (RAO) tests are conducted using CRAFT as well as FAST, another commonly used code. The obtained results are compared with experimental results to verify the capability of CRAFT. Viscous drag force induces higher harmonic pitch resonance, which is most prominent when the wave period is three times the natural period of the pitch and the wave height reaches a threshold. Springing motion is identified and found to be caused by this resonant pitch motion. Time-domain statistics show that extreme increases in tendon loads caused by springing as well as pitch and tendon tension probability distributions are non-Gaussian in random sea states. In addition, the resonant pitch motion is significantly reduced by aerodynamic damping.     

Validation of a hybrid modeling approach to floating wind turbine basin testing

Hybrid modeling combining physical tests and numerical simulations in real time opens new opportunities in floating wind turbine research. Wave basin testing is an important validation step for floating support structure design, but current methods are limited by scaling problems in the aerodynamic loadings. Applying wind turbine loads with an actuation system controlled by a simulation that responds to the basin test offers a way to avoid scaling problems and reduce cost barriers for floating wind turbine design validation in realistic coupled conditions. In this work, a cable‐based hybrid coupling approach is developed and implemented for 1:50‐scale wave basin tests with the DeepCwind semisubmersible floating wind turbine. Tests are run with thrust loads provided by a numerical wind turbine model. Matching tests are run with physical wind loads using an above‐basin wind maker. When the numerical submodel is set to match the aerodynamic performance of the physical scaled wind turbine, the results show good agreement with purely physical wind‐wave tests, validating the hybrid model approach. Further hybrid model tests with simulated true‐to‐scale dynamic thrust loads and wind turbulence show noticeable differences and demonstrate the value of a hybrid model approach for improving the true‐to‐scale realism of floating wind turbine basin tests.     

Dynamic wind turbine output power reduction under varying wind speed conditions due to inertia

The inertia of wind turbines causes a reduction in their output power due to their inability to operate at the turbine maximum co‐efficient of performance point under dynamic wind conditions. In this paper, this dynamic power reduction is studied analytically and using simulations, assuming that a steady‐state optimal torque control strategy is used. The concepts of the natural and actual turbine time‐constant are introduced, and typical values for these parameters are examined. It is shown that for the typical turbine co‐efficient of performance curve used, the average turbine speed can be assumed to be determined by the average wind speed. With this assumption, analytical expressions for the power reduction with infinite and then finite turbine inertia are determined for sine‐wave wind speed variations. The results are then generalized for arbitrary wind speed profiles. A numerical wind turbine system simulation model is used to validate the analytical results for step and sine‐wave wind speed variations. Finally, it is used with real wind speed data to compare with the analytical predictions. Copyright © 2012 John Wiley & Sons, Ltd.