Wind Energy

To study wind characteristics in complex terrain, we installed three dimensional sonic anemometers at 67 m and 45 m above ground level in Iwaya wind park of Aomori prefecture in Japan. High resolution analysis of wind direction shows that the flow distortion arises along the undulating topography. Absolute values of vertical wind speeds observed at lower observation point are more or less larger than those at higher point, which is dependent on wind directions. The difference of vertical flow between different heights would relate to increase of the wind shear and the turbulent intensity. Due to such flow characteristics and large roughness on the ground, the turbulent intensity seems to be larger than IEC category A, in the wide range of wind direction keeping steep slope and complex topography.

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In this study, the relationship between tightening torque and axial force of bolt at the tower top flange was investigated through a laboratory test with flange section models and a 750 kW wind turbine which had experienced the damages of bolt at the tower top flange. Tightening torques and axial forces of bolt in these tests were measured directly by using torque wrenches and strain gauges inserted in the bolts. Existence of lubricant had a great influence on the axial force of bolt, while types of torque wrench had a small impact on those. The expected axial force of bolt was obtained by torque wrenches and the variation was around 10%. A method for quantitatively evaluating decrease in axial force of bolt was proposed by measuring rotation of bolt. The damaged bolts in the wind turbine were also investigated and the axial forces at the damaged bolts were completely lost, while those at the loosed bolts were more than 30% of the initial value. It was found that the axial force of bolt decreased by about 20% after 500 hours from the exchange of bolts. This indicates that retightening is necessary for wind turbine tower bolts.

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We conducted feasibility study of detection system for unexpected lightning damage for wind turbine blade. In this study, puncture of blade skin and disconnection of down conductor are considered as the lightning damage. Aimed at detect these damages, three systems are examined. The first system consists of two down conductors and current measuring devices that installed at root part of the conductors. The first system is appropriate to detection of puncture of blade skin caused by lightning strike of wintertime thunderbolt. The second system consists of two down conductors and optical fiber current sensors which are installed at tip and root of each conductors. The second system has the merits of enable to simple judgement system for lightning damage and periodic inspection (to confirm the soundness of the down conductors). The third system is based on acceleration measurement by sensors that installed at the blade skin. The acceleration sensors measure the elastic wave caused by lightning strike (puncture of the blade skin). Feasibility of the third system was confirmed by model experiment. The third system can be retrofit to an existing wind turbine blade.

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L-type flanges used for wind turbine towers need high bearing resistance when they install in storm and earthquake active areas. Petersen, Seidel, and RISO/DNV introduced methods for evaluation of L-type flange ultimate strength based on elastoplastic theory, however they didn’t consider the elastic deformation of flange and caused underestimation of the load acting on bolts. VDI2330, Petersen, and Seidel also showed the location of equivalent flange reaction force by using flange thickness direction bending moment equilibrium, but these methods are complicated. An action point correction factor of L-type flange is proposed to consider the effect of flange deformation and to expand the applicable range of flange dimension. The results show good agreement with those by the formula of Seidel at the ultimate condition by using load transmitting factor for L-type flange proposed in this paper.

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