| With the continuous expansion of energy demand and the transformation of energy structure,the exploration of clean and renewable energy has become a top priority.President Xi Jinping proposed the strategic goal of "carbon peak by 2030 and carbon neutral by 2060",which points out the direction for the next stage of China’s energy transformation.Wind energy is one of renewable energy,which has the advantages of clean,environmental protection and abundant reserves.It has been vigorously developed by all countries in the world and has a good development prospect.Large-scale horizontal-axis wind turbines are currently the most widely used wind turbines.They usually run in the upwind direction,and use a nacelle wind anemometer to measure the real-time wind speed.The conventional nacelle wind anemometer is located downstream of the rotor and on the upper surface of the nacelle.The incoming flow reaches the position of the anemometer after passing through the blades,and the measured wind speed will be different from the actual incoming wind speed in front of the rotor,showing strong unsteadiness.To improve the accuracy of the nacelle wind measurement,it is necessary to conduct a detailed analysis of the flow structure above the nacelle.In addition,the operation control and power prediction of wind turbines usually use 10-minute average nacelle wind speed data.This time scale is suitable for some wind farms with gentle wind speed fluctuations;but for wind farms with strong wind speed fluctuations,problems such as low operating efficiency of wind turbines caused by untimely adjustment response and the inability to avoid extreme wind conditions in time will occur.In the current studies,taking a 2MW wind turbine as the research object,the minimum average time scale of wind turbine response control under three wind conditions:high wind speed,low wind speed and rated wind speed,is studied firstly using CFD simulations.It is found that under the premise of ensuring the accuracy of wind speed measurement,the average time scale can be reduced to a minimum of tens of seconds.The frequency spectrum analysis of the nacelle wind speed is performed in order to study the flow structure above the nacelle under steady and time-varying incoming flow conditions.It is found that besides the fluctuation of the incoming flow,the rotation of the blades has the greatest influence on the nacelle wind speed.The flow around the nacelle and the hub,and the flow separation,also have a certain degree of influence on the nacelle wind speed.The frequency domain data onto the nacelle wind speed is filtered using low pass filter.After filtering,the nacelle wind speed can restore the incoming wind speed to a certain extent.The factors affecting the nacelle wind speed are studied.The study found that the relative rotation of the blades and the nacelle enhanced the unsteady characteristics of the flow.The incoming flow produces to flow separation from the root of the blade,and the separation vortex continues to develop downstream with the wake of the blade.The flow around the hub due to the presence of the nacelle hinders the tendency towards fluid to develop behind the hub,causing the flow structure near the nacelle to become more complicated.At the same time,the nacelle geometry also restrains the spread of the blade root separation flow to the rear of the hub,making it develop along the nacelle wall.When the separated flow pass through the anemometer,the measured nacelle wind speed is lower than the incoming flow wind speed.Above the current nacelle anemometer,there is a certain height of the upper surface of the nacelle where the flow structure is more stable.At this position,the wind speed fluctuation reflects a strong periodicity,indicating that the wind speed here is only affected by the rotation of the blades,and the fluctuation range is small,which is more suitable for the installation of the nacelle anemometer.The current studies provide theoretical support for wind turbine optimal control method based on nacelle wind speed. |
PDF Full Text Request