The Evolution Characteristics Of Space Charge In The Neighboring Domain Of Rotating Wind Turbine Blade Under Positive/Negative Thundercloud And Lightning Leaders Propagation Model

With continuous increase of the wind turbine capacity,the height of the wind turbine tower and blade length grow significantly.Generally standing in a very high position,the wind turbine blades are prone to lightning strikes.The presently lightning protection technology for wind turbines can’t provide enough and effective protection ability for the rotating wind turbines,which is due to some principal factors,including unclear distribution characteristics of the space charge near the rotating blade tip and unclarified lightning attachment mechanism with rotating wind turbines,as a result,optimal design of the lightning protection system for rotating wind turbines still lacks of theoretical support.To properly address the above pending issues,this thesis makes comprehensive use of both experimental research and simulation modeling as to systematically study the spatial charge distribution feature in the neighboring domain of rotating wind turbines,the lightning attachment characteristics as well as lightning risk assessment methodology of the attachment system for rotating wind turbines,which is expected to provide theoretical basis for secure and reliable operation of the wind turbines from the perspective of effective lightning protection design.A combined convection and diffusion model with dynamic meshing of the charged particles in the neighboring space of a large-scale rotating wind turbine under thundercloud stress was established,which is used to clarify the impact of blade rotation on the space charge distribution near the blade tip,and to characterize the distribution of the charged ion flow in the neighborhood as well as its impact on the spatial electric field.A scaled-down experiment for rotating blade tip corona was conducted to validate the proposed model.It is found that,the space charge in the neighborhood of the rotating blade tip shows an "ellipse-like" distribution,and a charge deflection coefficient was presented to characterize the deflection of the charge distribution near the rotating blade tip,based on which the deflection of the space charge was found to increase with the growth of the rotational speed.The electric field variation on the blade tip affected by the space charge distribution at different rotation speeds and angles was also analyzed to derive the dynamic corona initiation characteristics at the rotating blade tip.A Ushaped critical curve was defined by analysis of the combined impact of the rotating speed and angle on the electric field of corona inception,from which a reliable rotation speed variation scope of the wind turbines under different thundercloud fields was achieved.An experimental platform for rotating blade tip corona observation was established so as to study the corona discharge characteristics under either positive or negative thunderclouds determined electric fields.Compared with the random pulse discharge pattern for stationary wind turbines,the rotating blade tip was found to present a corona pattern with periodically separated discharge clusters.Further study showed that,the corona discharge current and its repetition rate increased with the rise of rotation speed.Typical discharge patterns of the rotating wind turbines were determined by analyzing the corona discharge spectrum,namely,positive corona presented periodically separated strong discharge clusters,while negative corona gave separated discharge clusters superimposed by weak continuous discharge pulses.Combined with the corona characteristics of wind turbines under different lightning polarities,a calculation method for potential distortion coefficient induced by the initial streamer area at the rotating blade tip was put forward,and a upward leader initiation model for rotating wind turbines based on the dynamic potential distortion coefficient was further proposed.In the meantime,a potential-weighted lightning current shunting model for branch channels was established,based on which the three-dimensional stochastic development process of the lightning downward leader was implemneted.Hence,dynamic simulation for the whole process of the lightning strike to wind turbines was realized by combined numerical calculation of the upward leader initiation and the downward leader development.Based on the simulation of the wind turbine lightning attachment process under different rotating angles with vertical direction,it was found that,the lightning strike distance decreased with the increase of the blade angle.Furthermore,the lightning strike distance turned to be shorter under positive lightning leaders than that of the negative ones,which cases of the latter rendered more difficult to initiate the negative leader at the blade tip.An exemplar analysis was carried out for a 125m rotating wind turbine,which indicated the lightning strike distance increased with the increasing rotational speed,which confirmed that rotating wind turbines are prone to attach lightning strikes.The effectiveness of the model was verified by the statistical observation data from in-situ wind farms.According to the simulation model of the whole process of lighting strike for rotating wind turbine,a calculation method of lightning shielding failure rate for the attachment system on blades considering spatial charges evolution was proposed as to quantitatively asses the lightning failure risk of the attachment system.In addition,for the cases under atmospheric impacts,it was found that,an increase of the relative air density and humidity,or with salt fog adhesion conditions,would enhance the lightning shielding failure rate of the attachment system on the wind turbines.To counteract the deficiency of the conventional lightning protection system prone to form a band-like risk area on the blade surface,an optimized design with metal mesh-covered blade attachment system was presented.The lightning shielding failure rate of the optimized design was quantitatively evaluated,and its lightning shielding effectiveness was verified by long gap outdoor discharge experiments.The above research work provides theoretical basis and also effective methodologies as to enhance reliable operation of the rotating wind turbines under thunderclouds conditions.