The Polarity Effect Of Lightning Attachment On Wind Turbine Blade And Risk Assessment Method

The renewable energy has become the future direction of the energy source,due to the increasing demand of consumption,the shortage of fossil fuel and the accelerated greenhouse effect.The wind power generation is one of the most promising technologies among all the renewable energy sources.However,it is the lightning accident that is one of the most serious threats to the safe,stable and economic operation of wind farms,where the damage of the wind turbine(WT)blade caused by lightning is striking.The existing lightning protection system(LPS)faces new challenges because of the increasing of the size and capacity of wind turbine.The reason lies that the design of LPS of WT blade is based on empirical principles,which indicates that the fundamental research is badly needed.In order to reduce the probability of lightning accident of wind farm,the performance of WT blade LPS should be improved and the design should be optimized.In this thesis,the attachment manner of wind turbine blade was investigated by experimental method.The reason of LPS interception failure was analyzed.A method of lightning risk assessment was proposed.The conclusions of this thesis show prominent theoretical value and guiding significance for the design of wind turbine blade LPS.The experimental platform for lightning attachment manner of wind turbine blade was built.The long spark experiments were conducted to simulate natural lightning attachment process.The influence of different blade orientations,lateral distances of downward leader(DL)and polarities of discharges on the attachment manner were investigated.The type of attachment point was also classified.A strong polarity effect of attachment manner was found by the experiments,which shows that under positive impulse,it is more difficult for upward leader(UL)to initiate from receptor,the attachment point moves away from blade tip,and the interception efficiency of receptor decreases dramatically.The influence of space charge on the leader inception under different polarity was studied,and the mechanism of polarity effect was explained.Three typical patterns of interception failure were concluded.The positive lightning strike is more dangerous,compared with the negative,which may lead to LPS failure and the blade damage.In order to investigate the influence of blade rotation to the attachment manner,a scale wind turbine was built,and the same blade tip linear speed was used to simulate the same operation condition of real wind turbine.The rotation influences both the process of attachment and return stroke.During the attachment process,due to the rotation,the receptor close to the blade tip fails to initiate upward leader and the attachment point moves away from blade tip.The performance of LPS declines.Besides,during the return stroke process,the path of discharge channel near the attachment point changes into arc-shape on surface of blade,which causes damage on WT blade.There is also a strong polarity effect under the rotation condition,which the influence of rotation is prominent under positive impulse but insignificant under the negative one.The mechanism of polarity effect was studied from both the characteristics of leader inception and the attraction capability of upward leader.A physical model for negative leader was developed,which can successfully simulate the propagation of stepped leader,including the space stem,space leader and primary leader channel.The distribution of space charge for different part of discharge channel was analyzed,and the method to simplify the calculation of space charge originally from positive leader model was used.Several different long spark experimental results were used to verify the proposed model,and the results fit well.The attachment process of natural positive lightning flash was simulated and the results are consistent with field observation.A criterion for negative upward leader inception was established.The model and criterion in this thesis provide a useful tool to analyze the polarity effect of attachment process.The leader propagation model was improved by considering the influence of downward leader to the local electric field,and taking the relationship between the leader velocity and the charge to sustain the leader channel into account.The condition when unstable leader transforms into the stable one was also improved.A new criterion for the inception of upward leader,called dynamic critical length of initial streamer,was established,which is the expansion of the existing criteria.The proposed criterion also reveals the relationship between the leader velocity,peak current and the condition necessary for the inception of upward leader.Besides,a new method of lightning risk assessment for wind turbine blade was developed by considering the different difficulty of UL initiation on blade surface.The influence of blade orientation,the type of receptor and the lateral distance of downward leader on the performance of LPS were analyzed.The comparison of the distrubtion of lightning risk factor of wind turbine blade under positive and negative lightning strikes was made.The influence of polarity effect on the performance of receptor was investigated.The proposed lightning risk assessment method provides a useful tool for the optimal design of wind turbine blade LPS.