| The society becomes increasingly rely on the electricity with the development of society.The result from observation shows that the number of lightning strike incident increases with the height of the object.Therefore,how to protect the power equipment against lightning damage,and make sure the safe and stable operation of the grid and avoid distribution accidents become an urgent problem.As for the widely use electric geometric model from the lightning protection standard,for the reason of oversimplifying and equivalent to the physical process,hence lack of clear physical meaning,result in a large difference between the calculation result and the actual phenomenon finally.So,based on the series of problems,this work intends to start with the physical process of attachment,establish the upward leader inception and development model to obtain accurate striking distance and attractive radius,to improve the prediction of lightning strike probability that is closer to the fact,and provide the guidance to the lightning protection work.This work starts with the physical inception process of the positive upward leader.Establish the stem-leader thermal transition model based on the positive leader thermodynamic model and the energy balance equation considering the cone shape transition region.Then a simplified inception and development model of upward leader was established based on the finite element method,and discussed the subsequent development state after upward leader initiation under the influence of the stepped leader.A more precise calculation method of strike probability for wind turbine is proposed by using the model which verified with the observation data.At last,the influence of rotation of wind turbine blades on the discharge corona charge ability is carried out by using the high voltage test platform.A model of conical stem-leader thermodynamic transition was established.Based on the finite element method by and adopting the variable mesh technique.The inception condition of the upward leader which considering the thermal dynamics and the second corona is proposed.The simulation result shows that the more corona charge is generated during the initial corona process,it accelerates the transformation process but on the other hand the increasing space charge will enhance the shielding effect,which result in the second corona delay.The stem-leader transition time is very sensitive to the air humidity,the higher humidity condition would take less time to convert.It can achieve the thermal transition temperature of 1500 K with only 2 μs when the humidity is 20 g/m3,but the process could take over 13 μs when the humidity reduces to 20 g/m3.The upward leader inception simplified model is proposed by using the method of simple geometric analysis of the potential distribution to calculate the second corona charge.Compared with the results of strike distance between the thermodynamic transition model and the upward leader inception simplified model,there is only 3.61%difference at the height of the building at 90 m.The relationship between attractive radius and structure height and return stroke peak current is also established.The simulation result shows that the traditional electric geometry model is not fully satisfied the need of protection when strikie to the tall building.The development status of the upward leader after inception is simulated,and the calculation method of attractive radius for wind turbine considering the upward leader development is proposed.The simulation results are compared with observation data which shows the velocity and the length of the upward leader are consistent with the observed values,the reliability and accuracy of the model are verified.Then the influence of the return stroke peak current,the speed of stepped leader and the relative position of the channel on the upward leader is also discussed.The result demonstrates that the upward leader channel length is negative correlated with the speed of the stepped leader.The final length of the upward leader channel is 134.79 m when the stepped leader speed is 1×105 m/s,and the channel length is only 32.40 m when the leader speed reach 1×106 m/s.Meanwhile,the maximum speed of upward leader is 7.8×104m/s when the return stroke peak current is 15 kA,and as the return stroke peak current reach 50 kA the speed of upward leader accelerate to 1.8×105m/s.The simulation result is compared with the electrical geometry model which shows the result form our model is 77.03%higher than EGM;in response to the lower return stroke peak current,the difference still shows 41.14%.The attractive radius is calculated by using the upward leader inception and development model,and a more accurate equivalent lightning collection area is obtained for the wind turbine.In addition,the number of receive direct lightning strike is estimated,,the result shows that there is only a difference of-7.1%between the calculated value and the average observation(0.7761 times/year).The accuracy of the predict model of lightning strike probability for wind turbine is verified.This paper discusses Then the influence of the different rotation angle of wind turbine blades and the return stroke peak current and the speed of stepped leader on equivalent lightning collection area is discussed.The results indicate that calculation method proposed by IEC 61400 standard does not include any physical basis that leads to overstating the equivalent lightning collection area.Due to the upward leader channel is not considered to the calculation of lightning strike distance in IEC 62305 standard,which leads to the equivalent lightning collection area is rather small.The maximum difference between calculation result with the IEC 62305 standard is 313.12%.The rotation of the wind turbine blade can change the structure of the corona shield.A research is carried out to study the influence of rotation factors under the specific electric field environment with indirect measurement of corona current by using high voltage platform.According to the description of conceptual model,the corona layer to be thin due to the rotation of the blades.The smaller the charge above the tip,the higher the electric field will be.The results obtained through experiment shows that,the strongest ability of release corona charges into the surrounding space under the static blade rotation angle for 0°,which gradually decreases with increasing rotation angle,the difference is between 10.3%to 16.2%.Because of the rotation can enhance the local electric field of blade tip,which is also enhance the ability to release corona charges into space,resulting in the measurement of the corona current under rotating condition is larger than the average value under static state. |
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