Research On Mechanism And Prediction Model Of Icing On Overhead Transmission Lines

As one of the most serious threats to the safety of power grid,icing on overhead transmission lines has a big influence on the operation of power grid.The accidents caused by icing bring huge economic losses.It's necessary to do research on icing mechanism and prediction model,which is helpful for design of transmission line,icing prediction and develop of anti-icing and de-icing technology.However,there are still problems existing in the study of icing mechanism on transmission lines.The theory of icing is not good enough and the original icing prediction model is not so accurate and effective.Regarding the key problems of icing,the mechanism of icing and the prediction model were studied.The main contents and achievement are as follows,(1)First,the governing equations for air and water droplet were built based on fluid mechanics theory.And then the method to calculate the collision efficiency was presented.Second,the heat transfer equation for icing on the conductor surface was improved based on heat transfer theory.An improved method to calculate the freezing fraction was proposed.Finally,the method to calculate the ice mass and shape was proposed based on both above,and the icing prediction model was presented.Compared with the experimental results,the simulated results were validated.(2)Considering that the conductor shape is usually simplified as a circle,the strand model which is closer to the shape was established.The collision efficiencies by the circle model and the strand model were calculated.The errors caused by the circle model were analyzed.Besides,the influence on the collision efficiency by the conductor shape was analyzed.The results show that the collision efficiency calculated by the circle model is less than the one by the strand model,which will lead to a big difference for the ice mass and shape.The change of the conductor shape caused by changing the number of the conductor strands,will result in the change of the collision efficiency,and this change is influenced by the environment conditions.Therefore,the strand model is recommended to improve the accuracy of the calculation.And the best conductor strands number and layout can be calculated to reduce the ice mass when designing the transmission lines.(3)Considering that the existing theory is not suitable to calculate the collision efficiency at a small wind velocity and a large conductor,a new method was proposed by building the governing equations including the history term and applying the size distribution of the water droplet.The experiments were carried out in the wind tunnel and the theory was validated by comparing the theoretical and experimental results.(4)A novel water droplet size parameter(Volume Weighted Diameter,VWD)was presented.VWD is simple in calculation and has a good approximation.Compared with Median Volume Diameter(MVD),VWD has a more stable performance than MVD.VWD has an almost fixed relative error 10%,while the relative error by MVD changes from 90%to 1%.When the collision efficiency is small,WD has a higher approximation than MVD.Mathematical explanation for this was given,and the limitation of this parameter was discussed.(5)First,the model for icing on bundled conductor was established,and the ice mass and shape was calculated.Then,the results were compared with those on a single conductor.At last,the experiments were carried out to verify the conclusions.The results show that ice accretion on the windward conductor is the same as that on single conductor.They have the same ice mass and shape.For 2-bundled conductors,when the connect line of two conductors is not in the same direction with the wind velocity,the windward conductor does not have an impact on the leeward conductor.Two conductors then have nearly the same ice mass and shape.When the connect line of two conductors is in the same direction with the wind velocity,the windward conductor has an impact on the leeward conductor.Comparing with the windward conductor,the leeward conductor has a smaller local collision efficiency and ice mass.This impact is influenced by the distance of two conductors.When the distance increases,the differences of collision efficiency and ice mass between two conductors decrease.