Research Of Dynamic Characteristics Of Surface Charge On Dielectric Film Under The Needle-Plate Electrode

High Voltage Direct Current(HVDC)transmission system has unique advantages for long-distance and bulk-power transmission,it can optimize the allocation of resources,thus solve problems caused by unbalanced energy resource distribution and economic development.So HVDC projects are composing higher proportion in Chinese power grid.Together grows the public awareness of electromagnetic environmental problems.In a big agricultural country like China,greenhouses are widely used.With the construction of HVDC,HVDC transmission lines may pass over the greenhouses.When the greenhouse is under the HVDC transmission lines,charges generated by corona discharge can distort the electric field.Some charges will accumulate on the greenhouse,and can't be measured online,so this boundary condition is unavailable when modeling the ion flow field.With the off-line measurement of the surface charge companies charge dissipation,so the dynamic characters of surface charge on dielectric film should be studied.In this paper,an experimental platform is designed to charge the suspended film and measure potentials below it,with which surface charge distribution can be obtained.Thus the accumulation and dissipation process of polyethylene film are studied.The effects of polarity,magnitude and charging time of applied DC voltage on charge accumulation are studied.Experiment results shows that the charge accumulation saturation time is about 40 seconds.The saturation charge increases linearly with the increase of charging voltage,and more negative charges will accumulate on the film under the same voltage.The ways and affecting factors of surface charge dissipation are also analyzed.It is proposed that the main way of surface charge dissipation is through air.The dissipation curve is exponentially decaying,and the time constant in exponential fitting can be used to characterize the dissipation speed.The dissipation speed of charges is affected by the initial charge,temperature and humidity.Through the regression analysis of experimental results,regression equation of dissipation time constant is obtained.With the equation,influences of these factors on the dissipation velocity are studied quantitatively,thus the dissipation velocity under any combination of these factors can be predicted,thus the result of off-line measurement of surface charge can be more accurate considering the dynamic characters of surface charge,so as to further study the accumulation character of surface charges.