| The space charges generated by the corona discharge of High-Voltage DirectCurrent(HVDC)transmission lines move directly by the electric field force,forming an ion flow field problem.Space charges will accumulate on the dielectric film covered on the greenhouses and then distort the electric field nearby when the HVDC transmission lines are near the agricultural greenhouses.An in-depth study of the dielectric film charge characteristics in the ion flow field is needed,and a prediction method of the ion flow field needs to be proposed when the HVDC transmission lines are near the greenhouses.At first,the charge characteristics of the dielectric film in the ion flow field and its measurement methods are researched.Based on the probe measurement principle,the influence mechanism of a passive electrostatic probe on the surface charge of the tested sample is investigated by simulations.The mirror charges are introduced in the charge inversion calculation.The degree of the pathology of the inverse equation is analyzed.They provide a theoretical basis for the rational construction of the experimental platform.This paper builds a resistivity measurement platform.The electric field distribution is improving by optimizing the electrode structure of film surface resistivity measurement.The effective measurement of dielectric film surface resistivity is achieved.The influence factors on dielectric charge are clarified based on the simplified charge model.A platform of dielectric film charge characteristics in ion flow field is set up to research the influence law of the ion flow field,the volume resistivity of film,and the surface resistivity of film on dielectric film characteristics.The mechanisms of gas-side conduction and surface conduction charging of dielectric films in the ion flow field are proposed,laying the foundation for modeling dielectric film charging in the ion flow field calculation.Furthermore,based on the dielectric film charge mechanisms achieved by experiments,this paper establishes the calculation model of ion flow field considering dielectric film charge characteristics.The boundary conditions of the charged film are clarified.The saturation charge condition of the gas-side conduction model is proposed that the film surface potential should satisfy the second type of homogeneous boundary condition along the outer normal direction.The control equation of the surface conduction model should follow Ohm’s law.Based on this model,the ion flow field calculation method considering dielectric film charge characteristics is proposed in this paper.The charge accumulation of the film surface is equivalent to the summation of the right terms of Poisson’s equation.The surface conduction of the film is equivalent to the charges transfer between adjacent film nodes.Further,the calculation method is extended to 3-D,while a 3-D treatment of the film charge is proposed.The difference between the 2-D and 3-D calculations are analyzed to clarify the applicable conditions of 2-D calculations.The prediction of the ion flow field of HVDC transmission lines in the vicinity of greenhouses is achieved.Finally,this paper analyzes a practical engineering problem of ±800kV HVDC transmission lines near the greenhouse based on the calculation.The influence mechanism of charged dielectric on ion flow fields is revealed by separating the total electric field based on the relationship of sources and fields.The ion flow field characteristics of HVDC transmission lines considering the charged dielectric film are analyzed.The influence laws of the angle between film and conductor,film length,film geometry,film size,position,support structures,and film surface resistivity on the ion flow fields are analyzed.Based on the above characteristics and laws,a practical method to weaken the ground-level electric field near the charged greenhouse is proposed.This paper’s research provides feasible modeling tools and technical support for optimal design,engineering construction,and environmental assessment when HCDC transmission lines cross or are adjacent to the agricultural greenhouses. |
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