| The composite electric field and ion current are important reference indexes for evaluating the electromagnetic environment of HVDC(high voltage direct current)transmission lines.Due to the limitation of the transmission corridor,there are many objects with different electrical properties adjacent to HVDC lines,such as houses,trees,human bodies,agricultural greenhouses and so on.The existence of these objects will distort the ion flow field near the HVDC lines.In order to improve the electromagnetic environment of transmission lines and the technological level of HVDC projects,it is necessary to study the calculation methods and distribution characteristics of ion flow field when the lines are adjacent to complex objects.The upwind FEM(finite element method),which is widely used in ion flow field calculation,is optimized in this dissertation.The electric field is calculated on the basis of the generalized complementary variational principle,and the space charge density is calculated by Galerkin scheme,which improves the calculation accuracy of 3D problems.An converting relationship of the roughness coefficient between the bundle conductors and the equivalent single conductor is proposed.The corona discharge degree of bundle conductors can be reflected after conversion and the calculation complexity of 3D problems is significantly reduced.On this basis,the typical 3D ion flow field problems including the crossing of two circuit HVDC lines and the HVDC lines adjacent to houses or trees are studied.The distribution characteristics of ion flow field on the ground and in space are calculated when the ±800kV lines crosses the ±500kV lines.The residential house is modeled as a grounding conductor,and the 3D electric field distribution near the house with steeple roofs is analyzed.The difference between 2D and 3D calculation methods is analysed,and the relationship between 2D and 3D electric field distribution of residential houses is summarized.The calculation model of trees in ion flow field is studied.The effects of the shape and location of a tree on the electric field and the field distribution near a forest are analyzed.The calculation and measurement of charging potential of conductor grounded with high resistance in ion flow field are studied.In terms of calculation,in view of the grounding resistance approaches to infinite,a calculation method of charging potential based on the space charge density on the surface of conductor is proposed.In terms of measurement,the application scope of field mill is extended,and the charging potential is measured by non-contact method.The accuracy of measurement values is improved.The shielding effect of metal return lines on ground-level electric fields when the HVDC system is in normally operating.one-pole grounding or potential floating conditions is studied.The charging potential and electrostatic discharge of a metal human body model under the HVDC lines were studied by calculations and measurements.A calculation method of ion flow field considering dielectric film is proposed.The barrier effect of dielectric film on ion current is equivalent to the accumulation and movement of space charge moving to the nodes on the surface of film.The electric field under the film was measured in the laboratory,and the validity of the calculation method was verified.The surface charge of the film is obtained based on the inversion of measured ground-level electric field,which makes the measuring instrument not affect the charge accumulation of film.The measurement results provide further support for the calculation method of ion flow field considering the dielectric film.Influences of the dielectric film of agricultural greenhouse on the electric field near HVDC lines is studied.The research in this dissertation can provide reference for the design,construction and environmental assessment of HVDC transmission lines. |
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