| In order to ensure the supply of energy sources and the full utilization of the resources, long distance transmission of electric power by way of ultra high voltage (UHV) transmission lines have been well developed in China. Meanwhile, corona discharge occurs easily on UHV direct current (DC) transmission lines due to high field strength, especially under harsh weather conditions. As a result, the ionized field is formed and the influence of it on the electromagnetic environment and the corresponding ecological effect become a subject concerned deeply.The fog-haze is a kind of air pollution that frequently occurs in recent years. It contains a large amount of suspended particulate matters and different water droplets under different humidity conditions. These are the key factors causing the corona discharge. Therefore, it is of important theoretical and actual significance to study the corona discharge on ultra high voltage direct current (UHVDC) transmission lines and the ion flow field due to the corona with consideration of the fog-haze.In this dissertation, the effect of the fog-haze on the corona occurred on UHVDC transmission lines and the corresponding ion flow field have been studied numerically. A model of calculating the corona on UHVDC transmission lines and the corresponding ionized field which taking into account the fog-haze effect has been structured. Under the different fog-haze situations, different arrangements and parameters of conductor, the ground-level total electric field strengths(TEFSs) and the ion flow densities (IFDs) for a ±800kV UHVDC transmission line have been calculated and analyzed with the use of the finite element method (FEM). This dissertation includes the following contents and obtained results:1. The basic principles and characteristics of the corona discharge in the fog-haze weather are introduced. The adoption of the model, which dividing the fog-haze into fog and haze, is suggested in the relevant calculations of this study. The effect mechanism of the fog-haze on the corona discharge is described through three factors, i.e. the surface situations of conductor, the field inception and the ion mobility.2. A model of calculating the ionized field when the corona occurs on UHVDC transmission lines in the fog-haze environment has been structured, and the calculations for the TEFSs and the IFDs due to the corona are implemented by means of finite element method (FEM) with considering the structured model. The effects of different fog-haze situations on the ion flow field induced by the corona on a ±800kV UHVDC transmission line have been calculated and analyzed. The obtained main conclusions are as follows:(1) The TEFSs and IFDs near ground increase with the pollution degree of the fog-haze weather with high humidity, and the growth rate of their respective maximum value in comparison with those in fine weather is in a behavior of linearly increasing with the pollution degree.(2) The variety of the TEFSs and IFDs in the dry haze weather or in the fog-haze weather with low humidity with the pollution degree is similar to those with high humidity, but the increase of their respective amplitude relative to those in fine weather is very small.(3) The influence of the humidity on corona inception and the charging effect of the fog-haze particles are the main reasons for the variation of the TEFSs and IFDs near ground, and the change of the ion mobility restricts the growth of ion flow density under the fog-haze weather. Thus, the ion flow density is smaller than the limited maximum value in relevant national standard, in spite of its increase.3. Based on the different design parameters, which refer to conductor arrangement, conductor height, horizontal and vertical spacing for the same-tower double-circuit ±800kV transmission lines, and taking into account the different pollution degrees of the fog-hazy weather with varying humidity, the ionized field due to the corona has been investigated numerically. The following results are obtained:(1) With different conductor arrangements, the differences between the TEFSs and between the IFDs are minor for different pollution levels below the 4th grade. However, as the pollution degree rises, for vertically-layout double circuits, when the conductors with the same pole arranged on the same side, it shows a better ability to suppress the change of the ground-level field effect.(2) Conductors with larger cross section can be favorable to the improvement of the ion flow field in heavier fog-haze weather. But, the external diameter of conductors should not be too large in engineering for the reason that the field effect is enhanced when the pollution becomes above grade 6.(3) It is not an ideal way to suppress the ionized field effect in the fog-haze environment by increasing the spacing between circuits, thus the circuit lines should be as close as possible if meeting the requirements of insulation. Though the increase of spacing between poles can suppress the ionized field effect to some extent, the pole spacing still should be limited to a certain range to satisfy the line corridor limits.(4) The influence of the conductor height on the corona-caused field is greater than that due to the pollution degree in the fog-haze weather. Therefore, it is an effective way to suppress the TEFSs and IFDs in the fog-haze condition via increasing the conductor height. |
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