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The converter quadrivalve is one of the key devices in a ±400kV converter station valve hall at an altitude of 3820m in western China. The complex structure of quadrivalve may lead to serious electric field distortion when without well-designed shields. Besides, in view of high altitude, corona discharge is easier to occur, which threatens the secure operation of HVDC system. In this paper, the structure of ±400kV converter quadrivalve is introduced. A 3-D electric field calculation model of three quadrivalves with corona shields was built with the finite element method (FEM) analysis software ANSYS. Considering the influences of hybrid AC-DC operating voltages, the FEM transient electric field analysis with discrete loading voltage waveform is adopted. The distributions of potential and electric field on quadrivalve corona shields were calculated in a period under normal operating condition. A series of DC corona tests on typical electrodes were carried out at different altitudes in order to obtain the altitude correction formula for DC corona inception electric-field (E-field) strength. The correction formula is determined through the statistical analysis of test results, which provides a reference for the electric field evaluation. Based on the FEM calculation results, the maximum surface E-field strength on the quadrivalve corona shields in the ±400kV valve hall is lower than the critical value. Hence it can be concluded that the design of the corona shields satisfies the external insulation and electromagnetic environment requirements in high altitude area under normal operating condition.

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Epoxy Resin is one of the most popular insulation materials used in the electrical equipment. In order to enhance the toughness of epoxy resin, the elastic rubber is usually added into the formulation system, but the dielectric properties of the epoxy resin will be changed while the toughness is enhanced. In this paper，hydroxyl-terminated butadiene nitrile liquid rubber (HTBN) is used to enhance the toughness of epoxy resin. By comparing the parameters of glass-transition temperature, AC conductivity and relative permittivity with different weight proportions of HTBN, the changes of dielectric properties of epoxy resin polymer is studied, and the action mechanism of the rubber particles as the "Island" structure is analyzed. The study shows that, after the HTBN is added, the glass transition temperature decreases significantly, the relative permittivity and the AC conductivity rises with the increasing of the proportion of HTBN mixed in the epoxy resin. The relative permittivity and the AC conductivity shows a peak in each of the curves. The main reason for the change of dielectric properties is that the relaxation polarization of the natural dipole moment in the rubber particles occurs at the certain temperature and frequency. The research result shows that, when add the toughened rubber particles to the epoxy resin formulation, the impact to the dielectric properties should be considered and used.