E-field distribution analysis on three types of converter double valve in 800kV valve hall
Converter valve possesses the function of AC-DC voltage conversion and power transmission. It is an important device in the valve hall of UHVDC converter station. Since the corona discharge has a serious impact on the safe and stable operation of converter valve, it is necessary to analyze its surface electric field (E-field) distribution. In this paper, the structure characteristics of three typical double valve designs have been analyzed, which were respectively used in three high voltage (400-800kV) valve halls of one ±800kV UHVDC transmission project in China. Based on the project drawings, the equivalent model of each type of double valve with shielding structure was built with three-dimensional (3D) finite-element method (FEM) software ANSYS. Considering the interaction of double valves in actual valve hall, three double valve FEM models of same type in one hall, which formed the upper 6-pulse bridge in the 12-pulse converter unit, were built for the analysis of E-field distribution. Due to the hybrid AC-DC operating voltage, the transient E-field analysis with discrete loading voltage waveforms was adopted in these models. After calculation, the E-field distribution characteristic of different double valve types were summarized and compared under the normal operating condition. The results showed that the surface E-field strength on the shielding structure of all the double valves was lower than the critical value of corona inception and could meet the project requirements. This research could provide valuable references for shielding structure design and optimization of converter valve in the UHVDC transmission project.
Electric field calculation and optimization of the UHVDC converter valve shield case
The converter valve is one of the key equipments in the ultra-high voltage direct current (UHVDC) transmission systems. Shield case design is an important part in the process of converter valve design, appropriate shield case configuration can improve the electric field distribution of converter valve, extend the service life and enhance the long-term operation reliability of the UHVDC systems. In this paper, an electric field distribution equivalent calculation model of the double converter valve was discussed, and a 3-D transient calculation model of ±800 kV double converter valve was established by using a finite element method (FEM) analysis software ANSYS. The calculation results of potential and electric field distribution of the double converter valve were presented, the law of electric field distribution of the shield cases which run on different working conditions and locate in different positions of the double converter valve was revealed, the effect of electric field and potential distribution of different shield cases configuration schemes was discussed, and an optimal scheme was put forward. Verified by the computer simulation, the optimal scheme satisfied the external insulation and electromagnetic environment requirements of the double converter valve. The results provide an important basis on the design of the UHVDC double converter valve.
Transnational variation in linguistic politeness in Vietnamese : Australia and Vietnam
Over the past three decades, the Vietnamese language has undergone substantial changes, both in Vietnam, and in diasporic contexts such as Australia. Yet the nature of the variation resulting from those changes at the sociopragmatic level in expressing politeness is little researched. 
The question of whether there are differences in the politeness expressed by Vietnamese speakers living in Vietnam and Australia is the focus of this research.