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.
Research on corona inception electric-field strength and critical electric-field values for valve hall fittings
Valve hall fittings' corona characteristics, corona inception electric-field strength and the critical electric-field values are the key factors to consider while designing fittings. Furthermore, the characteristics and value have guiding significance for engineering application. However it is difficult to obtain electric-field strength by theoretical calculation as there are varied types of valve hall fittings with irregular and complex shapes. This paper adopted the research method of combining DC corona test with FEM (finite element method) calculation. Spherical electrodes were tested with a wide range of diameter, thus corona inception voltage and corona characteristic curve of spherical electrode with diameter from 33 to 1000mm was obtained. Based on the corona electric field FEM simulation with the inception voltage as an input, a formula similar to Peek formula and the change regulation of corona inception electric-field strength with electrode diameter were obtained. The article consulted China's national standard modification of fittings' discharge voltage and actual size of fittings in valve hall, and consequently proposed the differentiated critical electric-field values for various fittings. The critical electric-field value is 2000V·mm-1, when the fittings' equivalent diameter is below 200mm, e.g. valve tower shield and arrester grading ring, and for fittings with equivalent diameter above 200mm, e.g. shield ball, bushing shield, the value is 1200V·mm-1. The research method and conclusion of the article will provide a reference for designing fittings.
Finite element formulations of Maxwell's equations-advantages and comparisons between available approaches
The various vector wave equations derived from Maxwell's equations are examined. Formulations involving electric field intensity and magnetic field intensity are explored. The curl-curl equation is developed into a finite-element code. Rectangular waveguides are examined as an example problem. The results demonstrate that errors in boundary conditions can give rise to spurious solutions, while solutions with good boundary conditions do not. For waveguides with attenuating media the spurious solutions do not appear. >