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.