Modeling of the Electric Field in High Voltage Direct Current Gas Insulated Transmission Lines

Abstract

High voltage direct current (HVDC) gas insulated systems are utilized when space is limited. The composite of SF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> gas insulation and solid spacer insulation results in high electric field values in the vicinity of the sheath under DC conditions. To reduce high field values functionally graded materials (FGM) are applied, where the electric conductivity of the spacer $\kappa$ shows a defined spatial variation ($\kappa$-FGM). Numerical simulations are used to analyze the spatial variation. In order to enhance the accuracy of the results, models for the electric and thermal conductivity of the SF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> gas are developed.Measurements from literature show a significant temperature dependency of the electric and thermal conductivity of SF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> and a negligible electric field dependency in the expected field range of ≤12 kV/mm. Considering a two-dimensional radial symmetric gas insulated transmission line, a numerical analysis of $\kappa$-FGM indicates that a spatial variation in r-direction, compared to variations in z-direction, has a higher influence on the electric field and can reduce maximum fields by a factor of up to 2.7.