Extended numerical simulation model for HVDC gas insulated systems considering gas humidity and gas convection

Purpose The application of high voltage direct current (DC) gas insulated lines in the power grid is expected to increase in the near future, due to its advantages considering safety, compactness and high voltage capacity. However, there are challenges in the design of the system which need to be addressed, such as space charge accumulation and high electric field stress. Therefore, precise simulation models of the gas insulated line need to be developed for examination purposes. One important aspect is the accurate representation of the conduction behavior of the insulating gas. Factors influencing the electrical conductivity, such as electric field strength, temperature, gas pressure and gas humidity, must be considered. In addition, an accurate determination of the temperature is essential for precise simulation results, since practical applications show different temperature distributions in the gas, depending on a horizontally or vertically arrangement of the gas insulated line. This study aims to develop a simulation model, which incorporates such phenomena. Design/methodology/approach A three dimensional model of a high voltage DC gas insulated line is developed, with the application of a nonlinear electrical conductivity model for the insulation gas, sulphur hexalfluoride. The conductivity model is derived from experimental measurements. The electric field distribution is investigated with a focus on the humidity content of the gas. For the determination of the temperature distribution, heat conduction is considered in the solid domain of the system and heat convection and heat radiation is considered in the gaseous domain, where the dynamic gas flow is explicitly modeled. Findings The simulation results reveal distinct electric field distributions for dry (2% humidity) or humid gas (30% humidity), where the difference of the maximum electric field value is up to 35%. The inclusion of heat convection and thermal radiation in the gaseous domain allows to consider the effect of a vertical or horizontal arrangement of the gas insulated line, where higher temperature values are seen on the upper parts in a horizontal construction, compared to a vertical arrangement. However, the consideration of such physical phenomena increases the complexity of the model and results in up to 12 times longer computation times. Originality/value In most scientific studies, the electrical conductivity of the sulphur hexalfluoride gas is assumed to be constant, and heat conduction is considered the dominant heat transfer mechanism throughout the system. In contrast, the present work describes the conduction behaviour of the gas by a nonlinear electrical conductivity model, e.g. enabling a comparative analysis of dry and humid gas conditions. In addition, heat convection and thermal radiation is considered as a heat transfer mechanism in the gaseous domain, to account for the influence of the geometrical configuration of the system on the temperature and electric field distributions.