Time constant evaluation of transient AC – DC field distribution

HVDC (High Voltage Direct Current) technology is particularly suitable for the transmission of high-power energy over long distances. However, some technical/technological problems have to be addressed/managed when passing from HVAC to HVDC due among others to the fact that the electric field distribution in gas insulated systems (GIS) operating under DC stress is very different from AC conditions. For a given geometry, the electric field distribution, under DC stress, changes from a capacitive state which depends only on permittivity to a resistive state which depends on both permittivity and conductivity of gaseous and solid insulations. Therefore, the duration of AC – DC transition in a GIS is governed by conductivities and by geometries and time constant for this transition can be defined.

This paper focuses on the time constant of transient AC – DC field distribution. As the resistive field depends on the conductivity of solid and gas surrounding in the GIS, these parameters need to be evaluated. Therefore, the physics of nonlinear conduction in the gas is considered as result of natural charge generation by radiation and recombination. The time constant of the system is then evaluated using simulation model that takes into account gas model, the surface and bulk conductivity of solid insulator. The influence of different parameters as the nonlinear conductivity, the shape and dimensions of solid insulator are investigated. Variations of the surface and bulk conductivity of solid insulator with dependency on electrical field and temperature can reduce strongly the time constant for usual field conditions in GIS.

Simulation results presented give a better understanding of the AC-DC field transition under DC stress for two typical shapes of solid insulators in GIS, disk and conical, and they can guide for the definition of DC GIS tests procedures by considering realistic time constants.

Cong-Thanh VU1,2, Abderrahmane BEROUAL1,2, Alain GIRODET1 and Paul VINSON1
1 SuperGrid Institute, 130 rue Leon Blum Villeurbanne 69100, France
2 Université de Lyon, Ecole Centrale de Lyon, AMPERE CNRS UMR 5005, 36 Avenue Guy de Collongue, 69134 Ecully, France