Macroscopic Modeling of Inhomogeneous Field Breakdown in SF6 Under Lightning Impulse Stress

Abstract

Gas Insulated Substations (GIS) have proven to be a viable solution for compact and environmentally clean substations. SF6 gas has been widely used in GIS as insulating medium because of its excellent dielectric and arc quenching properties. During the course of their operation, GIS are subjected to Lightning Impulse (LI) and Very Fast Transient (VFT) over voltages which quite often are found to cause the SF6 insulation failure. Experimental investigations in this regard have suggested that the breakdown of insulation is by the leader mechanism. the phenomena of leader inception and propagation leading to breakdown in SF6 are currently being studied because of their relevance to the design of GIS. Attempts have been made in the recent past to macroscopically model the leader growth process for theoretical studies. the leader propagation is guided by the electric field at the leader tip, which inturn is dependent on the nature of voltage applied, electrode geometry and charge distributed in the leader. the charge build up process in the leader is governed by Resistive-Capacitive (R-C) transient fields. Modeling of breakdown in SF6 based on field approach involves modeling the leader mechanism in SF6 and computing the associated R-C transient field. in the present paper, an attempt made in that direction is discussed. Inhomogeneous field is realised by a hemisphereically tipped point-plane electrode system. a Laplace solver based on the principles of Charge Simulation Method (CSM) was coded and used in implementing the algorithm for the transient field computation. in order to reduce the computational burden without causing the numerical oscillations of the computed field quantities, a nonlinear numerical filter was employed. the final outcome of the work in terms of V-t characteristic for an inhomogeneous filed breakdown in SF6 under LI stress is also presented.