The study of very fast transient over voltages (VFTO) for the project of 400/220kV GIS substation with one and half circuit breaker configuration

Abstract

Gas insulated switchgears (GIS) are playing an important role in today's power systems. These are being used in many transmission and distribution substations because of their compactness and durability. The basic insulation coordination levels are well defined in the international standards. The ratio of lightning impulse withstand voltage level to per unit voltage varies for different system voltages. This ratio decreases when we go for extra high voltages. Therefore the insulation for extra high voltages should be carefully designed to avoid the dielectric failures. Very fast transient over voltages (VFTO) which are caused by the switching operations are the important factors to be considered in the design of GIS. The disconnector (DS) operation is slow when compared to the circuit breaker operation. Therefore, DS switching is the main source for generating VFTO in GIS. The transients are characterized by their short duration and very high frequencies. VFTO study has been carried out for the project 400/220kV GIS Substation in accordance with the methodology and guidelines as per CIGRE WG D1.03 — TB 519 [1] recommendations, IEEE standard 1313.2-1999 [2], IEC 60071-2 [3], IEC 60071-4 [4], and IEEE technical papers presented in various forums. Accordingly the study has been carried out to assess overvoltages at various surge impedance transition points. The results of this study are represented in this paper.