22 / 2024-03-28 00:35:06

The impact of gas pressure on the very fast transient overvoltage (VFTO) ratio

The impact of gas pressure on the very fast transient overvoltage (VFTO) ratio,gas insulation; eco-friendly switchgear; insulation coordination; electric field distribution; compound insulation

Disconnectors in gas insulated substations (GIS) are important components for safe operation of the substation. Once disconnectors open or close, re-strikes and pre-strikes occur between the contacts, which is a normal operation mode. Due to the fast collapse of the voltage during switching, travelling waves are generated in the GIS, leading to so called Very Fast Transients (VFT). The VFT might lead to overvoltage, which is usually referred to as very fast transient overvoltage (VFTO) and is expressed by the VFTO ratio in units of the applied voltage peak (VFTO_peak/AC_peak=VFTO_R). In the present contribution this phenomenon was investigated in a GIS test setup filled with CO2/O2/C4-FN gas mixture, which is the most suited alternative to SF₆ in high-voltage switchgear. The VFTO_R depends not only on the detailed layout of the GIS but also on the resistance development of the spark channel across the contacts at the instant of prestrike or restrike. The paper investigates the impact of spark channel development on the VFTO_R in a test device under conditions similar to disconnector bus-charging switching in GIS. This was done at different pressures ranging from 0.5 to 0.9 MPa (abs). The test circuit has been simulated to clarify the impact of the spark channel resistance decay on the VFTO_R.