Very Fast Transients in a 500 kV Gas-Insulated Substation

This paper performs thorough investigations of very fast transients (VFTs) in a 500 kV gas-insulated substation (GIS) by adopting complete earth-return impedance and admittance formulas. VFT simulations are performed with wideband (WB) models in a simulation tool for electromagnetic transients. The effects of various GIS parameters on VFTs are investigated. The maximum core and pipe voltages along the GIS are presented. The frequency spectra of VFT voltages are evaluated, and the effects of GIS length and transformer capacitances on the VFT frequencies are analyzed. The frequency decreases as the GIS total length increases. The frequency is decreased significantly by stray capacitances of transformers because those are far greater than the GIB capacitance. The effect of pipe grounding on the pipe voltage is very noticeable, but on the core voltage and the oscillating frequency is minor. The position of an operating DS influences the VFT over-voltages and frequency significantly. Approximate analytical formulas are derived for physical and qualitative interpretation of simulation results.