Experimental Research and Simulation on VFTO Mitigation by Ferrite Rings in UHV GIS

Experiments were performed on two different ultra-high voltage gas-insulated switchgear test circuits to evaluate the mitigation performance of ferrite R2KB rings with very fast transient overvoltages (VFTOs). Significant decreases in the overshoot factor and the VFTO level were observed, although the steepness was not much improved owing to saturation. It was found that ferrite rings are more effective for VFTO mitigation at the source side than at the load side. Simulations were carried out with a new circuit model of the ferrite ring, and the simulation results were consistent with the experimental results. The method of drawing the contour plot of VFTO and very fast transient current in the time-space domain was proposed to facilitate the analysis. With this method, the principle of determining the optimal installation location of ferrite rings for the best mitigation performance was clarified.