Calculation and Analysis on Circulating Current in Grounding System of UHV Hybrid Gas Insulated Switchgears

Due to high voltage class and huge capacity of UHV power transmission system,there are relatively evident circulating currents in the case of gas insulated switchgear(GIS) and in grounding grid,and the circulating currents lead to circulation loss during steady state operation of UHV transmission system.Taking a certain UHV switching station in China for example and based on basic principle of electromagnetic instrumental transformer,equivalent circuit models for the simulation of both cases for GIS and hybrid gas insulated switchgear(HGIS) are proposed.The grounding elements of grounding grid are represented by Π-shape equivalent circuit and the optimization calculation of electric parameters of the grounding system is performed by numerical calculation.Analysis results show that utilizing the grounding grid with the two-layer structure the circulating current level in the main grounding grid can be limited to less than 20 A,and the highest circulating current in the sub-grounding grid is less than 700 A,however the current in the corresponding short-circuiting wire is higher.The influences of case parameters of GIS and HGIS on the circulating in the cases are analyzed,and the effect of deeply buried vertical grounding rod on decreasing circulating current in grounding grid and the effect of short-circuiting wire on decreasing circulating current in grounding grid are obtained,and on this basis an optimal design scheme is put forward.Finally,the values of circulating currents in all parts of UHV grounding grid and corresponding temperature rises are given.