Experimental study on suppressing VFTO in 550 kV GIS by great and small magnetic rings

Very fast transient overvoltage (VFTO) caused mainly during switching operations of the disconnectors has been a serious threat to gas-insulated switchgear (GIS). The paper presented a new approach to suppress the VFTO by great nano-crystalline rings and small ferrite rings. In this research, the test platform consisted of a steep fronted impulse device and a real 550 kV GIS. Then, series of comparative experiments were carried out in the absence of magnetic rings, in the presence of 40 cm small rings cluster, 80 cm small rings cluster and 80 cm small rings cluster together with 10 cm great rings cluster under the charge voltage of 420 kV, 480 kV, 500 kV and 560 kV. The statistics and analysis of the experimental data show that the combined action of the great and small rings might get the best suppression effect in these tests and the longer the small rings cluster length, the better the mitigation. The test results verify that magnetic ring is feasible to suppress the VFTO in extra high voltage (EHV) GIS and the applicability of this new approach which suppress VFTO by great and small magnetic rings.