Bypass Sheath Voltage Limiters at Cable-to-GIS Terminations: Geometry-Resolved Connection-Lead Impedance for Insulation Coordination Under Very Fast Transient Overvoltages

Very fast transient overvoltages (VFTOs) from disconnector operations in gas-insulated switchgear (GIS) stress the sheath sectionalizing insulation at cable terminations, where a bypass sheath voltage limiter (BSVL) protects it. At these frequencies, the inductive drop across the short connection leads can match or exceed the arrester residual voltage and cancel the protection. This article answers one design question: does the BSVL still protect once real lead geometry is accounted for? The generic 1 µH/m allowance of IEC TR 60071-4 leaves this question open. This article proposes a method that uses the geometry-resolved impedance Z(f) of the leads and the enclosure return to find the connection geometry with the lowest voltage across that insulation. The dominant frequency is taken from a measured record or from a simulation, and a single inductance from the actual route represents each lead and the enclosure return. The transient voltage then divides between the arrester and the leads, which gives the voltage remaining across that insulation. Rearranging the same expression gives the pass or fail decision and the largest lead inductance, and hence the longest lead, that keeps this voltage below the withstand. At an operating substation, the worst arrangement leaves 66% more voltage on the insulation than the best one. The main contribution is a methodology that screens the BSVL and its connection geometry together.