Induced voltage on control cables in a GIS due to the transient EM fields generated during switching events

Very fast transient overvoltages (VFTO) generated due to switching operations in a gas insulated substation (GIS) may propagate out into external environment through discontinuities like SF₆ to air bushing, SF₆ to cable termination, nonmetallic viewing ports and flanges, etc. For the reliable operation of substation controls, it is essential that the transient electromagnetic (EM) fields radiated due to VFTO be within permissible levels. Since these transient fields have frequency components up to 200 MHz, they may induce currents on the metallic sheath of the shielded control cables. This in turn may induce undesired voltages on the central conductor of the cable depending on the transfer impedance of the cable. In the present paper, the influence of transient EM fields, type of grounding (single end grounding or both end grounding), contribution of vertical and horizontal components of transient fields, length of the cable and height of cable above ground on induced currents and voltages have been studied. Finally, the role of transfer impedance of the cable on the induced voltages for different types of groundings is analysed and reported.