An Analytical Approach for Estimation of Induced Voltage on Control Cables in a GIS Due to Switching Transient Fields

In Gas Insulated Substations (GIS), VFTOs are generated mainly due to switching operations. These VFTOs have a rise time of about 3-10 ns and peak magnitude of about 2.8 p.u. (∼1000 kV for a 420 kV substation) for a worst case switching scenario. These transient voltages radiate electromagnetic (EM) fields during their propagation along the co-axial high voltage (HV) bus of the GIS as the asscoated frequencies are in the range of a few MHz to about a few hundreds of MHz. The transient electromagnetic fields in turn leak into the external environment through apertures like SF6 gas-to-air bushing, gas-to-cable termination, non-metallic viewing ports, insulated flanges etc. and couple to the control equipment/data cables present within the GIS. This may result to transient current/voltage on the shield of the control cable..The coupling of these currents into the interior of a co-axial cable is via the mechanism of transfer impedance and pigtail coupling. The transfer impedance (Zt) is a characteristic parameter of the control cable. Shielded cables have low transfer impedance. The pigtail coupling is an inductive coupling between metallic sheath termination (pigtail) and the load circuit of the control cable. The induced voltage at the output of the cable may appear across the relays, logic circuit etc., of secondary circuit and may cause mal-functioning of the primary equipment. Earth switching operation produces multiple transient fields and the number of transients may vary with the related voltage of the substation, type/speed of the switch and the electrical characteristics of the high voltage bus being operated. The transient field levels, in general, decrease with increase of distance from the high voltage bus. Further, it is very often seen that electronic equipment are present in the transient electromagnetic field environment of the gas insulated switchyard. This raises the level of concern equipment with the EMI environment in which they must operate. These transient fields contribute to significant radiated fields and can couple energy to the components/equipment present in the gas insulated switchyard.