Transients induced on control cables and secondary circuit of instrument transformers in a GIS during switching operations

The transient fields generated during switching operations in gas-insulated substations (GIS) can have rise times in the order of a few nanoseconds. Transient fields, which are developed internally to the gas-insulated modules, leak into the external environment through discontinuities and interfere with nearby electronics. Depending on the configuration of the GIS and the observation point, amplitudes of the electric and magnetic fields could be of the order of a few tens of kilovolts per meter and a few hundreds of amps per meter, respectively. These transient fields, in turn, induce voltages in the control cables and secondary circuit of the instrument transformers, which result in malfunctioning of the primary equipment. In this paper, induced transient voltages in the control circuitry and their frequency spectra have been calculated due to the transient fields, leaking out from the gas-to-air bushing and the gas-insulated bus section of a 245-kV-rated GIS, during switching operations. The effect of the length of control circuit, type of grounding of the cable sheath (both-end/one-end), characteristics of the transient fields, type of secondary circuit (i.e., current transformer/potential transformer) and LC loading of the control circuitry on the induced voltages have been analyzed and reported.