A High-Frequency Circuit Model of a Potential Transformer for the Very Fast Transient Simulation in GIS

The high-frequency circuit model of potential and current transformers (PT/CTs) is indispensable in simulating the voltage stress in secondary circuits through the stray capacitances of the PT/CT particularly caused by the very fast transient overvoltages in gas-insulated subsystems. This paper presents a systematic methodology for establishing the high-frequency (0.1 to 11 MHz) circuit model of PTs based on the scattering parameters. First, the two-port S parameters of the PT are transformed into the Y parameters so that the equivalent ldquopirdquo circuit model can be established. Second, the vector-fitting code is used to approximate the admittances of the equivalent ldquopi rdquo circuit model by rational functions consisting of real as well as complex conjugate poles and residues. At last, a circuit synthesis method for the rational functions is applied to build the circuit model. The proposed method is general and robust. The simulation and measurement results are presented, confirming the validity of the proposed model.