Research On The Setting Up Method Of Strictly Passive Circuit Model For Power Transformer Under VFTO

Power system electromagnetic transient simulation, as well as electromagnetic compatibility analysis needs to build electromagnetic transient model for elements and equipments in power system. Because the transformer type device presents nonlinear behavior as well as frequency-dependent effects under transients, it may be one of the most difficult equipment to model in power system for a transient simulation. Standard EMTP transformer models, can accurately be used for transient calculation at power frequency and low frequency (below 10kHz). However, the main frequency component of lightning over-voltage and very fast transient over-voltage (VFTO) generated in gas insulated substation can reach up to a few or even tens of Mega Hertz. The given models is far from meeting the requirements of lightning and VFTO transient simulation, therefore, it is necessary to establish the high frequency circuit model of transformer type device.Supported by Natural Science Foundation of China (No.50977031) and Natural Science Foundation of Hebei Province (No.E2008001243),based on the black-box technique, a systemic methodology was proposed to setup the wideband strictly passive circuit model for transformer type device for the very fast transient simulation. To guarantee the simulation stability of the circuit model, the modified vector fitting method was used to approximate the Y-parameters by rational functions transformed from S-parameters by measured, firstly. And then, the residues of the rational functions were optimized by sequential quadratic programming to meet the strictly passive conditions. Moreover,πequivalent circuit model based on Y-parameters was built by Foster circuit synthesis method. The pattern search algorithm was further applied to optimize the values of the lumped components of theπequivalent circuit. The strictly passive circuit model composed of R, L, C, G with only positive values was finally achieved. Taking a continuous type transformer windings and a 10kV/0.2kV single-phase power transformer as examples, wideband strictly passive circuit models were setup. The simulation and measured results were presented, confirming the validity of the proposed method.This modeling method is simple, effective and general,which is suitable for any transformer type device, as long as their parameters of the multi-port network are obtained. The model can be used for lightning and VFTO electromagnetic transient calculation.