Wideband Modeling of a Single-Phase Transformer Considering Correction for Current Sensor and Cable Effects

This paper describes a methodology to obtain a linear frequency-dependent black-box model of a 1 kVA 220 V/127 V single-phase transformer considering compensation of errors due to limitations of the considered current sensor and influence of measuring cables. The limitations of the current sensor are assessed by determining the frequency response of carbon resistors with nominal values from 0.5 Omega to 10 k Omega and its deviation from the frequency response of ideal resistances. The measuring cables can be seen as capacitive elements whose admittances are added to the transformer circuit. The proposed wideband model, which can be used to analyse transferred voltages through power transformers, is based on measurements performed in the range 10 Hz to 10 MHz of both admittance matrix and transferred voltages from high-voltage to low-voltage transformer terminals. The proposed model is validated via MATLAB simulations assuming the application of impulse voltages of different characteristics at the high-voltage terminal of the transformer for different resistive load conditions. Comparison between measurements and calculations indicates that the proposed wideband model is sufficiently accurate for studying transferred impulse overvoltages from the high-voltage side to the low-voltage side of the transformer.