Determining the Broadband Loss Characteristics of Power Transformer Based on Measured Transformer Network Functions and Vector Fitting Method

A method is proposed in this paper to determine the frequency-dependent loss of power transformer for very fast transient simulation. The attenuation factors and the corresponding natural frequencies of the transformer at measurable range were extracted using measured power transformer network functions and the vector-fitting method (VFM). The approximate frequency-dependent formula of attenuation factors was established by analyzing the theoretical transformer model. Then, the parameters in the formula were optimized by fitting the real part (attenuation factors) and imagine part (natural angular frequencies) of poles obtained by VFM. The time-domain simulation process can be divided into two steps. First, the lossless transformer transient response, which could be expressed as the sum of the natural frequency components, is obtained by solving the network differential equations. Second, the attenuation factors calculated by the approximate formula are added directly on the corresponding frequency components. By comparing with the measured results, it is shown that the proposed fitting formula for calculating the attenuation factors could improve the simulation accuracy in comparison with the existing methods.