Detailed lossy-transformer model

Detailed transformer models have been extensively used by engineers as an important tool in the design of the internal insulation structure of large power transformers. During the beginning of the 90's the inability of existing models to represent the transformer under a wide range of transient phenomena motivated researchers to look for faster, non-linear and lossy-models. Improved detailed transformer models satisfying utilities and manufacturers requirements were presented at that time [1, 2, 3, 4, 5]. Nevertheless, time domain detailed models, based on the calculation of the open circuit inductance matrix [2, 3, 4], were incomplete in terms of their ability to predict the frequency-dependent damping effect of losses in the transformer internal voltage transient response. The work presented in this thesis extends this particular model to represent the lossy nature of the electromagnetic phenomena experienced by a transformer under transient conditions. The losses due to eddy currents in the core, skin and proximity effects on the winding conductors as well as the dielectric losses in the transformer insulation structure are included in the proposed model.; The new lossy-transformer model requires the calculation of the impedance characteristics of each loss mechanism as a function of frequency. Analytical formulas are used to represent losses related to the eddy current phenomena in the transformer's winding and core. Losses in the insulation structure are represented using lossy-parallel plate capacitors and available empirical data in the form of the dielectric material properties as a function of frequency. The different frequency characteristics representing each loss mechanism are approximated in the S-plane. Each component in the transformer winding lumped parameter model is replaced by its corresponding frequency-dependent representation. The time domain model is achieved by applying the bilinear transformation and the properties of the z-transform.; A detailed description of the methodology employed in this thesis to construct the new model is presented. The lossy-transformer model is verified using available experimental data for four different transformers. The predicted damping as well as the frequencies of oscillation of the transformers internal voltage response showed very good agreement with the measured waveforms.