| Power transformers, in service, are subjected to voltage transients. The transformer insulation should be designed to withstand the internal stresses initiated by these transients. To determine these stresses during the design stage, lumped parameter models of the transformer are employed. Computer models are the best tools for predetermination of these stresses. To date, in spite of the great efforts employed, lack of accurate and consistent transformer models are felt to contribute to the relatively high failure rate of the EHV transformers. This thesis is based on a detailed transformer model developed by G.E. for transformer insulation design. Although this model has provided the designer a suitable tool for transient study during design stage, it assumed a linear characteristic for iron core and employed an empirical method to model damping effect of transformer losses. It is felt that the core's non-linear magnetic characteristic has an appreciable effect on the transformers transient response.;This thesis develops a detailed transformer model which accurately models the nonlinear, saturable, characteristic of transformer magnetic core. This model is based on linearization of the core's saturable characteristic during each time interval of solution. Then the linear model will be solved within that interval and based on the magnetizing flux in the core, the inductance model will be adjusted for the next time interval of solution, and the updated linear model will be solved for the interval. A stable solution routine is developed to solve the nonlinear model's system of O.D.E.'s. Supplementary features needed for transformer design such as external circuits, switches and nonlinear resistors are developed for this solution routine.;The necessary methods are developed for verification of linear transformer model in the frequency domain. An autotransformer (765/345/34.5 KV, 500 MVA, single phase) is employed for linear verification in frequency domain, and for non-linear verification in time domain. This thesis provides a nonlinear, single phase, detailed transformer model and verifies it against the available measured data. |
