MODELING OF THE POWER TRANSFORMER FOR ELECTROMAGNETIC TRANSIENT STUDIES IN POWER SYSTEMS

This thesis obtains a model for the transformer which is suitable for transient system studies and takes into account eddy currents in the core. A very extensive survey of the state of the art of transformer modeling is included.; In modeling the transformer, saturation and frequency dependent effects due to eddy currents in the core play an important role. Transformer models for system studies derived without taking into account the frequency dependency of its parameters (particularly its magnetizing branch) may not be adequate for representing the transformer performance over a wide range of frequencies and operating conditions.; The thesis describes a method of taking into account the frequency dependent characteristics of the transformer core. The method is based on the analysis of the electromagnetic field distribution within the laminations of the transformer core. A one-dimensional linear diffusion problem is analyzed, and a simple linear equivalent circuit of the Norton type is synthetized. This circuit is an exact representation of the transformer core for any type of excitation at any frequency lower than the frequency "f(,c)" at which the electromagnetic field propagates mainly in the winding insulation and the interlamination insulation. For frequencies higher than "f(,c)", displacement currents and consequently capacitances are of primary importance.; Saturation and hysteresis are included by modifying the equivalent linear circuit.; The resultant model is comparable to models for synchronous machines recently obtained from field tests.; It is determined that at high frequencies the effects of eddy currents in the core, namely those of frequency dependent core losses and limited penetration of the flux, are significant. It is also determined that, at low frequencies, the demagnetizing effects of eddy currents in the core can be neglected, and the effect of eddy currents can be represented by a resistor obtained from no-load test losses.; As an alternative, this magnetizing branch resistance can be obtained from an assumption of uniform electromagnetic field distribution throughout the core.; Application examples include the modeling of: (a)Switching-on of frequency dependent linear inductors. (b)Switching-off of frequency dependent linear inductors. (c)Overvoltages in HVDC systems caused by transformer saturation. (d)Ferroresonance problems in transmission systems.; The model proposed in this thesis can be considered as the first step towards an accurate general purpose transformer model.