Electromagnetic processes in a three-phase transformer during heating

The features of heating three-phase power transformers by non-sinusoidal alternating currents of frequency below industrial are considered when preparing them for seasonal work. It is noted that the analysis of currents in the primary and secondary windings when the transformer is powered in the short circuit mode from a three-phase source of non-sinusoidal alternating voltage has specific differences from the methods for analyzing electromagnetic processes in a single-phase transformer, for which the equations of instantaneous and root-mean-square current obtained by exact analytical methods are known. The relationship of electromagnetic processes in three-phase transformers with the processes in single-phase transformers as well as the reasons for the differences in the processes in them are shown. The use of precise analytical methods for describing electromagnetic processes in a three-phase transformer and, in particular, the use of the operator method with Heaviside transformations is hindered by the high order of the characteristic equation of an electric circuit and the operator equations of current, the solution of which is achieved by numerical methods. However, when using the method of analogy of processes in a six phase windings electric circuit with processes in a two windings circuit, it is possible to obtain partial equations for the instantaneous current in the windings. The equations of instantaneous current in the phase windings of a three-phase power transformer are presented, obtained on the basis of the accepted assumptions that idealize the electromagnetic process in the frequency converter and transformer. The article presents the calculation schemes of the process of heating the windings and oscillograms of the transient currents during switching of the power elements of the converter, which make it possible to explain the nature of electromagnetic processes in the "converter-transformer" block and to give a qualitative assessment of the possibility of controlling these processes. The obtained results of calculations of instantaneous current in the windings give convergence with experimental data with an error of no more than 10%. The obtained equations make it possible to reasonably solve the problem of choosing a frequency converter to provide a given intensity of heating or drying of the transformer.