Research On Problems Of Core Saturation Instability And Temperature Rise In Converter Transformers

Interaction between the second harmonic quantities on the AC side of converter and fundamental frequency quantities on the DC side of converter will lead to a type of harmonic instability on high voltage DC transmission system. The instability can be reinforced by core saturation of the converter transformers. The eddy current loss and the local overheating problem in the power transformer is a very important problem. The transformer capability is larger, the leakage magnetic and the eddy loss are greater, so the calculation of local overheating caused by the eddy loss is more distinct.In this paper, the general mechanism of converter transformer core saturation instability has been studied and the property of impedance-frequency of the AC and DC system has been analyzed. An equivalent circuit of converter transformer for the analysis of core saturation instability has been established and saturation stability factor has been derived by the harmonic transfer matrix. The method proposed in this paper can be used to predict the core saturation instability of converter transformer. Analytical results have been verified by using the digital transients simulation program PSCAD/EMTDC.According to the transformer electromagnetic theory, a2-D leakage magnetic field model of a power transformer is built by the finite element software-Ansoft. The eddy current loss is analyzed combining with the distribution of the magnetic flux density of high voltage winding and low voltage winding. At the same time, the eddy current loss of the winding caused by the leakage is calculated on the basis of the analysis of2-D leakage magnetic field of the transformer. Because the results are basically consistent with experimental data, so it proved that the finite element method is feasible.The physical model of heat transfer and oil flow is set up in this paper. On the base of the result of eddy loss calculation in the windings and the structure parts, this paper considers the Joule loss as the load of fluent analysis by using the method of direct coupled field and the coupling in magnet-fluent-temperature rise fields in the power transformer is analyzed. Then, the distribution of temperature rise field in the winding and structure parts can be obtained by solving fluent field and temperature rise field. At last, the calculative results are compare with trial ones, and the erroneous reasons are analyzed. The scheme proves that the coupling in magnet-fluent-temperature rise fields is feasible.