Research On The Temperature Rise Of ±800kV Converter Transformer Caused By DC Bias

The ±800kV converter transformer in ultra high voltage direct current system has extreme large capacity which makes the leakage magnetic field of the flitch plate,oil tank and other supporting parts very strong,and the distribution of eddy current loss is not quite uniform.When HVDC system works in single pole-ground mode or unbalanced double ploe-ground mode,coupled with the geomagnetic storm,there will be dc current flowing into the converter transformer and therefore cause dc bias,prompting the leakage magnetic field much stronger and local overheating,which will pose a threat to the safe operation of the transformer.Therefore,it is of great theoretical significance and practical value to study the dc bias of ±800kV converter transformer and overheating of the structural parts.Based on the difference between converter transformer and regular transformer,the magnetic-circuit coupled model is established by finite element analysis software Ansoft Maxwell and Simplorer,to calculate the excitation current without load under different levels of DC bias.Considering the large short circuit impedance and special structure of the flitch plate,the leakage magnetic field and eddy current loss of both converter transformer and regular transformer are obtained for further understanding of its electromagnetic characteristics under DC bias.In order to study the temperature distribution of converter transformer,the magnetic-thermal coupled method has been used to establish the three dimensional thermal model of converter transformer by both Ansoft Maxwell and Steady State Thermal.Moreover,based on the empirical formula the convective heat transfer coefficients of the inner surface of oil tank and the surface of each structure are calculated,and thus obtained the rule of temperature distribution of each metal structure with and without DC bias,as well as the specific hotspot temperature.According to the magnetic and thermal simulation result of the ±800kV converter transformer,the eddy current loss is more intensive on the top of the oil tank where appeared local overheating under 10 A dc current,based on which the electromagnetic shieldings are installed to the high leakage magnetic field area,and more efforts are made to optimize the length and number of the electromagnetic shieldings to obtain better cooling effect.The research conclusion has important significance to solve the transformer temperature rise caused by dc bias.