Research On Electromagnetic Characteristics Of Single-phase UHV Autotransformer Under DC Bias

It affects the safe operation of the transformer seriously by the DC bias magnetic effect caused by the high voltage direct current transmission and geomagnetic storm.UHV power grid with eight split wire and small resistance is more vulnerable to DC disturbance damage.UHV transformer as a key equipment of UHV power grid,the reliability and safety of its operation are directly related to t he normal operation of the entire grid.For this reason,the DC bias problem of UHV transformer has been calculated and analyzed in this dissertation.The main research contents and achievements are as follows:Based on the high permeability and strong non-linear characteristics of UHV transformer core material,this dissertation put forward the segmented analytic method based on the no-load simplified circuit model,and studied and analyzed the accuracy and stability of the calculation results under the DC bias condition.The numerical results of the Fourth-Order Runge-Kutta method was compared and analyzed based on the analytic solutions.The results show that,in the numerical calculation of the DC bias of the UHV transformer,the more accurate DC bia s characteristic can be obtained by increasing the series resistance and reducing the calculation time step.The results of this study provide a different solution for the field-circuit coupling calculation of the actual UHV transformer.According to the actual parameters of UHV transformer,the field coupling model of transformer is established,which is used to analysis the influence of the value of series resistance and step size on the results of DC bias calculation.Increasing of the series resistance artificially has changed circuit structure of the transformer itself and result in the voltage behind the series resistance deviating from the nominal voltage.A voltage compensation method was proposed to implement the iterative calculation of voltage compensation.The results show that the voltage compensation can effectively eliminate the current calculation error caused by series resistance.On this basis,this dissertation has completed the no-load DC magnetic bias simulation calculation and obtained the exciting current and magnetic field characteristics under different DC current level of UHV transformer.In order to improve the calculation efficiency of no-load DC magnetic bias calculation of UHV transformer,this dissertation proposed a fast method of no-load DC magnetic bias calculation based on the dynamic inductance-excitation current curve,which avoid the repeated returning to the magnetic field model and calculating dynamic inductive under different DC bias current level.The correctness and validity of the method is verified by comparing with the results of the field –circuit coupling algorithm.Simultaneously,compared with the conventional field–circuit coupling method,this method can greatly save the calculation time and improve the calcula tion efficiency,which can provide a quick and simple calculation method for transformer DC bias calculation.Research and analysis of DC bias problem for load operation mode of UHV transformer is carried out.Increasing the series resistance,the DC component of primary current approximate the DC bias current,the accuracy of the calculation results is improved.The influence of different load resistance on the high and medium voltage winding current of the UHV transformer is analyzed.The variation of the current in the high and medium voltage winding from the overload to the rated load and then to the light load until the no load is summarized.DC bias is calculated and current characteristics is analyzed in load operation mode with different DC bias current.Based on the no-load and load current calculated by different DC bias current levels,the reactive power loss and eddy loss is calculated,and the relationship between loss and DC bias current is summarized,which can provide reference for the evaluation of DC bias capability of UHV transformer.The research work carried out in this dissertation lays the foundation for the next step in calculation of eddy current loss and the calculation of temperature rise under the DC bias of UHV transformer.