Research On Harmonic Characteristics And Structure Optimization Of UHVDC Converter Transformer

With the increasing of power consumption,the imbalance between energy supply and power demand,and the new guidance of energy production policy,UHVDC technology has become the main way of power transmission.The application of high-power nonlinear power electronic devices in AC-DC conversion of UHVDC transmission technology raises more particularity in power quality than AC transmission technology.AC harmonics in converter station can be generated by DC converter,which influence the windings on both sides of converter transformer that cannot be ignored.Converter transformer is one of the most important parts of converter station,which is one of the core devices to complete AC-DC conversion.The windings on both sides of converter transformer are easily affected by harmonics and reactive power deficiency caused by valve side,that resulting in winding temperature rise and fault,the vibration and noise of core and shell,the aggravation of electrical loss of winding and so on.Based on the harmonic condition of UHVDC transmission system,The harmonic characteristics in the winding of converter transformer while working were analyzed and discussed.The main sources of harmonics,the development and the present harmonic filtering methods of UHVDC transmission system are analyzed under the new policy of electric energy development in China.Secondly,according to the characteristics and the basic working theory of the converter structure,the harmonics in converter transformer winding are analyzed.Through the classification of different characteristics of harmonics,the calculation methods and national standards of converter transformer winding harmonics were studied,the harmonic filtering methods commonly used in UHVDC system were also analyzed theoretically.Thirdly,the simulation analysis were carried out in PSCAD/EMTDC simulation platform,and the correctness of the established model were also verified according to the harmonic characteristics of converter transformer windings under different operation modes by solving practical problems.Through the analysis of the unified magnetic equivalent circuit model,the structure of converter transformer is optimized,and a new type of converter transformer model and its electromagnetic coupling circuit were proposed under the background of UHVDC simulation model.The harmonic filtering function was realized by using the equivalent circuit of electromagnetic coupling circuit,the additional winding can shunt the harmonics by adding equivalent negative flux of winding.Therefore,the influence of harmonic currents transmission from valve side to grid side was weakened.The simulation results show that the proposed converter transformer can effectively improve the harmonic condition and suppress the harmonic transmission process to the grid side.The correctness of the electromagnetic coupling model was also verified by the simulation results.Finally,by designing the hysteresis model and the core structure,the magnetic distribution of the transformer with different core structures excited by harmonic power supply were simulated and analyzed by Comsol Multiphysics simulator.The simulation results verified the harmonic filtering theory of the new converter transformer and the characteristics of simulation results with different core structures were described.The research results have practical significance and reference value for theoretical designing of converter transformer and filter in UHVDC transmission project.