Research On Broadband Nonlinear Modeling Method For HVDC Converter Transformer

As one of the important equipments to realize the energy conversion between AC and DC in HVDC project,the safety,dependability and stability of converter transformer is of great acount to the reliability and stability of HVDC project.At present,abnormal operation conditions of converter transformer still occur frequently,such as the phenomenon of medium and high frequency transient overvoltage caused by frequent operation of converter valve,the pollution of medium and low frequency harmonic caused by power electronic device and the abnormal excitation condition of low frequency caused by core saturation.Therefore,in view of this phenomenon,this paper focuses on the establishment of a wide-frequency electromagnetic transient nonlinear model of converter transformer considering the core nonlinearity and wider frequency range adaptability,which provides better theoretical support for the study of electromagnetic transient characteristics of converter transformer.In this paper,the research content is as below:Firstly,the paper analyzes the relevant factors of converter transformer model in detail,focusing on the influence of core loss,capacitance effect,core saturation and high frequency skin effect on the modeling process of converter transformer.T he nonlinear characteristics of converter transformer core are characterized by magnetic circuit equivalent coupling model,and the wide frequency non-linear characteristics mechanism of converter transformer characterized by winding dispersion and multi capacitance are proposed.The calculation method of inductance,resistance and nonlinear inductance parameters caused by core saturation are given,and the calculation method of capacitance parameters under the condition of winding dispersion is proposed.Secondly,based on the above calculation method,the parameters of the transformer model are calculated by large-scale finite element analysis software,and contrast to the measured data to test the correctness of the parameter computing method.Considering the influence of winding skin effect and the leading role of the winding nonlinear resistance in high frequency,the nonlinear resistance of winding at different frequencies is segmented The results indicate that the means can ameliorate the quality factor of harmonic impedance waveform effectively.The excitation inductance current characteristic curve is obtained by waveform conversion method,which can effectively characterize the nonlinear characteristics of the core.In the low frequency state,the effectiveness of the core nonlinear model is verified by no-load test.Finally,in order to solve the problem of winding dispersion at high frequency,the winding is divided into different segments for simulation,and the number of segments suitable for the transformer studied in this paper is determined as 4 sections.In conclusion,a wide-frequency nonlinear model of converter transformer is established considering core nonlinearity,winding nonlinearity,multi capacitance characterization and winding dispersion.The frequency response method and sweep impedance method are selected as the broadband impedance test method.The simulation calculation of the broadband harmonic impedance is obtained and contrast to the measured value,and the accuracy of the prop osed model is verified.Based on the model,the impact test circuit is built to simulate lightning shock and operation shock fault,and the short circuit fault is simulated by the method of leading out tap.The approximate range of fault point is determine d by the change of impact response.The results show that the proposed wide frequency nonlinear mode of converter transformer is practical in the calculation of impact response.