Dynamic Phasor Modeling For Converter Considering The Influence Of Ac Asymmetry

HVDC transmission technology has been widely used due to its advantages in long-distance large-capacity transmission.In 2020,nearly 40 DC transmission projects will be put into operation or construction.By then,China will become the country with the most extensive application of DC transmission.As the core device in the DC system,the model accuracy and efficiency of the converter have a direct decisive effect on the accuracy and effectiveness of the DC system analysis.In the traditional electromagnetic transient modeling actuarial scale is large,the electromechanical transient model is only applicable to the deficiencies of the three-phase symmetry state.This paper analyzes the commutation process of the inverter under asymmetric operation state,based on which the establishment can reflect The switching function model of the converter action process,and further proposed a correction model based on quasi-steady state.The research content of this paper is mainly from the following three parts:1)Based on dynamic phasor theory,the application of dynamic phasor method in converter modeling is summarized and analyzed.2)Based on the modulation theory,the commutation process of the converter under three-phase asymmetric operation is analyzed in detail,and the actual commutation voltage zero-crossing offset and the DC second harmonic are simulated.Based on the influence of duration,an improved calculation method of commutation angle dynamic phasor is proposed.Based on the analysis of commutation process,an improved converter switching function model is established.3)On the basis of the improved converter switching function model considering the AC asymmetry,a simplified converter dynamic phasor model is established by ignoring unnecessary harmonics.On this basis,the low-order non-characteristic harmonics on both sides of the inverter under asymmetrical conditions are neglected,and the dynamic phasor model is converted into a sequence component form,which is further simplified and a modified quasi-steady-state model is established.