Research On Asymmetrical Line Commutated Converter Model Simplification And Commutation Failure Discrimination Method

With widely application of the HVDC transmission system in our country,the fast and accurate modeling of HVDC of line commutated converter,as the key component of HVDC transmission system,is critically important.Traditional electromagnetic tran-sient models shows many disadvantages,such as slow calculation speed,and large amount of data required,while electromechanical transient model is only suitable for ideal symmetrical state.To solve this shortcomings,this paper analyzes the switching function of the converter under asymmetric conditions,The switching function of the course of action is modeled,on the basis of which,the improvement and simplification are carried out,and the mechanism of inverter commutation failure is further analyzed.The main contents of this paper include the following three parts:1)The actual triggering and commutation process of the converter under asymmetric conditions are analyzed,and the calculation formula of the actual firing angle and com-mutation angle considering the harmonic content with more content on both sides of the converter are put forward.On the basis of the original tradition switch function model,the improved switching function model which is suitable for the asymmetric situation is proposed.The accuracy of the switch function model is verified by an ex-ample.2)Based on the proposed improved switching function model for asymmetric condi-tions,an accurate dynamic phasors model is established.Sampling the AC side voltage and the DC side current information of the converter in the electromagnetic transient state,the accuracy of the dynamic phasors model is verified by means of AC side cur-rent and DC side voltage calculating by the dynamic phasors model.By further neglect-ing the harmonics on both sides of the inverter,the dynamic phasors model is simplified and deduced to obtain a more simplified modified quasi-steady state model.3)Judgment of commutation failure caused by asymmetric fault of AC system in HVDC transmission system is deeply studied.Through the analysis of the actual trig-gering and commutation conditions of each valve in the asymmetric situation,the actual extinguishing angle is solved,and finally the commutation failure is judged based on the actual extinguishing angle.At the same time,the effect of harmonic on both sides of converter on commutation failure is analyzed.The above mechanism analysis pro-vides theoretical reference for the study of commutation failure.