Protection Scheme And Improved Protection Research Of AC/DC Hybrid System With MMC-HVDC

In recent years,with the development of DC transmission technology,voltage source converter based high voltage direct current has been developed rapidly.Voltage source converter based high voltage direct has strong controllability,no commutation failure and easy to build multi terminal DC system.Modular multilevel converter has a highly modular structure,can be extended to any voltage level,high waveform quality and fault handling ability and other characteristics,widely used in voltage source converter based high voltage direct engineering.With the increase of the capacity of the DC system,there are many new problems in the power system dispatching and coordinated control.For the AC / DC hybrid system with MMC-HVDC,the mechanism and configuration of relay protection are different from the traditional AC system.This paper carries on the research to this question,summarizes the existing relay protection configuration,and puts forward the improvement scheme on the basis of the analysis of the incorrect action of the AC/DC hybrid system with MMC-HVDC.In this paper,according to the different protection objects,the AC / DC hybrid system with MMC-HVDC is divided into different protection areas.Then,from the influence of harmonics,the existence of distance protection and zero sequence voltage protection misoperation,the problem of incorrect action in the AC/DC hybrid system with MMC-HVDC is analyzed.In the aspect of harmonic effect,the relationship between MMC level,AC system parameters and output harmonic characteristics is studied.On this basis,the preventive measures are put forward to avoid the occurrence of misoperation.In the aspect of distance protection,the influence of MMC-HVDC system operation mode on the distance protection of AC line is studied in depth.Based on the tuning method,an adaptive distance protection method based on the measurement information at the common connection point is proposed to overcome the influence of MMC-HVDC on the second section of distance protection.This can be avoided due to protection under the cover line passing through short,end risk caused by short circuit fault distance protection II section protection tripping the circuit,reduce the fault clearing time,has an important significance to the stable operation of the power system.In the aspect of misoperation of zero sequence voltage protection,the MMC average model and zero sequence equivalent network are established The zero sequence voltage relationship was established between the MMC-HVDC system and analyzes the mechanism of zero sequence voltage protection misoperation,and puts forward the improved protection scheme.Finally,the effectiveness of the proposed schemes is verified by simulation analysis.