Research On Sub-module Topology Of Modular Multilevel Converter With DC Fault Blocking Capability

The high-voltage direct current transmission technology based on modular multi-level converter(MMC-HVDC)has attracted the attention of academic and engineering circles due to its unique technical advantages,and has broad development prospects.When the MMC based on the traditional half bridge sub-module(HBSM)fails on the DC side,the AC side fault overcurrent is fed into the DC side through the anti-parallel diodes at both ends of the IGBT in the structure.Self-clearing of DC-side faults cannot be achieved.Through the improvement of the sub-module topology,it has DC fault blocking capability,which is one of the feasible solutions for DC-side fault clearing.As the basic theory for carrying out the research of this subject,this paper firstly introduces the basic operating principle of MMC and the control strategies of various levels of the system.In view of the MMC sub-module topology with DC fault blocking capability proposed at this stage,this paper proposes a new MMC sub-module topology,namely asymmetric full bridge sub-module(AS-FBSM).At the same time,in order to improve the simulation speed and efficiency of AS-FBSM in electromagnetic transient simulation,this paper establishes a full-state fast simulation model of AS-FBSM by using the Thevenin equivalent method.Compared with the AS-FBSM detailed model based on the switching device,the simulation shows that the fast simulation model is highly consistent with the simulation results of the detailed model,which can simulate the AS-FBSM in real and reasonable conditions under normal operation and fault blocking.The response characteristics,and the simulation run time is greatly reduced compared to the detailed model.Secondly,the fault traversal protection strategy of MMC-HVDC system based on AS-FBSM is introduced.At the same time,in the background of the most serious DC bipolar short-circuit fault,the DC fault blocking mechanism of AS-FBSM topology is analyzed,and the analytical formula of capacitor voltage and DC current in DC fault development process and ASare derived by analytical method.The calculation method of DC fault blocking time of FBSM.The double-ended MMC-HVDC system model is built in the PSCAD/EMTDC electromagnetic transient simulation platform to verify the correctness of the fault blocking process and analytical expression derivation of AS-FBSM.Finally,by comparing with various MMC topologies with DC fault blocking capability,AS-FBSM not only can block DC fault current relatively quickly,but also pays well in economy.