Modeling And Simulation Of Flexible HVDC Transmission System Based On Modular Multilevel Converters

Modular Multilevel Converters(MMC)are easy to manufacture with many good qualities including high modularity,low switching frequency,low loss,small step voltage,and high output waveform.Therefore,they are suitable for Modular Multilevel Converter Based HVDC(MMC-HVDC).Due to the late introduction of MMC-HVDC,the traditional two-level or three-level voltage source converter control method can’t be fully used in MMC.Therefore,it is very meaningful to make an investigation on mathematical modeling,strategy control and reduction of the loss of the sub-module capacitor voltage switch of MMC-HVDC system.To begin with,the working principle of the voltage source converter and the control strategy of MMC-HVDC is analyzed,especially the converter topology of voltage source,the basic working principle of MMC,the theory analysis and verification of carrier phase-shift modulation and the nearest level approximation modulation,and finally the selection of the nearest level approximation modulation.The MMC-HVDC mathematical model is deduced,and the MMC-HVDC control strategies and start-up control strategies are studied,which provided a theoretical basis and basis for building PSCAD simulation model.Thereafter,according to the mathematical model,a double-ended active MMC-HVDC model is built on the PSCAD/EMTDC simulation platform including the direct current decoupling control module for the inner loop of the converter station stage,and the creation of self-defined sub-module voltage-sharing control module in PSCAD according to the valve level control module of the nearest level approximation.A pressure-equalization control strategy based on the coefficient of variation quick-sort state switching is adopted,and a start protection module is established.The simulation results show that the defined voltage-sharing module can significantly reduce the switching frequency and the switching losses.Finally,the double-ended active MMC-HVDC model is artificially verified,which includes the unipolar earth faults,power steps,DC voltage steps and tidal current response on the DC side of MMC.The simulated results are in accordance with the theoretical analysis.The model active step and DC voltage step response complied with DL/T 1526-2016 dynamic performance test standards.