The Study Of HVDC Transmission System Using Modular Multilever Converter Based On Real-time Simulation Of RT-LAB

Flexible HVDC transmission due to its economy, flexible, high controllability in somearea such as the interconnection of different frequency AC system, new energy powertransmission，supply power to the passive load on the distant islands and Other aspects hasa wide range of applications.because of many defects on the performance, control andeconomic,in the HVDC field, the traditional two-level and three-level voltage sourceinverter has become increasingly can not meet the economic development needs of thecommunity.Therefore, in recent years, multi-level inverter has been a very gooddevelopment.This paper based on the analysis of the traditional diode clamped multi-level converter,flying cross-capacitance multi-level converter and cascaded H-bridge multi-levelconverter,Emphasis considerate research the topology,running principle and controlstrategy of modular multi-level converter (MMC). Based on the analysis method ofswitching function coupling, accurate mathematical model of MMC internal current path isdeduced.Through analysis and comparison of the various control strategies, pointed outthat nearest level modulation(NLM) and carrier phase-shift Modulation(CPSM)is moresuitable for MMC topology,and detaile analysis the control principle of the two modulationmethods. And this paper analysis of hierarchical control of flexible DC transmissionsystem, including system-level control, converter-level control and valve-level control.Rectifier system-level control using constant DC voltage and constant reactive powercontrol, the inverter side of the fixed AC voltage control; converter-level control usingdirect current control method, it have good dynamic effects, and fast response.This paper also research MMC HVDC system real-time simulation method on theRT-LAB (also known eMEGAsim) real-time simulation platform.And in order to ensurethe accuracy compared with real-time simulation results and PSCAD/EMTDC offlinesimulation results, these two simulation results are consistent.Real-time simulation isproved has great advantages in terms of efficiency and accuracy.Research results show thatthe MMC is a very suitable multi-level topology for VSC-HVDC because it have thecharacteristics of the cascade converter, easy to implement multi-level, modular design,scalability and redundant control.