| With the development of economy, the needs of high-capacity and long-distance power transmission is growing. The AC transmission has shown some disadvantages when voltage level becomes higher and capacity gets larger. Modular Multilevel Converter based High Voltage Direct Current, MMC-HVDC has low switching frequency, power loss as well as harmonics. It has high reliability and suits high-capacity long-distance power transmission. MMC-HVDC has promising application and becomes hotspots in recent years.As voltage level getting higher, the number of submodulars (SMs) increase rapidly. The electromagnetic transient model contains large number of switches, which make simulation very slow. The electromagnetic transient model does not apply in power flow calculation and stability analysis of MMC-HVDC. This put off the research and application of MMC-HVDC. Therefore the electromechanical transient model needs to be built for fast simulation.This paper first analyze the topology and mathematic model of MMC and its SM. The outer voltage controller and inner current controller based on dq rotating coordinates is built to control active power and reactive power separately. The nearest level modulation (NLM) and capacitor voltage ranking control are studied. The circulating current suppressor based on negative sequence double basic frequency is derived to eliminate circulating current among phases. Then the electromechanical transient models of AC side, DC side and control unit are built. The simplified model is considered and built in Matlab and compared with electromagnetic transient model in PSCAD/EMTDC, to verify the accuracy of proposed model. Furthermore, the simulation time are compared to show the efficiency of proposed model. Lastly, the dynamic phasor model of MMC is built. It is applied in circulating current analysis. The general matrix for current and voltage harmonics calculation is built in Matlab and the results are compared with electromagnetic transient model in PSCAD/EMTDC to verify the accuracy of dynamic phasor model and its circulating current analysis strategy. |
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