Research on Main Circuit Analysis of VSC-HVDC under Over-modulation Method

Over-modulation can increase the voltage RMS of the component with fundamental frequency produced by the voltage sourced converter based high voltage direct current (VSC-HVDC). It has economic and technical advantages since it can reduce the current flowing through the VSC valve, the power losses of VSC valve and the requirement of sub-module capacitance. However, the present theory cannot describe the characteristic electric quantity of modular multilevel converter under natural over-modulation condition precisely. Taking the MMC based on full-bridge sub-module for example and taking advantage of generating negative voltage by full-bridge submodule, the analytical method of MMC with using natural over-modulation is studied. By introducing the AC- and DC-modulation ratio, the mathematical models of voltage ripple, current ripple and circulating current of MMC are derived. Meanwhile, impact on the requirement of the total number of sub-modules caused by natural over-modulation is analyzed. Comparison of the simulations by PSCAD/EMTDC and calculation results shows that the derived mathematical model is capable of describing voltage ripple, current ripple and circulating current of MMC accurately and has value on MMC theory and industrial application.