Level number selecting and verifying method based on flexible direct-current transmission system of MMC (Modular Multilevel Converter)

The invention discloses a level number selecting and verifying method based on a flexible direct-current transmission system of an MMC (Modular Multilevel Converter). The level number selecting and verifying method is characterized by comprising the following steps of: 1) establishing a simulation model of the flexible direct-current transmission system by an equivalent model when a three-phase voltage is symmetrical according to the flexible direct-current transmission system in a grid-connected operation process, so as to form a simulation model of flexible direct-current transmission harmonic analysis based on the MMC, which is established according to an optical level approaching NLM (Network Link Module) method, and form a PSCAD (Power System Computer Aided Design) simulation platform of the flexible direct-current transmission system; 2) inputting a plurality of groups of numerical values with a modulation ratio M so as to obtain various main parameters through the PSCAD simulation platform; and 3) describing a curve of changing each order of characteristic harmonic content of the NLM with a level number and a curve of changing a THD (Total Harmonic Distortion) content with the level number through the PSCAD simulation platform, combining voltage class verification to obtain the feasibility and accuracy of a cascade unit number, and obtaining the level number corresponding to a critical point meeting a harmonic requirement simultaneously. By utilizing the level number selecting and verifying method provided by the invention, the accuracy and reasonability of selection of the level number and the corresponding cascade unit number of the flexible direct-current transmission system of the MMC can be verified through the simulation platform and a calculation procedure.