Improved Modular Multilevel Converter Equivalent Method based on Thévenin’s equivalent

The traditional equivalent method is based on the Modular Multilevel Converter (MMC) steady state, without considering the half-bridge sub-module blocking, but in the electromagnetic transient simulation, when the module multilevel Converter is in the start-up phase or fails, there are many sub-modules in the blocked state. When the half-bridge sub-module is blocked, the current direction of the bridge arm will change frequently. Due to the improper simulation of the diode interpolation, the bridge arm current has a distortion point, resulting in low simulation accuracy and poor effect. This paper proposes an improved modular multilevel converter equivalent method based on Thévenin's equivalent, which is different from the traditional bridge arm equivalent method which only considers the normal working state of the system. Moreover, when the half-bridge sub-module is in the locked condition, the diodes embedded in the PSCAD simulation software are used for modeling, so that the problem of diode numerical interpolation is effectively solved, thereby improving the precision of the simulation waveform and the speed-up effect is obvious.