This paper proposes an improved fast model for the modular multilevel converter (MMC) which

compared with existing fast models

is capable of accurately simulating the converter arm with DC converter blocked. The proposed fast model replaces the converter arm with an equivalent sub-module. Under un-blocked simulation

the converter arm can be treated as a Thevenin’s equivalent circuit

which is similar to the existing fast model. While under the DC converter blocked condition

the converter arm is simplified as an uncontrollable sub-module with lumped parameters. Two-stage start-up simulation and DC line fault simulation are carried out to verify the accuracy of the improved fast model. By comparison

the existing fast model has appreciable computational errors under DC blocked simulation because of the negligence of interpolation of diodes. However

the proposed improved fast model

which takes the interpolation of diodes into account

has very good accuracy under DC blocked simulation.