Improved Arm Average-value Simulation Model of Modular Multilevel Converter with Blocking Capability

The arm average-value model (AAVM) of MMC assumes identical SM capacitor voltages and ignores the sorting of capacitor voltages and therefore greatly reduces the simulation burden. However, the existing AAVM cannot consider the effect of anti-parallel diode and fails to correctly simulate the behavior of arm blocking. In view of this, an improved AAVM which considers the effect of anti-parallel diodes is proposed to simulate the arm blocking. Three thyristors are used to associate the arm current circuit and capacitor circuit in AAVM. In normal state, all thyristors are controlled off and the improved AAVM is performing the same with AAVM. At the instant of arm blocking, all thyristors are controlled on to allow the arm current to charge the SM capacitor, which emulates the effect of anti-parallel diodes. The improved AAVM is compared with the detailed switching model of MMC based on a benchmark system established in PSCAD/EMTDC. The simulation result shows that the improved AAVM behaves the same as the detailed model of MMC at both the steady state and the transient state and the improved AAVM can correctly reflect the behaviors of arm blocking while the simulation time is greatly reduced.