Unequal damping of the average sub-module capacitor voltages in modular multilevel converters

摘要

A modular multi-level converter (MMC) used for HVDC application is expected to handle sudden reversal of power flow through the converter. This work reports an unequal damping that may arise in the average voltage of the sub-module (SM) capacitors of an MMC during this power reversal. In the absence of any closed loop control for average capacitor voltages, the damping is found to be significantly less in the inverting mode of operation as compared to the rectifying mode and this inequality is dependent on the HVDC line inductance. Analysis using an equivalent circuit of the MMC, developed in this work, leads to a correction factor for the active power reference which helps to remove this unequal damping. A mathematical justification of this phenomenon using small-signal analysis is also presented. Further, it is shown that with closed loop control, higher and equal damping is achievable. The analysis is validated using the EMTDC/PSCAD simulation.