Recovering the modular multilevel converter from a cleared or isolated fault

The modular multilevel converter (MMC) is one of the best potential candidates for high-voltage direct current (HVDC). Various kinds of faults may occur in an MMC-HVDC system. The system should be restarted by deblocking the converter after a temporary fault is cleared or a fault is isolated, thus recovering the power transmission as soon as possible. This study analyses the recovery process after the converter is deblocked. The analysis shows that the capacitor voltages of the three upper arms or the three lower arms can be rebalanced quickly, and the power transmission and the DC voltage can be recovered with the original main controller. However, the speed of rebalancing the capacitor voltages of the upper arm and the lower arm depends on the arm resistance, the modulation index and so on, and is probably slow. To improve the damping characteristic of the MMC, a dedicated supplementary controller is proposed based on the concept of 'virtual arm resistance'. Time-domain simulation studies based on PSCAD/EMTDC are performed to verify the analysis and the proposed controller.