A Reverse-Biased Voltage Controlling Method for Mitigating Arm Overcurrent and Submodule Overvoltage in Hybrid MMCs During DC Faults

Blocking all submodules (SMs) of the hybrid modular multilevel converter is a simple way to clear dc fault currents. However, each arm's reverse-biased voltage (RBV) is uncontrolled in this method. In this case, the dc fault current will concentrate into two of the six arms. Thus, the maximum arm current will increase to the fault current in the dc line, which will lead to arm overcurrent. Moreover, full-bridge submodules (FB-SMs) will be charged by the large arm currents and may suffer from severe overvoltage. The arm overcurrent and FB-SM overvoltage problems have not been solved properly. This letter proposes a method to control the RBV of each arm during the dc fault-clearing process to relieve the arm overcurrent and FB-SM overvoltage. Thus, the safety of the converter can be improved. In the meantime, the impact on the dc fault clearing time is well limited. Simulations and experiments validated the proposed method.