An Active DC Fault Current Limiting Control for Half-Bridge Modular Multilevel Converter Based on Arm Voltage Reconstruction

For the modular multilevel converter (MMC) based dc grid, dc fault current suppression is a key issue. The existing fault current limiting methods by controlling half-bridge MMCs will result in valve-side overcurrent and arm overcurrent. Thus, limiting the fault current with minimizing the impacts on the ac side is necessary. Generally speaking, arm dc reference voltage determines the contributions of the sub-module capacitors to the fault current, whereas arm ac reference voltage influences the ac-side currents. Therefore, the idea of reducing arm dc reference voltage but keeping arm ac reference voltage unaffected is born. However, arm dc reference voltage is only composed of the dc component and cannot be reduced lower than the peak value of arm ac reference voltage. This constraint limits the suppression effects of the above idea on the fault current. To overcome this obstacle, this article tries to further suppress the fault current through arm voltage reconstruction. First, the principle of suppressing the dc fault current without affecting the ac side by only reducing arm dc reference voltage is investigated. Then, a way to reconstruct arm dc reference voltage is proposed. Through reconstruction, arm dc reference voltage can be further reduced to a value lower than the peak value of arm ac reference voltage. Thus the fault current can be deeply suppressed. Finally, simulation verification and comparisons with the existing methods are conducted. The results show that the proposed method can deeply suppress the dc fault current with little effects on the ac side.