A DC Fault Current Limiting Method for HB-MMC Without Losing Control of AC Currents

Employing converters to limit the dc fault current is a good choice. By reducing the dc voltage, the fault current can be limited. Modular multilevel converters (MMCs) based on half-bridge submodules (HBSMs) are widely employed in high voltage direct current (HVDC) transmission systems. Most of the existing methods for HB-MMC to limit the dc fault current are at the cost of reducing the ac output voltage, which will lose control of ac currents and increase the ac grid's instability. A dc fault current limiting method for HB-MMC is proposed in this paper. The ac output voltage is not reduced during the dc fault current limiting period. Thus, the method will not disturb the ac grid. The common-mode voltage of each arm decides the dc voltage of HB-MMC. Common-mode voltages of the three phases are controlled separately to realize a significant current limiting effect. To avoid the arm overcurrent caused by increasing circuiting currents, the minimum common-mode voltage among the three phases is limited.