Optimum Injection of Second Harmonic Circulating Currents for Balancing Capacitor Voltages in Hybrid MMC during Reduced DC Voltage Conditions

Hybrid modular multilevel converter (Hyb-MMC), which consists of both half-bridge (HB) and full-bridge (FB) submodules (SMs), has the capability to withstand wide variations in dc-link voltage ranging from nominal to dc short circuit conditions. With around 50% FBSMs, the Hyb-MMC can block the dc short-circuit current while exchanging reactive power with its ac grid. Other than faults, often, the high voltage direct current system may have to operate at reduced dc voltage levels with rated dc-link current, which results in arm currents becoming unipolar. This phenomenon affects the cyclic charge/discharge of the HBSM capacitors. To overcome this problem, it is proposed to inject second harmonic circulating current (SHCC) for making the arm currents bipolar and, hence, ensuring capacitor voltage balance. However, the choice of suitable amplitude and phase angle of the SHCCs is crucial to avoid increase of losses in the converter. Hence, a method to optimize the amplitude and phase angle of the SHCCs is also presented. The proposed method is verified through simulations performed on PSCAD/EMTDC platform followed by experiments on laboratory scale prototype.