An Operating Mode Based on Variable DC Voltage and Current for Enlarging P-Q Capability Range of Unidirectional Current H-Bridge Submodule-Based MMC

Modular multilevel converters (MMCs) are widely utilized in dc applications. With the development of MMCs, many applications require the MMCs to have additional functions and characteristics, such as dc fault clearing capability and low submodule capacitance. A unidirectional current H-bridge submodule (UCH-SM) has been proposed, which uses low submodule capacitance and possesses dc fault clearing capability. However, the reactive power capability is limited during low active power because of the inadequate dc bias in arm currents caused by the low dc current. Although retaining a rated dc current even at zero active power has been proposed, it results in increasing the power losses and large capacitance. This study proposes a new operating mode based on variable dc voltage and variable dc current (VVVCM) for point-to-point dc systems. The basic idea is that both the dc voltage and current vary on the basis of active power and a lower limit is set for the dc current to provide adequate dc bias in the arm currents during reactive power exchange. Analysis shows that the UCH-MMC in VVVCM has an enlarged P-Q capability range and still retains low capacitor usage. Simulation results verify the effectiveness of the proposed mode.