PWM Control Scheme for a Buck/Boost Modular Multilevel DC/DC Converter With Reduced Submodule Capacitance

High-voltage direct current (HVdc) grid which incorporates multiple dc transmission lines will have extensive application since it has better operating flexibility and enhances the power trade. DC/DC converters are important components in the HVdc grid. The buck/boost modular multilevel dc/dc converter (MMDCC), which uses half-bridge submodules to reduce the voltage stresses across the power switches, is a good choice for the nonisolated applications in the HVdc grid. This paper proposes a pulse width modulation (PWM) control scheme for the buck/boost MMDCC, which uses much smaller submodule capacitors than those in the existing phase-shifted (PS) control scheme, thus reducing the converter volume and weight significantly. The operating principle of the proposed PWM control scheme is elaborated, along with the submodule capacitor voltage balancing strategy and the closed-loop control flowchart. Besides, the capacitance requirement of the buck/boost MMDCC in the PWM control scheme is calculated and compared to that in the PS control scheme, showing a dramatic reduction. The reason for the capacitance requirement difference of the two schemes is also explained. Simulation and experimental results validate the effectiveness of the proposed PWM control scheme and its ability to reduce the submodule capacitance.