A Novel Sensorless Capacitor Voltage Balancing Method for the Modular Multilevel DC Transformer With Diode-Clamped Circuits

This article proposed a novel sensorless capacitor voltage balancing method for the modular multilevel dc transformer (MMDCT), which is a promising solution for interconnecting dc grids. The MMDCT comprises two modular multilevel converters (MMCs) and a medium frequency transformer (MFT), where the MFT provides galvanic isolation between the input and output dc grids and the MMCs offer high-voltage handling capability. In order to produce medium frequency sine-wave voltages at the MFT terminals, the phase shift carrier pulsewidth modulation (PSC-PWM) is employed for the MMCs. Typically, a feedback controller is required for each submodule of the MMC to regulate the capacitor voltage to its nominal value, resulting in increased costs due to the requirement for a large number of voltage sensors and fiber optic transceivers. To address this issue, low power rating clamping circuits are implemented between each adjacent submodule of the MMC, and the capacitor charge differences caused by the PSC-PWM are analyzed. Sequential balancing and nonsequential balancing are proposed to achieve capacitor voltage self-balance. Both simulation and experiment results have shown that the capacitor voltages are well-balanced with the proposed method.