A Capacitor Voltage Balancing Method for a Modular Multilevel DC Transformer for DC Distribution Sys

Abstract

A capacitor voltage balancing method for a modular multilevel DC (MMDC) converter is proposed. The MMDC inherits the desirable features of both a modular multilevel converter (MMC) and a dual active bridge (DAB), namely high voltage capability, fault tolerance, ZVS-on of all the power devices, and a wide operating range. These features are ideal for a DC transformer in any DC distribution system. One key MMDC control issue is balancing of submodule capacitor voltages. Conventional MMC balancing techniques cannot be used as arm currents of an MMDC change direction within each switching cycle. This paper establishes that the average capacitor voltage of each arm is self-balanced and proposes a method to balance voltages within an arm by assigning the gate signals with higher charge difference to the submodules with lower voltages and vice versa. Charge difference is obtained by the voltage difference, hence only the SM capacitor voltages are required for balancing control. Different from the existed methods, the proposed technique is not limited by voltage gain or load conditions.

Existing System

Trapezoidal Modulation.

Proposed System

To ensure stable MMDC operation, a necessary condition is balance of submodule capacitor voltages. This paper has established that the average capacitor voltages of each arm are self-balancing, and within an arm, charging of a submodule capacitor only relates to which gate signal is assigned to this submodule when the MMDC operates in steady state. The proposed balancing method: reassign gate signals with higher charge difference to the submodules with lower voltages and vice versa. The charge difference is obtained by the voltage difference, hence only the submodule capacitor voltages are required for balancing. Simulation and experimental results have shown that capacitor voltages are balanced at different output powers, different voltage gains, and also when an MMDC starts-up. The proposed method can be used for both single phase or multi-phase MMDCs.

Architecture

Schematic of the single phase MMDC