Neutral-Point Voltage Balancing Method for Three-Phase Three-Level Dual-Active-Bridge Converters

Three-phase three-level dual-active-bridge (3L-DAB3) converters are a potential topology for high-voltage and high-power applications. Neutral-point voltage balancing is a complex and important issue for three-level (3L) circuits. Compared with the single-phase 3L dual-active-bridge converter, the self-balancing capability of the 3L-DAB3 is limited. To guarantee the reliability of the converter, a neutral-point voltage balancing method for the 3L-DAB3 is proposed in this paper. First, the neutral-point voltage balancing principle of the 3L-DAB3 is analyzed. Then, the relationship between the duty ratio adjustment and injected neutral-point charge is described. In order to guarantee accurate neutral-point voltage balance, the proposed balancing method adopts a sign-hysteresis control with a dead zone. The dead zone is responsible for whether the duty ratio adjustment is activated, and the sign-hysteresis control guarantees a correct adjustment direction. The proposed neutral-point voltage balancing method only needs to sample the capacitor voltages, thus avoiding a complex parameter design and making it easy to implement. The transmission power of the converter is not affected during the adjustment process. The proposed balancing method has a rapid response speed and does not have problems with respect to stability. Finally, experiments were conducted on a 3.6 kW laboratory prototype. The validity and performance of the proposed neutral-point voltage balancing method were verified on the basis of the simulation and experimental results.