Transient DC-bias Suppression Strategy of Three-level Dual-Active-Bridge Converter with Five Control Degrees of Freedom

By introducing the three-level (3-L) neutral-point-clamped (NPC) bridge arms into both the primary and secondary sides of the dual-active-bridge (DAB) converter, both sides of DAB can adapt to higher DC bus voltage, and more control degrees of freedom (DoFs) are provided to make it more flexible to control the 3L-NPC-DAB converter and easier to optimize the system performance. In the conventional DAB converter, the transient DC-bias in the current stress and the magnetizing branches are the severe problems need to be solved. The transient DC-bias problem could also occur in the 3L-NPC-DAB converter under the load change, resulting in transformer saturation, affecting the reliability of the converter. Originated from DC-bias is generated from the bias introduced in the integral of voltage in the 3L-NPC-DAB converter. In this paper, a DC-bias suppression strategy is proposed in the 3L-NPC-DAB converter modulated with 5 DoFs to suppress the DC-bias on the current stress and magnetizing branches. Between the two steady states, a transient state is added to suppress the DC-bias, and the values of the control degrees are determined and applied in the PWM generation technique without extra components. With the proposed strategy, the DC-bias in the integral of voltages of the primary side and the secondary side are both eliminated, then DC-bias on the current stress and magnetizing branches could be suppressed after one switch cycle. Experiment results under various conditions are carried out to verify the effectiveness of the proposed DC-bias suppression strategy.