Research On Three-level Bi-Directional DC-DC Converter For Fuel Cell Power System

Fuel cell has the advantages such as high efficiency, low pollution and good adaptability, and it is a kind of potential generation device. In fuel cell power system, energy storage device and bi-directional dc-dc converter (BDC) are employed to improve the system dynamic performance. Therefore, fast dynamic response is an important issue in BDC. This thesis disusses the design of BDC in the aspects of topology and control, and the soft-start of BDC when the input and output sides are respectively connected to an independent voltage source is also discussed.In the aspect of topology, the three-level (3L) switch cell is introduced to buck-boost BDC and thus a 3L buck-boost BDC is obtained. The inductor of 3L buck-boost BDC is significantly reduced, so the dynamic response is improved. The operation principles, characteristics and parameter design are discussed in this thesis, and the theoretical analysis is verified by a 1kW prototype.In the aspect of control, the sideband effect of pulsewidth modulation (PWM) is presented, and the multi-frequency model is introduced to analyze the bandwidth of two-level (2L) and 3L BDCs. It is deduced that the sideband effect in 3L BDC is eliminated by adopting the interleaved control, and hence the bandwidth of 3L BDC can be moved up to as high as the switching frequency, twice of that of 2L BDC. Consequently, 3L BDC is expected to have better dynamic performance. Simulation results are provided to compare the dynamic performance of 2L and 3L BDCs, and it is shown that the later is better.In fuel cell power system, the input and output sides of the BDC are respectively connected to an independent voltage source. Using the traditional soft-start method will lead to huge inverse current and it will damage power devices. This thesis proposes a two-phase soft-start method, which lets the duty cycles of the active switch and passive switch increase slowly from zero in two successional phases of time, and thus prevents the inverse current effectively. The proposed soft-start method is applied to the 2L and 3L BDCs and the effectiveness of the method is verified by the experimental results.