Research On Topology And Control Strategy Of Bidirectional Half Bridge Three-Level DC-DC Converter

Single phase line frequency transformer is necessary for existing traction applications on electric locomotives to get lower voltage. However, the line transformer is contributing to about 15% of the weight of a whole locomotive. To decrease the weight and size of the traction applications and increase transportation capability, medium frequency transformer (MFT) isolated power electronic transformer (PET) can be used. For Chinese 25KV/50Hz railway grid, a diode-clamped three-level multi-module based power electronic transformer is contributing to improve interface voltage and power scale, meanwhile, the number of cascaded modules and medium frequency transformers is decreased. The bidirectional diode-clamped three-level half-bridge DC-DC converter, as an important role of the equipment, provides voltage isolation and adaptation from high voltage to low voltage DC-link output. A minimum RMS current control algorithm is studied in three-level half-bridge DC-DC converter to improve system efficiency.First, the advantage of the bidirectional three-level half-bridge DC-DC converter in structure has been discussed, compared to the dual active bridge DC-DC converter (DAB) and the dual half bridge DC-DC converter. The operating principle, power characteristic and zero voltage switching (ZVS) range of the three-level half-bridge DC-DC converter with phase-shift (PS) control strategy have been analysed. To take the advantage that the two bridges of the converter can generate three-level voltage, a dual three-level modulation is proposed. The modulation strategy is verified by Matlab simulation.Based on above analysis, the operating principle, power characteristic and ZVS range of single three-level control strategy have been analysed. The superiority of the single three-level control strategy is that all switches can achieve ZVS under whole power range. And then, the RMS current in DC-DC converter can be decreased by coordinating control the duty ratio D1 and phase shifting ratio D However, Considerable circulated current is existing under non-load condititon by adopting this control strategy.In order to further reduce the RMS value of the current in DC-DC converter, The operating principle, power characteristic and ZVS range of the DC-DC converter with Dual three-level control strategy have been analysed. The relationship of D1、D2 and D which minimize the RMS current when transferring certain power is solved. Theory analysis shows that the solved result can significantly reduce RMS value of the current under light load. To verify the optimal control strategy, a simulation model has been built by Matlab/Simulink, and the simulation result is consistent with the theoretical result.Finally, an 800W laboratory prototype has been constructed to verify the optimal control strategy. The experimental results show that the dual three-level modulation, the single three-level control strategy and dual three-level control strategy is feasible. At the same time, the optimal dual three-level control strategy can significantly reduce RMS current and improve the efficiency of the converter under light load.