Power Electronic Traction Transformer Based on No Phase-locked Loop Control

MW-class Power Electronic Traction Transformer (PETT) has excellent prospects in 25kV traction network which adopts Cascaded H-Bridge Rectifier (CHBR) and LLC resonant DC/DC converter. However, the electrical transient problems caused by harsh condition of traction network bring higher requirements on the control strategy. Hence, a topology based on no Phase-locked Loop (no-PLL) control is proposed in this paper. By constructing a new control mathematical model, which is not limited by the voltage phase angle of the PLL, the unity power factor operation under various operating conditions can be realized quickly and effectively. Besides, Line side converter (LSC) adopts phase-locked loopless voltage equalization control and quasi-PR control. It works well in the presence of harsh conditions such as traction network voltage sag, frequency mutation and phase fluctuations. Galvanically isolation adopts resonant converter, realizing soft-switching to reduce switching loss. Simulation results show that the PETT under proposed control not only achieves the energy transfer in 25kV traction network, but also works well under the common traction network harsh condition.