Electrical isolation in advanced power supply systems typically relies on power frequency transformers or high-frequency isolation DC-DC converters. However, the transformers result in multiple converter stages and increase the system’s complexity and cost. To reduce the cost of advanced traction power supply system, this paper proposes a two-phase to single-phase non-isolated power electronic transformer (NI-PET) topology based on the existing traction transformer and has the advantages of fewer transformation stages and higher system efficiency.

Some switch states can result in short-circuit paths of the DC-link capacitance in NI-PET topology. The traditional modulation strategy fails to avoid the short-circuit paths. A three-dimensional space-vector pulse width modulation (3D-SVPWM) strategy is proposed based on the 3D space vector distribution diagram, taking the vectors of three ports as the coordinate axis. According to the number of available vectors, the 3D space is divided into different ranges. In addition, the proposed strategy determines the range of reference voltage vectors and selects available space vectors to complex the demanded reference vector. Finally, based on the V-v traction transformer, the simulation model and experimental platform are built.

Simulation and experimental results show that compared to the traditional space pulse width modulation (SPWM) strategy, the proposed modulation strategy can realize the stable operation of the system. When the load and grid-side voltage fluctuate repeatedly in a short period, the two-phase to single-phase NI-PET system restores a steady state within 0.2 s, the grid-side power factor remains above 0.99, and the THD of input and output current is less than 3%. With the same load, the three-phase current unbalance degree of the proposed topology is about 45% less than the traditional power supply system. It is verified that the proposed topology and modulation can adapt to harsh conditions such as continuous load and grid-side voltage fluctuations. Compared to PET, NI-PET avoids the loss caused by the isolation stage, thus significantly improving the efficiency. In the low-power experimental platform, the efficiency of NI-PET is about 10% higher than PET.

The following conclusions can be drawn. (1) The proposed two-phase to single-phase NI-PET topology can adapt to the harsh conditions of advanced traction power supply systems. It has the advantages of low cost and good power quality. (2) Compared to the traditional modulation strategy, the proposed one can avoid the short-circuit paths of DC-link capacitance. There is no short-circuit current that is much larger than the load current on the cascade line. (3) The proposed topology can achieve about 10% efficiency improvement in a low-power experimental platform and is expected to increase the efficiency by about 2% in industrial PET.