Study On Power Electronic Traction Transformer

Chinese railways are becoming faster, lighter, and more comfortable. It is raised new demands and challenges in electric locomotive technology. Power electronic traction transformer is the most important and most bulky part in the electric locomotive traction drive system. So, it has a great significance to improve efficiency while reduce its size and weight. Power electronic traction transformer integrates transformer and power electronics, which replaced the traditional frequency traction transformer. It can manage power flow and power quality at the same time. The high frequency transformer can be used to provide galvanic isolation, and it also permits the transformer to be much smaller and lighter. Power electronic traction transformer makes it possible to increase available space, to reduce weight, and to be more comfortable.Power electronic traction transformer is essence a converter, which can be divided into three parts:input stage, isolation stage, and output stage. Different topologies can be used in these three stages. Input stage is consist of active front end converter that performs ac-dc conversion and power management; Isolation stage is act as galvanic isolation and voltage converter; Output stage provides suitable output power and frequency for the traction motor. Different topologies are compared, and input stage H-bridge cascade, isolation stage LLC resonance circuit with a high-frequency transformer, output stage three-level traction inverter is determined in this paper.For the proposed main circuit topology, the input stage control methods and isolation stage design method is studied in this paper. The controller features two main loops on the active front end, an inner line current control loop and an outer dc-link voltage control loop. Cascade unit voltage vector is added to its basis equalizing control, to ensure that the voltage balance between its units. Its principle and stability is analyzd in this paper. For the isolation stage, a high frequency transformer has been designed, and its leakage inductance is calculated by finite element method in this paper. Transformer leakage inductance and LLC resonant converter can be integrated in design. The LLC resonant converter also can reduce switching losses on the primary side through zero-voltage switching (ZVS) over the entire load range.A power electronic traction transformer model in Chinese electric locomotive traction power system parameters has been built in this paper. It is simulated in different conditions, illustrating some of the characteristic operating regimes of the power electronic traction transformer, and provide insight into converter capabilities and improved power quality aspects toward the railway grid. It laid the foundation for the power electronic traction transformer prototype in the near future.