Research On Single-Phase NPC Cascaded PWM Rectifier And Its Control

There are some inherent disadvantages of line frequency transformer (LFT) in high-speed railway traction system, such as bulky, noisy, and fixed out voltage problem. The power electronic traction transformer (PETT) is an alternative solution to substitute the LFT using power electronics technology, and the issues of LFT can be solved. The neutral point clamped (NPC) three-level cascaded PWM rectifier based power electronic traction transformer is studied in this thesis, and feasibility of the system is also verified by theoretical analysis, simulation and experiment.The NPC three-level cascaded PWM rectifier topology and its control system are designed for power electronic traction transformer in this paper. The topology and working principle of NPC three-level PWM rectifier is studied. On this basis, the mathematic model, overall voltage control strategy and voltage balance control strategy between modules of NPC three-level cascaded PWM rectifier are analyzed. According to the present research status of multilevel modulation technology, the phase opposition disposition (POD) PWM modulation and carrier phase-shifted (CPS-PWM) modulation is mainly studied. On the basis of characteristics of NPC three-level cascaded PWM structure, the modulation strategies are combined POD-SPWM with CPS-SPWM modulation.According to the research aforementioned, the simulation system of sixteen-module NPC three-level cascaded PWM rectifier applied in PETT is built based on MATLAB/Simulink. The results of single phase grid input current, DC output voltage of rectifier, voltage balance control between cascaded modules and inner-module capacitance voltage balance control are analyzed respectively when the simulation system is operating on startup, non-load, load and load mutation conditions. The simulation results show that the system can work normally and stably under different conditions. The system can respond quickly, the DC voltage overshoot is small, and the adjusting time is short when the system operating conditions change. In the case of load operation, the cascaded rectifier could operate under unit power factor operation, and the harmonic distortion (THD) rate is low.In order to further verify the reliability of the system, a low power experimental platform is built based on field programmable gate array (FPGA) control system, and relevant experimental system structure, the experimental parameters and the experimental program have been designed. With the low power experimental platform, experiments of single phase single module PWM rectifier, single phase three-module cascaded rectifier and single phase nine-module cascaded rectifier are completed. Experimental results prove that the feasibility of the main circuit structure, control strategy and modulation strategy of the system.In conclusion, the main circuit structure, control system and modulation strategy of NPC three-level cascaded PWM rectifier are designed for power electronic transformer. The simulation and experimental results verified that the system voltage level and equipment capacity can be improved through cascaded connection scheme; the equivalent switching frequency of the cascaded system is quite high, every module switching frequency can be properly reduced to minimize the switching loss. Furthermore, the control strategy and modulation strategy adopted is easy to extend to more cascaded modules. The NPC three-level cascaded PWM rectifier for power electronic transformer to replace the traditional locomotive AC traction transformer is feasible in engineering practice.