Harmonic Analysis Of Substation Based On Cascaded Converter Of Advanced Co-Phase Traction Power Supply System

In this thesis, harmonic problems of this traction power supply system is studied based on theoretical analysis. Correctness of derivation is verified by simulation and experiment, and output harmonic characteristics of these types of cascaded inverter are also researched in this thesis.First, this thesis introduces topology, control and modulation strategy of cascaded structure based advanced co-phase power supply system. Then mathematical model of three-level converter rectification in three phase static frame is introduced. And working principles of single-phase PWM rectifier and cascaded inverter are analyzed, and it is controlled by inner current loop controller and outer voltage loop based double closed loop control strategy. And carrier disposition SPWM strategy and carrier phase-shifting SPWM strategy are also introduced.Research on traction power supply system based on Diode-Clamped 5-level H-bridge Cascaded Inverter, particularly in the field of harmonic analysis is still scarce. This paper analyses harmonic of the output voltage generated by the 5H Bridge cascaded inverter based substation for the first time. Using double Fourier function, this article deduces the output harmonic distribution of Diode-Clamped 5-level H-bridge Cascaded Inverter with CPS-SPWM strategy. After that,5H Bridge cascaded inverter is modeled, simulated and verified, using Matlab/Simulink. The simulation results show that the total harmonic distortion rate of the output voltage meets the national standard, and the filter circuit can be canceled.For harmonic problems of three-level neutral point clamped cascaded inverter (NPC-CI), Design CPSPOD-SPWM modulation strategy for the first time applied to NPC-CI, and big triangle carrier equivalent disposal method is put forward.Thus, the analysis can be simplified and complexity of computing can be reduced by equivalent disposal of triangular carriers and modulation wave with double coordinate system and bipolar big triangular carrier. Then, switching function of output voltage with CPSPOD-SPWM (carrier phase shifted with phase opposition disposition SPWM)strategy is obtained. Utilizing double Fourier transform, this paper deduces the analytical expression of output voltage harmonics. By simulation through Matlab/Simulink platform and experimental verification, the output voltages of 1 module,2 modules cascaded inverter,3 modules cascaded inverter and 4 modules cascaded inverter are analyzed by FFT analysis, and characteristic harmonics are counted. Simulation and experiment results indicate that the distribution regularities of output voltage harmonics of 3H Bridge cascaded inverter with CPSPOD-SPWM strategy are consistent with theoretical analysis. Thus, correctness of deduction is verified and theoretical foundation for deeper research is obtained.Low power FPGA+DSP based experimental platforms of cascaded structure consisted of two 5H bridges and cascaded structure consisted of four NPC modules is established. Experiment is completed with appropriate control parameters and hardware parameters. Experimental results indicate:Output voltage and current waveforms are sinusoidal, cascaded inverter is functioning normally; Simulation and experimental results of output voltage spectrums of one module and cascaded structure are consistent with theoretical analysis.