Research And Design Of Cascaded Medium-High Voltage Converter

For the reasons of bulky,poor performance and more harmonics, the use of traditional high-power converter has been got more and more limited. Multi-level inverters have been drawing growing attention due to its high voltage operating capability, and developed rapidly. In particularly the cascede multi-level inverter has got widespread attention and application for its unipue structure and significant advantages. Therefore, the study of medium-high voltage converter has important theoretical significance and practical significance.First, this paper analysis several medium-high voltage converters with different typical topologies, and choose the project is cascade 9-level converter with 1200KVA/6KV capacity because of the advantages of its topology, adopting the technology of phase-shifting and multiplying rectification, then analyzing the principle of 24 pulses rectifier that can eliminate the line side harmonic current and reduce THD (Total Hannonic Distortion). Focus on the multi-level inverter circuit and its PWM (Pulse Width Modulation) technology, analysis the working principle of cascade nine-level inverter. Also, by contrast to several PWM techniques that can apply to cascaded inverter, propose the CO-SFO-PWM technology, which combine carrier-overlap PWM technology and switching-frequency optimal PWM technology, and use the freedom of carrier-overlap can achieve optimal control for output wave. In addition, according to the 1200KVA/6KV capacity of converter ,this paper also make a quantitative analysis and calculation with rectifier diodes, filter capacitor, IGBT etc to live up to the design requirements.Finally, this paper set up 1200KVA/6KV converter system by MATLAB / SIMULINK simulation technology. Multi-pulse rectifier adopt 24 pulses rectification technology, it was proved that use the technology of phase-shifting and multiplying rectification can improve the power factor and reduce THD of the line side harmonic current by simulation and experimental prototype debug. By comparing the simulation analysis of new CO-SFO-PWM technology and SHPWM technology for cascade nine-level inverter, proved that this new PWM technique can significantly improve quality of the output wave at low modulation ratio,and have low THD of the output wave at high modulation ratio. Moreover, it can increase utilization of DC-side voltage.