Cascaded Multilevel Converter And Its Application To Active Power Filter

Together with the development of high power self-turn-off devices and intelligent high-speed micro-controller, high power electronic equipment is more and more widely applied. Traditionally, multi-modular converter structure, the bulk, efficiency, cost and reliability of the power transformer or inductor become obstacles to exalt the power ratio of multi-modular converter structure. The appearance of multilevel converter gives an opportunity to overcome these obstacles. Another important obstacle that high power electronic equipment must face is that the large power device can't work at high frequency, so it's difficult to use good modulation technique such as Pulse Width Modulation (PWM). In order to overcome this problem, Carrier Phase-Shifted SPWM (CPS-SPWM) technique has been researched. In this paper, the cascade multilevel converter based on CPS-SPWM and its application to Active Power Filter (APF) has been deeply studied in theory and practice.In the beginning, the basic principle of CPS-SPWM technique and the operation principle of cascade multilevel converter have been deeply analyzed: CPS-SPWM technique can reach an effect of higher equivalent switching frequency under much lower device switching frequency. This technique improves the equivalent switching frequency through the counteract of lower order harmonics rather than simply processing the harmonic from lower order to higher order;Cascade multilevel converter has many advantages such as the least component amount, the easily balanced DC link voltage due to the separate DC source structure and the identity of each converter unit that is available to perform the modularized design and redundant design. The cascade multilevel converter based on CPS-SPWM technique has both of the advantages, so it has attractive perspective in large power application. A closed-loop detecting circuit of harmonic and reactive current based on instantaneous reactive power theory has been analyzed and used in this APF system. It can achieve the accurate and real-time detection, and make the system stable.In this paper, a prototype of shunt APF with cascade multilevel converter based on CPS-SPWM is presented. This system contains some modules such as multi-channel PWM waveform generator, non-linear load, main circuit, detecting circuit, sample & gain circuit and insulation & driver circuit. All design rules of these modules have been deeply investigated.The experiment, including single phase and three phase, is performed and the experimental result is given. Experimental results prove that the shunt APF system with cascade multilevel converter based on CPS-SPWM can achieve excellent compensation effect even under much lower switching frequency. We can easily get to the conclusion: this method has a perfect prospect in industry application.