Study On Three-phase Multilevel Rectifiers Of Current-Source

Multilevel converters offer benefits of increased power ratings, low switching frequency, reduced output harmonics, fast dynamic response and good electromagnetic compatibility (EMC), which will make it possible to enlarge the power rating and improve the output performance simultaneously. With the development of superconducting magnetic energy storage technologies (SMES) and the semiconductor devices with high blocking voltage (such as MCT and IGCT), the efficiency of the conventional CSI's will be overcome and hence they will be in a more attractive proposition, particularly for higher power applications.Carrier phase-shifted SPWM(CPS-SPWM) can help SPWM be used in higher power applications since it can do benefits for improving waveforms greatly and restraining harmonics.So it can solve the problem of power and frequency that can't be increased together.The topology construction,together with its PWM strategy are presented in this dissertation due to CPS-SPWM and module-parallelizing techniques.The contents are as following.After reading lots of relevant papers,the topology together with its PWM strategies were discussed.And 7 level rectifier was taken as an example to illustrate the working process,self current-balancing characeristics and so on,which was the basic theory of controlling strategies in closed-loop.On finishing analyzing the different harmonic characeristics of 2-logic and 3-logic CPS-SPWM techniques seperatelly, the 3-logic CPS-SPWM technique was chosen as the strategy in this dissertation.After learing the experience of people's above, a digital platform consisting of DSP and CPLD was set up on which a technique named phase locked-loop was studied.This technique can make the compared sine waveform synochronize with AC source both in phase and frequence.7 level three-phase rectifier of current-source was chosen as an example to illustrate its parameters. Three methods of 3-logic CPS-SPWM techniques were discussed and the most simple digital implementation method for 3-logic SPWM was presented in the thesis on the basis of which all the 18 PWM signals were generated.An experimental prototype of 7-level rectifier was set up to verify the proposition.In order to make line power factor equal to 1, an indirect-current-control stragegy based on the current back from AC-source was presented.After comparing the multilevel rectifier with multi-pulsation rectifier from various aspects, a conclusion was done which pointed out that the former one was much better and it had more practical value and wider application prospect.