Research On Some Theories And Application Techniques Of Multilevel Current Source Converters

Multilevel converters can offer many benefits such as increased power ratings, low switching frequency,reduced output harmonics 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(SMES) and the semiconductor devices with high blocking voltages(such as MCT and IGCT),the efficiency of Current Source Converters(CSCs) will be overcome,and hence their use is becoming attractive in high power applications.In this dissertation,some theories about the topology conformation and Pulse Width Modulation(PWM) methods of Multilevel Current Source Converters(MCSCs) have been studied.As an application example of MCSC,operation characteristics,parameters design and control strategies of a three-phase Multilevel Current Source Rectifier(MCSR) have also been developed. The following research works have been achieved.This dissertation explores the issue of constructing MCSC topologies using the duality principle.Three circuit cells for MCSCs are derived from the applications of the duality on the three common leg circuit cells of Multilevel Voltage Source Converters(MVSCs).Based on the presented three cells,some new or given MCSC topologies are constructed.Analytical results show that these derived MCSCs and their corresponding MVSCs present analogous features such as the ac side currents/ voltages and inductor/capacitor currents/voltages,though they are not direct dual structures.Based on this understanding,the research on MCSCs and MVSCs can refer to each other.The PWM methods for three-phase CSCs are also investigated in this dissertation. The indirect PWM method is proposed,which states that some modulation strategies of three-phase MCSCs can be got from those used for three-phase MVSCs by transforming the bi-logic signals to the tri-logic signals.Besides,based on the built-in mechanism of three-phase CSCs,the direct PWM method is also proposed.Compared to the indirect modulation,the direct PWM method has many advantages such as the linear transmission,less switch times and higher-usage of the dc current,etc.In addition,a novel low voltage-stress direct PWM method is developed to lower the voltage stress on the switches of three-phase CSCs.Low frequency and high frequency time space mathematical model of three-phase MCSR have been founded,and the characteristics of the three-phase MCSR in frequency space have been analyzed in detail.Based on analytical results, some comparisons between the three-phase MCSR and the multi-pulses thyristor phase-controlled rectifier are carried out from the characteristics,circuit parameters and cost,etc.Then,it is proposed to substitute the three-phase MCSR for the multi-pulses thyristor phase-controlled rectifier because of many superior features of the three-phase MCSR.According to the analytical results of characteristics of the three-phase MCSR, a circuit parameter design method for the dc-side filter inductor/capacitor is proposed. The dc-side equivalent circuit of MCSR can be looked on as the parallel connections of multiple dc-side equivalent circuits of a three-phase Current Source Rectifier (CSR),so for the constant dc current output,the dc-side circuit is a first-order system and the dc inductor can be designed according to the dc current ripple index and the dynamic response index.For the constant dc voltage output,the dc-side circuit is a second-order system and the design of dc filter inductor/capacitor can refer to those of the buck converter.After designing the MCSR parameters,in this dissertation we investigate the direct current double-loop control for the three-phase MCSR.For the constant dc voltage output,the problem of low bandwidth and gain,resulted from the LC-filter resonance frequency by the direct current double-loop control,makes the dc-side dynamic response bad.The load current forward feedback control strategy is proposed to solve the conflict between the fast dynamic response and the LC-filter resonance peak,and even adopting the less loop gain,the system can have a fast dynamic response and good steady state performance.Finally,a fully digital control system using cartier phase-shifted SPWM is built up based on DSP and FPGA hardware platform.The experiment on a three-module three-phase MCSR is carried out to test the operation performance when outputting the constant dc voltage/current and the experimental results verify the above proposition.