Topological Derivation And Improved Modulation Method Of Multilevel Converters With Common DC-link

With rapid development of the modern society and economy,power electronics technology has been widely used in various industrial applications and plays an important role in the national economy.In recent years,the contradictions between the growths of energy demand,energy shortage and environmental protection are becoming increasingly prominent,which makes many countries pay more and more attentions to develop and utilize the renewable energy.Meanwhile,the emergence of various industries provides power electronics technology with new opportunities and developments.However,due to the complexity and diversity of the applications and the increasing demand for power electronic equipment,the conventional two-level converter has been unable to satisfy the demand of the industrial applications,so multilevel technology has become one of the most representative schemes in the modern energy system.Taking the multilevel converters with common dc-link as the research objects,this dissertation focuses on the multilevel technology in power electronics field and launches research from two aspects of topology and modulation method.There are many types of multilevel converter topologies,and each topology has its own characteristics.Therefore,it is always the research hotpots in the multilevel field that how to quickly and effectively develop a new multilevel topology and reveal the internal connections between multilevel converters.This dissertation summarizes the characteristics of four typical multilevel topology derivation methods,and a voltage-level extension method is presented.The principle of this method is that based on a single-phase known multilevel converter topology,two additional semiconductor switches are added in series to the top and bottom of the single-phase structure,and are respectively connected to a higher level dc-link,forming a higher level topology.According to this derivation method,some novel flying-type multilevel converters have been derived.Among the series of the flying-type multilevel converters,three flying-type converters(four-level flying T-type converter,five-level flying hybrid-clamped converter and five-level flying active neutral-point-clamped converter)are further analyzed in this dissertation.For four-level flying T-type converter,the basic principle and operating mode are analyzed in detail.Meanwhile,compared with the existing four-level converters,the advantages and disadvantages of the four-level converter are summarized.According to the circuit and operating modes,all effective current paths are analyzed,and a set of optimal switching states are determined,which can reduce the irregular voltage jumps during the dead-time.Based on the different effects of switching states on the flying capacitors,all flying capacitor voltages can be controlled by selecting the appropriate redundant switching states to ensure the converter can operate normally.The five-level flying hybrid-clamped converter is a novel topology which is derived through the voltage-level extension method.Based on the characteristics of the existing five-level converters,the advantages and disadvantages of the novel five-level converter are comprehensively evaluated.According to the circuit and the operating modes,the effective current paths are analyzed and the optimal switching states are provided.Through analyzing the energy variations of all flying capacitors within one fundamental cycle,the capacitor voltage control ability of the novel converter is obtained,and a simple control method is proposed based on the different effects of each switching states on the flying capacitors,which can balance all flying capacitor voltages by selecting the least redundant switching states.For the five-level flying active neutral-point-clamped converter,the capacitor voltage control ability is analyzed in detail,and the issue of this converter is that the middle flying capacitor voltage is difficult to be controlled at high power factor and modulation index condition.In order to solve this issue,a hybrid capacitor voltage control method based on the redundant level is proposed,which can break the limitation of the capacitor voltage control ability and ensure this converter can operate normally and all flying capacitor voltages are controllable at various modulation index and full power factor.Based on the theoretical analysis of the three flying-type converters,the common characteristics of the flying-type converters derived from the voltage-level extension method are obtained,and the relationship between the newly-derived flying-type converters and the original converters is revealed.The characteristics of the flying-type converters and voltage-level extension derivation method are summarized.For the complex implementation of the multi-level space vector pulse width modulation(SVPWM),an improved multi-level SVPWM algorithm is proposed.The basic principle of the proposed algorithm is based on the two-level SVPWM algorithm,and the whole modulation process can be completed by only three steps with simple mathematical and logical operations.For the proposed algorithm,the center of the two-level hexagon where the reference voltage vector is located can be directly expressed through a simple calculation,and the decomposition does not require the layer and iterative operations.Meanwhile,the proposed algorithm is generic for any n-level converter and the computational times are almost the same.Compared with the same type of multi-level algorithms,the computational time of the proposed algorithm is shorter and the efficiency is higher.There are 88 figures,19 tables and 188 references in this dissertation.