| Due to their superior characteristics such as low harmonic of output voltage,low voltage stress of device and system EMI,multilevel converters are drawn much attention by researchers.They have been used in high voltage motor driver,high voltage direct current(HVDC)transmission,flexible AC transmission,renewable energy generation and other fields.There are mainly three types of multilevel converter topologies widely used: cascade multilevel converter,diode neutral-point-clamp converter and flying capacitor converter.However,the existing traditional multilevel converters have some limitations.With more and more widely application of multilevel converters,how to further simplify their structure,reduce the use of power devices,reduce volume and cost has been paid more and more attention.On the other hand,with the development of power semiconductor technology,how to make full use of high-frequency lower-voltage devices and high-voltage low-frequency devices is also widely consiered,which can make the system structure more optimized.However,with the increase of voltage levels,the number of power devices increases dramatically,and the complexity of circuit topology and control also increases,thus reducing the reliability of the converter system.Now reliability has become another important criterion to determine converter performance such as power density,efficiency and output power quality.Fault-tolerant technology is one of the main ways to improve the reliability of converter systems.Because the multilevel converter itself has a certain redundancy capacity,that is,by reducing the voltage level and output power can release part of the redundant resources,so as to ensure that the system still has fault-tolerant operation capability.Based on the aforementioned two aspects,this paper studies the optimization of multilevel converter topology and the use of fault-tolerant technology to improve its reliability.In this paper,the topology,control strategy,fault diagnosis and fault-tolerant control strategies of the new hybrid multilevel converter are studied.The main contents of the thesis include the following aspects:1.The topology derivation of multilevel converters is studied,which is used to obtain various multilevel converter topologies.Based on series-parallel/parallel-series of basic multilevel switching cells,three kinds of topology derivation methods of hybrid multilevel converters are proposed and extended to general topology or structure.The switching state and operating principle of the derived hybrid multilevel converters are studied and verified experimentaly.The performances such as the number of devices,voltage stress and power losses are analyzed comprehensively.2.Mathematical modeling of multilevel converters is studied.The high-frequency switching function model and low-frequency state average model are established for three-level T-type converter system,respectively.A general mathematical model is established for flying-capacitor based multilevel converters.The accuracy of the mathematical models is verified by simulation and experiment results.3.The modulation and control of hybrid multilevel converters is investigated.Based on the carrier-based pulse width modulation methods,an improved hybrid modulation strategy is proposed.On this basis,the balancing mechanisms of neutral-point voltage and flying capacitor voltage are analyzed,and three active capacitor voltage balancing control are proposed.Both the simulation and experiment results prove the validity of the proposed hybrid modulation and control strategy.4.Model predictive control of the hybrid multilevel converters is investigated.The discrete-time mathematical model of the hybrid multilevel converter system is established,based on which the predictive models are established.The control objectives include balancing capacitor voltage,output current,switching frequency control and orderly transmision of switching states.A model predictive control with different capacitor voltage reference is proposed.Experiment results verify the effectiveness of the proposed model predictive control method.5.Fault diagnosis and fault-tolerant control strategy of multilevel converter are investigated.Aiming at the open switch and short switch faults of three-level T-type converter,a fault-tolerant three-phase four-leg three-level T-type converter topology is proposed,and the corresponding fault isolation,fault location and fault-tolerant control methods are proposed.For the hybrid T-ANPC converters,the rules of open switch fault classification and localization are proposed,and several fault-tolerant control methods based on redundant switching state selection is proposed.Simulation and experiment results verify the effectiveness of the fault diagnosis and fault tolerant control methods.6.Based on the real-time d SPACE controller,a multilevel converter system experimental platform is built.A high integrated experimental prototype of three-phase hybrid T-ANPC converter is designed and developed,which lay a foundation for verifying the effectiveness of the proposed modulation and control,fault diagnosis and fault-tolerant control methods. |
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