| Nowadays,power electronic technology has been widely used in power transmis-sion,electric drive and many industrial fields.Because of the better modularity,har-monic characteristic and redundant features,cascaded H-bridge multilevel converter applications have extensively applied in such fields.However,several problems make power electronic converters the most vulnerable part of one system,such as higher power level,higher switching frequency,and more power semiconductors and so on.In the event of a failure of power electronic converter,the performance will degrade,and system may indeed shutdown for serious failures.Therefore,it is of great signifi-cance to study the operational reliability improvement strategy for cascaded H-bridge multilevel converter,and to make power semiconductors more valuable in practice.This project was funded by the National Natural Science Foundation of China project Research on Key Technologies of Novel High Frequency Isolated Cascaded Medium/high Voltage Converter(51577187),and the Fundamental research funds for the Central Universities project Key Technologies of Transformerless Cascaded multi-level Rectifier(2010YJ03).Relying on these projects,this dissertation takes cascaded H-bridge rectifier as the research subject to study reliable operation theories for power semiconductors of cascaded submodules.Specifically,the study content contains ac-tive thermal control strategy for pre-fault,fault diagnosis and fault tolerance strategy for post-fault.Active thermal control is an effective way to reduce the temperature stress when the conventional passive heat dissipation capacity is limited by internal or external conditions for power semiconductors of cascaded H-bridge rectifier.The temperature stress is directly related to the power loss generated by power semiconductor.In this dissertation,the power loss is analyzed through type composition and size distribution which is also written by mathematical expressions.Subsequently,the equivalent ther-mal network between junction and ambient of power semiconductor,as well as the lifetime model,are also discussed.The submodule of the cascaded H-bridge rectifier is composed of full-controlled devices which have redundancy characteristics,so that some operating modes correspond to a plurality of switching states.Based on the characteristic of redundant switch states,H-bridge cascaded submodule can be con-trolled to operate under bridgeless mode through corresponding switching states.Therefore,an active thermal control strategy based on operating mode conversion is proposed in this dissertation.It makes cascaded submodule switch to bridgeless oper-ating mode when the temperature indicator exceeds a threshold.Through alternately conversion of different types of bridgeless operating modes,the purpose of reducing the temperature stress of power semiconductors is achieved.Fault diagnosis is the prerequisite and foundation of fault tolerance,and is very necessary to improve the reliability of cascaded H-bridge rectifier.This dissertation focuses on the open-circuit fault of power semiconductors of cascaded H-bridge recti-fier,where the problems generalized as fault feature analysis,extraction and identifica-tion methods are studied in detail.Taking input current and DC-side output voltages as fault signals,the fault features are extracted using coordinate transformation analysis and frequency domain analysis.The voltage fault feature reveals fault location between different cascaded submodules,while the current fault feature realizes fault detection and location within one submodule.Together,a one-to-one mapping relationship be-tween open-circuit faults and fault features is formed.On this basis,a nearest neighbor algorithm based fault diagnosis strategy is proposed.The fault features are classified by calculating the normalizing Manhattan distance which begins from new sample data to constructed foregone sample data set.Then,the status number of the nearest dis-tance reveals the most probable open-circuit fault type.In order to make the cascaded H-bridge rectifier operating continuously and nor-mally after a power semiconductor failure,fault tolerance is studied after fault diag-nosis.The implementation of hardware and software optimization fault tolerance method is discussed here,as well as the advantages and disadvantages.Hardware based fault tolerance method is easy to implement,but the tolerant capability is di-rectly related to the number of redundant hardwares which will cost higher for better properties.Software based fault tolerance methods obtain tolerant capability by opti-mizing the modulation strategy,which cost lower but is more difficult to achieve a complete tolerant effect.Thus,this dissertation proposed a software optimization based fault tolerant method,which is achieved by bridgeless operating modes recon-struction.Different fault states have corresponding fault tolerant operation modes,so the rectifier is enabled to maintain a quasi-normal operation under fault conditions without additional components.To verify and analyze the theories and control strategies proposed in this disserta-tion,Matlab/Simulink and PLECS are used for software simulation,and an experiment platform is set up for practicing test.Taking one double submodules cascaded H-bridge rectifier as an example,these aspects study content are verified by simulation and ex-perimentation,such as power loss and temperature variation of power semiconductors,fault diagnosis and fault tolerant strategies of power switch open-circuit faults.The simulation and experimental results prove the following conclusion.Cascaded H-bridge rectifier has different power loss when operating in H-bridge or bridgeless mode,then the active thermal control strategy can effectively reduce the temperature stress of power semiconductors by alternately changing the operation types under bridgeless mode.The fault diagnosis method proposed in this dissertation can accu-rately diagnose the power switch open-circuit faults,and quickly tolerant the fault state by proposed fault tolerance control strategy. |
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