Research On Key Technologies Of Fault Diagnosis And Redundancy Operation For Modular DC-DC Converters

With the advantages of flexible structure,rapid manufacture,and high reliability,modular DC-DC converters have been widely used in high-power high-voltage DC-DC power conversion applications.It normally consists of several basic DC-DC converters,and it could be divided into four categories based on different connections on input and output sides.In normal mode,these basic modules are closely related with each other.If one fault occurs in one module and is not well handled in time,the fault may spread out quickly,threatening the safety of rest modules,or even leading to a collapse of the whole system.Based on the industry survey,the power switch and the capacitor prove to be the most fragile components in power electronics system.A large number of these components are involved in the multi-module applications,which is a high potential risk for the reliable operation.Therefore,to guarantee the safety and save the cost,this thesis focuses on the most vulnerable components(power switch and capacitor),and develops the fault diagnosis and redundancy operation techiques.The main content is organized as follows:On the fault diagnosis of power switch,there are two main limitations of the current research: how to select a proper diagnostic signal in modular DC-DC converters,which has abundant fault information and is easy to acquire;how to make the method more applicable and lower the cost,while maintaining the diagnostic speed and accuracy.To address these issues,a fault diagnosis method based on the synchronization of output inductor voltage cluster is proposed.By analyzing the common feature of typical DC-DC topologies and the operation characterisitcs of multi-module system,the output inductor voltage of each module is selected as the diagnostic signal.In normal mode,the diagnostic signals are synchronized with each other due to the symmetrical operation.However,the asynchronization appears when a fault happens in one module.Based on this feature,a simple diagnostic circuit is designed,which is able to identify the faulty module within one switching period.The proposed method is fast,low-cost,and easy for integration.It works well in different operating conditions,and is promising to be used widely in the future.The experimental result has verified the effectiveness of the proposed method.On identifying the degradation fault of capacitor,the urgent challenge in this field is how to extract the health information of capacitor accurately without interrupting the normal operation or adding extra sensors.To solve this problem,a noninvasive online capacitor condition monitoring method based on the selective module isolation is proposed for modular DC-DC converters.The capacitors on the series-connected sides are the main targets,and the good redundancy ability of modular system is fully used.By isolating the specific module,an operation mode which is convenient to get capacitor information is created during this transition.Based on this idea,the way of getting capacitance information in different structures and its implementation flowchart are presented.The possible factors which could affect the result are discussed.The proposed method is quite easy to implement.All the required information is from the already existing sensors.It is convenient to repeat this process regularly to collect more reliable data and improve the accuracy.The experimental result has verified the effectiveness of the proposed method.After receiving the fault alarm,the redundancy operation should be activated immediately.The main drawbacks of the current redundancy strategies are: the limited coverage range,the low efficient process,and the absence of solution considering the connection with fault diagnosis.To address these limitations,the systematical module-level redundancy operation strategy for different series-parallel structures is proposed.The fault feature on the terminals of different structures is analyzed,and a fault isolation circuit is added selectively.A reconfiguration timing sequence is presented to make the isolation of faulty module and the activation of redundant module happen at the same time while maintaining the normal output.In this way,the electrical stress during the transition is optimized,reducing the design margin and improve the redundancy efficiency.The experimental result has verified the effectiveness of the proposed method.In the last part,based on the fault diagnosis methods of modular DC-DC converters,a common rule for other modular systems is summarized,which is then applied in the submodule capacitor condition monitoring of modular multilevel converters as an example.The proposed method is based on an internal discharging path,which excludes the interference from the main circuit.The modeling and computational burden are reduced.The normal operation of the system is not affected,and the health condition of the capacitor could be monitored in the long term effectively.Besides,it is applicable in other submodule topologies.The experimental result has verified the effectiveness of the proposed method.