Submodule Fault-Tolerant Control Strategy Of Modular Multilevel Converter Applied In Power Electronic Transformers

With the increase of energy demand and the more serious of environmental pollution,distributed clean energy has become the mainstream of future energy structure.Traditional power transformers have been unable to meet the demands in the scale and function of power systems,and along with power electronics.Meanwhile,with the rapid development of electronic components and technology,power electronic transformers have emerged.Since the three-stage structure power electronic transformers could provide DC and AC ports,which facilitates the access of distributed energy sources,and does not require AC/AC inverters,it has become the most studied at present.It is also the most widely used type of structure.In a variety of cascaded topologies of three-stage power electronic transformers,the advantages of modular multilevel converters are highly modular,easy to implement for redundancy and expansion,high equivalent switching frequency,low withstand voltage,good harmonic characteristics of the output waveform,and can provide high voltage and low voltage DC ports,thus it provides solutions for the application of power electronic transformers in the medium and high voltage fields,and also specify the direction of power electronic transformers in the future energy Internet.However,due to the large number of sub-modules required for MMC in medium and high voltage applications,each component in the sub-module is a potential fault point.How to implement MMC's effective processing of sub-module faults through effective fault diagnosis,fault location,fault sub-module processing,become the key to ensure reliable and stable operation of the entire system.In this paper,on the research background of the power electronic transformer based on modular multilevel converter topology,do research and analysis of modular multi-point converter are carried out from the following aspects:Firstly,the topology of MMC system and sub-module are introduced.The working principle of sub-module and the working principle of three-phase MMC are analyzed.The corresponding mathematical model is established.The sub-module capacitance voltage,bridge arm voltage and current,The relationship between three-phase voltage and current,DC side voltage and current.After that,the mechanism of various faults of the MMC sub-module is analyzed in detail,which lays a foundation for the subsequent fault-tolerant control research of sub-modules.Secondly,the principle of carrier phase shift modulation and recent level approximation modulation commonly used in MMC is introduced in detail.The advantages and disadvantages of each phase are compared and the advantages of recent level approximation modulation in medium and high voltage applications are pointed out.An improved method is proposed.Recently,the level approximation modulation method can improve the harmonic characteristics of the output waveform without increasing the number of sub-modules,the switching frequency of the power switch tube,and the switching loss when the number of sub-modules is small.The SIMULINK simulation model is built in MATLAB software to verify the effectiveness of the proposed improved modulation strategy.Thirdly,based on Kirchhoff's law of voltage and current,a mathematical model based on current and DC current is established,and the relationship between DC current and sub-module capacitance is obtained.Based on this,a fault diagnosis based on DC current is proposed.Methods:Several common alternative schemes for redundant submodules are compared and analyzed.An alternative scheme for redundant submodules participating in the system precharge process is proposed.The scheme of coexisting cold standby submodules and hot standby submodules is ensured.After the fault sub-module is removed,the spare sub-module can be replaced in the shortest time.The simulation model is built in MATLAB/SIMULINK to verify the effectiveness of the proposed overall fault-tolerant control scheme.Finally,the construction work of the modular multilevel converter experimental platform is introduced,which focuses on the acquisition of voltage and current signals,multi-channel analog-to-digital signal conversion,overcurrent protection hardware circuit design,DSP and FPGA coordination controller design and other hardware.Circuit problems,as well as software design issues such as DSP and external AD communication,DSP and FPGA communication.Finally,the verification of the recent level approximation modulation strategy was carried out on the prototype.