Research On Circulating Current Control And Open-circuit Fault Diagnosis And Location Method Of Submodule Switching Device For Single-phase MMC

Flexible DC transmission has significant advantages in the field of large-scale renewable energy grid-connected power generation.Modular multilevel converter(MMC)is widely used in the field by virtue of its advantages such as high modularity and low harmonic content of output voltage.The circulating current is generated due to voltage imbalance in the MMC system.In addition,the system also uses a large number of sub-modules and power switches,which increases the possibility of system failure.Therefore,circulating current control,accurate diagnosis and location of open circuit fault of sub-modules have become current research hotspots.In this paper,the single-phase MMC inverter is taken as the research object to conduct in-depth research on the above problems,and the specific research contents are as follows:Firstly,the topology structure and working principle of the single-phase MMC system are analyzed in the normal operating state,and the equivalent circuit mathematical model is established.The principle of three main modulation strategies and the corresponding sub-module capacitance voltage balancing control methods are introduced.Quasi proportional resonance(QPR)method is used to effectively control the 2 times harmonic components in the circulating current.Secondly,the circuit working principle under short circuit and open circuit fault of the submodule switching device is analyzed in the faulty operation state.The QPR method and the repetitive control+QPR method are respectively used to effectively control the main harmonic components in the circulating current under open-circuit faults,and the overall control scheme of the MMC system is explained.Then,for the open-circuit fault diagnosis and location of the sub-module switching device,the circuit characteristics of the system are analyzed under different open-circuit faults.The preestimated value of capacitance voltage is calculated by Kalman filter algorithm,and the theoretical value of capacitance voltage is calculated by integral algorithm in real time,and according to the change law of voltage difference between them,the fast open-circuit fault diagnosis and location of the sub-module switching device are realized.Furthermore,the experimental platform of the MMC system is built,and the design of hardware circuits and software programs are introduced.The system adopts the control architecture of FPGA+ARM,and the data communication between FPGA and ARM is explained.Finally,simulations are used to verify the effectiveness of the main harmonic components control methods in the circulating current under normal and faulty operation states.And simulations and experiments are used to verify the effectiveness of fault sub-module diagnosis and location method under different open circuit faults,the results show that the various operation states of sub-module can be judged within 100 ms.Simulation and experimental results verify the effectiveness and feasibility of the research method.