Research On Fault Detection And Fault Tolerant For The Modular Multilevel Converter Submodule

With the increasing application of power electronics in the field of reactive power compensation,active power filter and high voltage direct current transmission systems,high voltage power electronic equipment has become a hot research topic.It is the most effective method for multilevel converter to solve the high voltage and high power conversion at present because of its advantages such as low voltages on switch devices,low harmonics in output voltage.Based on the Modular Multilevel Converter,this paper aims to research on the IGBT open-circuit fault detection based submodule and the fault tolerant control for submodule fault.The control strategies under normal operation are analyzed firstly.The topology and working principle of MMC are introduced,on this basis,the advantages and disadvantages of the four modulation strategies and model predictive control are analyzed.The method to realize pre-charging of submodule capacitor is analyzed.The submodule capacitor voltage equalization control strategy is studied.According to the mathematical model of MMC,the circulation current suppression strategy based on the voltage reference value of the arm is deduced.Which prepares for following analysis of control strategies under faults.The open-circuit fault condition of the switching device is studied and two fault diagnosis methods are proposed.One method which can find fault locations rapidly by the comparison actual value and ideal value about phase current and circulation.In addition,the submodule capacitor voltage is needed.The other method which can use the comparison of the actual and predicted values about the bridge arm current and the current flowing through the T₁ and T₂ modules to locate the fault.According to the open-circuit fault condition,the effective fault tolerance method is studied,and two fault tolerant control methods based on the zero-sequence voltage injection are proposed to maintain stable line-to-line voltage under submodule fault conditions.The proposed two methods can make full use of un-faulty submodules and reduce the negative influences caused by the submodule failure.In addition,the proposed two methods are with simple implementation and easy to be executed in digital controllers.Based on the theoretical calculation and simulation verification,the advantages and disadvantages of the corresponding modulation strategies and its application range are pointed out from the aspects of the harmonic analysis,switching losses and dynamic characteristics about the state of the fault tolerance.Last,based on the resource of laboratory,an experimental platform of MMC is built.Using this platform,the IGBT open-circuit fault detection based submodule and the fault tolerant control for submodule fault are completed.The experimental results verified the feasibility and effectiveness of proposed control strategies.