Study On Fault-tolerant Control Of T-type Three-level Converter

In recent years,with the advancement of power electronics technology,power conversion technology has also been developed rapidly,and the proportion of electric energy processed by the conversion process in the daily electricity consumption of the country is increasing day by day.In the latest 14 th Five-Year Plan,it is proposed to promote the development of green and low-carbon new energy technologies.Among them,photovoltaic and wind power technologies are the focus of development.In these two technologies,converters are responsible for the core tasks.High-voltage and high-power converters are the main requirements.The performance indicators of modern converter technology are currently mainly affected by the topology and control algorithm,although the traditional two-level system topology has a simple structure,it is mainly used in the environment of low-voltage low-power grid-connected inverters.To meet the current demand for high-power inverter,multilevel topology is one of the important ways to solve this problem,on the premise of meet the demand of large power converter,on the one hand,ensure the safety of equipment long-term stability work on the other hand,the safe operation of the fault tolerance of fault cases has become the one of the core issue of research scholars at home and abroad.The fault-tolerant control of the converter studied in this thesis is mainly aimed at the T-type three-level topological structure,and analyzes from two directions of normal operating mode and fault operating mode.For the normal operating mode,starting from the topology and working principle of the T-type three-level converter,a basic mathematical model is established,and the double closed-loop control strategy adopted in the case of no fault is the outer voltage loop which based on the PI regulator and the inner current loop of the deadbeat control(DBC).Regarding the modulation strategy of the converter,the sinusoidal pulse width modulation method(SPWM),the space vector pulse width modulation method(SVPWM),the virtual space vector pulse width modulation method(VSVPWM)and the discontinuous pulse width modulation method(DPWM)are respectively introduced.Finally,the core problem of the converter is analyzed and maintained in the middle of the potential balance.The necessary conditions for point potential balance,and based on this condition,the zero-sequence voltage injection method and the independent control method of upper and lower capacitors are introduced.As for the fault working mode,firstly,the working status of different switch tubes when open-circuit fault occurs is analyzed.Based on this analysis,a fault diagnosis algorithm is proposed,which prepares for the follow-up fault-tolerant control research.For the fault-tolerant control technology of the T-type three-level converter,the research is mainly carried out from the two directions of the open circuit fault of the outer tube and the inner tube.The self-balance analysis is carried out on the midpoint potential balance of the open circuit fault of the inner tube,in order to ensure the fault tolerance of the equipment The operation adopts two fault-tolerant strategies of two-level control strategy and hybrid modulation strategy.A common-mode voltage injection method based on linear programming is proposed to ensure the midpoint potential balance under fault conditions.Finally,detailed experiments and simulation analyses are carried out on the applicable scope of these two fault-tolerant strategies.For the open circuit fault of the converter’s outer tube,the article mainly introduces two fault-tolerant schemes based on space vector and voltage vector reconstruction,and has been verified by simulation to ensure the feasibility and stability of the fault-tolerant strategy.