Research On Nested Clamped Multi-level Converter Topology And Control Strategy

In the application and research field of medium and high voltage power electronic devices,the multi-level converter output voltage waveform is more moderate than the traditional two-level converter,the working frequency is low,and each switch tube is subjected to reverse voltage in series,which is very suitable for the use of high voltage and large capacity converter.The development of multi-level converters has formed a variety of different topological structures,these structures are distinctive,in their respective scope of application play an important role.Due to the large number of output levels of multi-level converter,the circuit is more complex and the number of internal active and passive components is large.For example,the cascade H bridge needs several isolated DC power supplies,and the diode clamp powered by a single power supply and the flyover capacitor clamp converter need a large number of diodes and capacitors,which makes the cost,volume,control,reliability and other related indicators of the multi-level converter poor.Five level T embedded midpoint clamp converter(T-Type Nested Neutral Point Clamped,T-NNPC)hereinafter referred to as T-NNPC,proposed in 2019 by the Canadian scholars,it only takes six to single-phase switch tube and two fly across the capacitor can output five level,structure is simple,reliable,low cost of small size,considering the T-NNPC a variety of advantages of this article will mainly by the embedded clamping of the multilevel converter as the research target.Firstly,through the analysis of T-NNPCs working principle,the common converter algorithms and the problems of these algorithms in T-NNPC are introduced.Because there is no redundant switching function to compensate the capacitor voltage,the output current and the capacitor voltage can only be controlled by FCS-MPC.However,MPC needs an accurate mathematical model of the system,and T-NNPC needs to control the capacitor voltage and output current at the same time.Moreover,the establishment and calculation of the coupling mathematical model between the two are relatively complicated.Considering this problem,this paper proposes an improved model predictive control algorithm suitable for T-NNPC.Compared with the traditional model predictive algorithm,this algorithm greatly simplifies the system model.Moreover,the improved model predictive algorithm has better output quality and higher stability.Aiming at the defect of high output common-mode voltage of conventional converter,a low common-mode voltage finite-set model prediction algorithm suitable for T-NNPC is proposed in this paper.The simulation results show that the peak value of output common-mode voltage of the converter is reduced by six times,which greatly reduces the harm of common-mode voltage to the system.For further mining multilevel converter performance,this paper summarizes T-NNPC characteristics of the converter and put forward a new kind of seven electric E embedded midpoint clamp converter(E-Type Nested Neutral Point Clamped,E-NNPC)hereinafter referred to as "E-NNPC",E-NNPC each phase need only eight switch tube and three fly across the capacitor can achieve seven level output,structure is simple and efficient.By applying the improved model predictive control of T-NNPC converter to ENNPC,the simulation results show that the total harmonic distortion of the output current of E-NNPC can be reduced by 50% compared with that of T-NNPC under the same external conditions.Given the limited number of sets that seven level model predictive control,digital system iterations,this paper puts forward two kinds of simplified model is applied to the E-NNPC model predictive control algorithm,through experiment,the two algorithms under the condition of the individual performance index fell slightly,seven level model prediction algorithm greatly reduces the calculation burden,has a strong practical value.