Study On Cascaded Multilevel Converter With Regeneration Capability

Constructing an efficient,clean,low-carbon,and safe green manufacturing system is the clear direction of industrial technological transformation and upgrading.The fourquadrant cascaded multilevel converter has advanced features such as green energy saving,high efficiency controllability,and grid-friendliness.Its products have been rapidly promoted in industrial applications.At present,the topological structure of the four-quadrant cascaded multilevel inverter presents a diversified development trend.The research in this paper is based on a cascaded multilevel inverter with a five-level H-NPC structure.This topology reduces the number of secondary windings of the gridside phase-shifting transformer,simplifying the design and manufacture of the transformer.At the same voltage level,the number of cascaded units of this topology is half that of a three-level H-bridge cascaded inverter,increasing the power density of the converter.Firstly,this thesis introduces the topology of a cascaded multilevel inverter based on a five-level H-NPC structure,and analyzes its application advantages and working principles.The parameter design of the grid-side inductance and DC-side capacitance of its single-phase five-level rectifier is carried out,and the influence of selecting different parameters on the system's speed and stability is analyzed.The principle of the modulation method is explained,and the relationship between the phase shift angle and the output voltage harmonics is analyzedSecondly,the control methods of the grid side and the motor side of the inverter are discussed.The mathematical model of the rectifier is established,and the design of its double closed-loop control system is completed.Based on the double closed loop control system structure,the principles and characteristics of the transient current control strategy and the active and reactive current decoupled control in the d-q synchronous reference frame are analyzed.The mathematical model of the motor is established,and the control method design of the asynchronous motor is completed based on the two-phase rotating coordinates.The control to the single-phase five-level rectifier was verified under the MATLAB / Simulink software environment.Thirdly,the energy transfer of the four-quadrant cascade H-NPC inverter is analyzed.Due to the fact that the instantaneous input and output powers are not equal,this leads to the second-harmonic ripple on the DC voltage.In order to solve this problem,the theoretical derivation of the grid side harmonics is carried out,and a partial power feedforward compensation method is proposed to achieve the suppression of DC side voltage fluctuations.The drive system based on the four-quadrant cascaded H-NPC inverter was simulated and verified.The simulation showed that the DC power voltage fluctuation suppression method with partial power feedforward decoupling was effective.Finally,a single-phase four-quadrant cascade H-NPC converter experimental platform based on DSP + FPGA controller was built.The design method of the inverter used in this paper and the suppression strategy of the secondary frequency fluctuation on the DC side were experimentally verified,and the experimental results confirmed the correctness and effectiveness of the design method and the suppression strategy.