Research On The Modulation Strategy Of Dual Three-Level Inverter Based On An Open-End Winding Induction Motor

Multi-level converters have the features of output power with high quality, power device with low voltage stress, less switching losses and so on. Therefore, multi-level converters can overcome the shortage of switching device with low voltage range, to some extent. After years of development, several mature topologies of multi-level converters have been formed. Among these topologies, neutral-point clamped(NPC) three-level converter is the most widely used. Dual three-level inverter speed control system researched in this thesis is constituted by two NPC three-level inverters. Compared with the traditional five-level inverter, dual three-level inverter speed control system has the advantages of DC link with low voltage, less independent DC power supply, easy to control the neutral-point potential and so on, which has a prospect in high power industrial applications.The mathematical model of open-end winding induction motor is re-established under the static three-phase coordinate system, the static two-phase coordinate system, and the two-phase rotating coordinate system, respectively. Contact between the electrical flow and signal flow is masterly achieved by using ideal linear transformer in MATLAB. According to the mathematical model of static two-phase coordinate system, simulation model of open-end winding induction motor is built and tested. Switching process of dual three-level inverter is analyzed and five kinds of output phase-voltage state and the corresponding switching state are obtained. According to the five kinds of output state of dual three-level inverter and definition of space voltage vector, space voltage-vector scheme of inverter is depicted, which lays the foundation for the study of modulation algorithm of dual three-level inverter.Reference-vector decoupled strategy and half-controlled strategy of dual inverter researched for dual three-level inverter speed control system both take the midpoint potential balance control, the common-mode voltage and zero-sequence voltage suppression into consideration. Firstly, principle of neutral-point potential balance for the two above modulation strategies is detailed. And self-balancing of neutral-point potential for reference-vector decoupled strategy during each switching cycle is dicoveried. While self-balancing of neutral-point potential for dual-inverter half-controlled strategy is achieved at least within a fundamental period. Therefore, the neutral-point potential fluctuation range of reference-vector decoupled strategy is significantly less than that of dual-inverter half-controlled strategy. Then, based on the definitions of common-mode voltage and zero-sequence voltage in dual-inverter speed control system, common-mode voltage suppression of each three-level inverter is analyzed. The abtianed conclusion is that transient fluctuation amplitude of common-mode voltage for dual-inverter half-controlled strategy is smaller while suppression effect of zero-sequence component for reference-vector decoupled strategy has more advantages. Finally, the calculation method of total harmonic distortion(THD) of a waveform is introduced. Based on the above calculation method, variation curves between output phase-voltage THD and modulation m and variation curves between output phase-voltage THD and fundamental frequency f1 of above two modulation strategies are depicted. Then inverter output harmonic content of both modulation strategies is compared.Two modulation strategies are based on a simplified and fast three-level algorithm. Therefore, over-modulation method of this three-level modulation algorithm is firstly researched, then, it is applied to the two modulation strategies. Over-modulation method proposed in this thesis is novel and simple, which can be realized by just adjusting the given amplitude of reference phase voltage.Control platform of open-end winding induction motor based on dual NPC three-level inverter is built and debugged, where the two modulation strategies and their over-modulation methods are verified. Correctness and validity of the theoretical content for this thesis are verified by simulation and experimental results.