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

As a novel topology,the dual three-level inverter fed open-end winding induction motor has the same output characteristics as the traditional five-level inverter by using two three-level neutral point clamped inverter connecting at the both side of open-end winding induction motor.Compared to the traditional five-level inverter topology,there are lots of advantages in this novel topology,such as lower DC voltage,better fault freedom,more flexible control methods and so on.The dual three-level inverter fed open-end winding induction motor has an extensive prospect in high voltage fields.In this paper,the current conversion process of dual inverter system is analyzed and five voltage output states of this topology are concluded.The mathematical model of open-end winding induction motor is re-established under the static three-phase asix,the static two-phase asix,and the two-phase rotated asix,respectively.Besides this,the ideal linear transformer is used to connect the electrical flow and the signal flow.Based on this,the mathematical model of open-end winding induction motor is established in MATLAB under the static two-phase asix.Based on the dual three-level inverter fed open-end winding induction motor,the traditional 180° vector decomposition SVPWM strategy is researched and the zero-sequence voltage suppression method is considered.Firstly,the principle of the 180° vector decomposition strategy is analyzed in detail,which simplified the modulation from five-level into three-level through resoluting the reference vector into two vectors with equal size and opposite direction.Next,the zero-sequence voltage is suppressed by adjusting the effect time of short vectors.As for the obvious hamornic spikes in voltage spectrum of the inverter,a modulation strategy based on Sigma-Delta converter is proposed,which can optimize the spectrum.However,because of the unfixed switch state and switch frequency,traditional method by changing short vector operation time to eliminate zero-sequence voltage is not adaptable,so the 120° vector decomposition method is proposed to eliminate the zero-sequence voltage of the system.The proposed method optimizes the voltage spectrum of the inverter by oversampling technology of the Sigma-Delta converter.Meanwhile,in three-level space vector diagram,the corresponding zero-sequence voltage value of two basic vectors is zero,when this two basic vectors have a phase difference of 120° but equal size.So in the modulation of the dual three-level inverter system,the reference vector can be decomposed into two equivalent vectors generated by each inverter respectively,with equal size but an angle of 120°,and the zero-sequence voltage is absolutely eliminated.To verify the effectiveness of theory,platform of open-end winding induction motor based on dual NPC three-level inverter is built and debugged,and the experiment of zero-sequence voltage suppression under reference vector 180° decoupled strategy and the Sigma-Delta converter are completed.The results of experiment verify the effectiveness of strategy which proposed in this thesis.