A Hybrid SVPWM-MPTC Control Strategy of Three-Level NPC Inverter-Fed PMSM under the Balanced and Unbalanced DC Links

Controlling a 3L-NPC inverter-fed PMSM drive is complicated because of the deviation of the neutral point voltage. That the DC-link voltage is not balanced on its upper and lower capacitors tends to result in the inaccuracy in the synthesis of the expected space voltage vectors. A hybrid SVPWM and MPTC method is proposed for this problem. Based on the deviation ratio of the NP voltage, the control strategy includes two stages. Firstly, the dwelling time of the redundant small vectors is rearranged as the traditional method when the deviation ratio is relatively low. Additionally, with the decomposition ratio factor being introduced, the middle voltage vectors are decomposed into small vectors to alleviate the NP voltage. Secondly, when the deviation of the NP voltage surpasses the predetermined threshold, the proposed MPTC is employed with only the vectors capable of regulating the NP voltage being considered and the duty cycle is directly engaged in the cost function to be optimized, which incorporates a flux constraint to eliminate the need for weighting factors. The experimental results show that the proposed method leads to the notable decrease in computational complexity while improving control effect on both the steady and dynamic performance of PMSM.