The voltage feedforward based pulse width modulation strategy under the fluctuated dc-link voltage for a unidirectional five-level ac motor drive system with open-end winding connection

In a unidirectional five-level ac motor drive system with open-end winding connection, because of the variation of operating frequency of ac motor, the instantaneous power fluctuation of rectifier stage and inverter stage will generate larger voltage ripple on each phase dc-link capacitors, and further lead to a large amount of current harmonics in output currents of inverter. To cope with this problem, an improved space vector pulse width modulation (SVPWM) strategy based on dc-link voltage feedforward is proposed. In this paper, the effects of zero sequence current harmonics caused by dc-link voltage ripple on ac motor are firstly analyzed, including the stator winding flux linkage equation, voltage equation, and electromagnetic torque. Then, the generating mechanism of dc-link voltage ripple and the current harmonics are discussed in detail in terms of instantaneous power fluctuation of each single-phase converter. Based on this, an improved SVPWM strategy by modifying the modulation signal is proposed to suppress the generation of current harmonics. Finally, a 2.2 kW prototype experimental results are given, which verify the feasibility and advancement of the proposed modulation strategy. In a unidirectional five-level ac motor drive system with open-end winding connection, an improved space vector pulse width modulation (SVPWM) strategy based on dc-link voltage feedforward is proposed to cope with the effects of larger voltage ripple generated by the instantaneous power fluctuations.image