Synchronous Overmodulation for Reduced Number of Switchings with Unit Voltage Gain Including Six-Step Operation

The six-step operation is widely used with its advantages in voltage utilization increased by 10.27% without boost circuits. However, the six-step operation causes non-ideal low-order harmonics in the currents due to the uneven ratio between the sampling frequency and the fundamental one. Moreover, transitioning between pulse width modulation (PWM) and the six-step operation needs consideration of factors such as current shocks and parameter dependencies in voltage limit conditions. The proposed overmodulation method facilitates a seamless transition to the six-step operation while maintaining the closed-loop current regulator. Furthermore, the non-ideal low-order current harmonics are eliminated through a variable time step control. The proposed method achieves a reduced number of switchings by utilizing edge voltage vectors. Additionally, the current ripple is reduced by solving a minimization problem targeting the weighted total harmonic distortion (wTHD) of the phase voltage.