FIG. 16 shows that when the grid voltages in three phases are unbalanced, and after the zero sequence is injected, the bridge arm voltages in three phases are saddle-shaped waves (correspondingly referring to vbrefABC), which improves the DC voltage utilization rate, without any distortion in the three-phase current (correspondingly referring to igABC). The third-order AC harmonics injected into the bridge arm voltages in three phases in the open-loop manner are different, in which the amplitude of the third-order AC harmonic injected into the phase with a higher grid voltage in the open-loop manner is larger, but the total third-order AC harmonic in each phase are basically the same, indicating that the third-order resonance controller enables the total three-phase AC harmonic to be synchronized automatically and implements the effects of zero sequence.
Part (A) and part (B) of FIG. 17 respectively show the suppression effects for the double frequency of the voltage DC-Link in a case in which the zero-sequence component is not injected and in a case in which the zero-sequence component is injected according to the present disclosure. As shown in FIG. 17, the third-order harmonics are injected such that the waveforms of the bridge arm voltages are closer to a square waveform. Also, the fluctuation in double frequency of the power obtained by multiplying the bridge arm voltage and the current is reduced. The peak value of in the fluctuation of the voltage Vdc in the figure is reduced from 80V to 50V, i.e., reduced by more than 30%.
The third embodiment of the present disclosure is the method for obtaining the mean value of all the first information applied to the three-phase Y-connected system.
