Synchronous Control Method of Three-phase Inverter Under Unbalanced Conditions Using the Energy Operator

Unbalanced three-phase voltages, harmonics and DC offset to the grid voltage will affect the speed and accuracy of a grid-connected inverter when estimating the frequency and phase angle of the grid voltage. Existing synchronization methods were difficult in accurately estimating the phase angle of the three-phase voltages. To solve these problems, this paper proposes a synchronous control method for three-phase grid-connected inverters using the energy operator. First, the cascade delay signal cancellation algorithm is used to eliminate harmonics and DC offset of the grid voltage, and the grid voltage is unitized to eliminate the adverse effects of unbalanced amplitudes. After that the energy operator is applied to obtain the frequency and the phase angle of the thee-phase input voltages. Therefore, the inverter and the grid voltage can be accurately synchronized even under unbalanced grid voltage conditions with harmonic distortion and DC offset. Finally, simulation results are used to verify the effectiveness of the synchronous control method proposed in this paper.