This paper proposes the grid synchronization method of inverter using a quasi Proportional+Multi Resonant (P+MR) control-based voltage controller a stationary reference frame. The inverter supplies a non-linear load under the islanded mode. In islanded mode, the inverter is defined as a voltage source to supply the full local load demand without a connection to the grid. On the other hand, if the grid is restored from a previous fault or the strategic islanding is unnecessary, the inverter needs to be synchronized with the phase of the grid before the transfer from islanded mode to grid-connected mode. When the system is modeled and controlled based on the stationary reference frame control, the AC reference voltage, which has a constant voltage and frequency in islanded mode, is substituted to the AC grid voltage. Significant error can occur due to the large phase differences between the phase of reference and that of the measured value. This error also can cause severe voltage dynamic problems. In addition, if any nonlinear local load is connected to the output of the inverter, it becomes more serious due to the harmonics generated from the loads. In this paper, the PR control under a stationary reference frame is used for voltage control under islanded mode considering the harmonic effects from the nonlinear load. The seamless grid synchronization method based on this PR control is proposed to solve the aforementioned problems. The validity of the proposed seamless grid synchronization method is verified through PSiM simulations and experimental results.
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