Phase Synchronization Algorithm for Seamless Mode Transfer from Stand-alone Mode to Grid-conncted Mode

When the AC grid is disconnected due to an accident, the interlinking converter linked to the DC microgrid supplies power to the AC load in standalone mode. When the AC system is recuperated at any time, the interlinking converter must again switch to the grid-connected mode for stable operation of the distribution. In this case, the phase of the AC grid and the phase of the voltage supplied to the AC load by the interlinking converter may be different from each other. If the mode is switched as it is in a state with a large phase difference, it is equivalent to a short circuit between two AC voltage sources having different phases, so a large inrush current flow. Therefore, to prevent such an accident, an algorithm that synchronizes the phase of the interlinking converter and the phase of the recovered AC grid is required. In addition, while performing such phase synchronization, electricity that meets the electricity quality maintenance standards must be supplied to the consumer side. Therefore, in this paper, we proposed a phase synchronization algorithm for seamless transition when switching from stand-alone mode to grid-connected mode. This algorithm was proposed based on the electricity quality maintenance standards. The suitability of the proposed algorithm was verified through PSIM simulation.