Microgrids can operate in both grid-connected and islanded modes. In order to seamlessly transfer from islanded to grid-connected modes, it is necessary to synchronize microgrid voltage and frequency, and phase to the main grid. However, since the microgrid is often based on power electronic converters, the synchronization process is quite different compared with the quasi-synchronism control in conventional power systems. First, in order to address this concern, the microgrid synchronization criteria are derived. Based on these criteria, a novel distributed active synchronization strategy is proposed, which takes into account not only the fundamental component, but also positive and negative sequences of the harmonic components. This way, a seamless reconnection to the main grid can be performed. The proposed method is implemented in the secondary control level of a hierarchical control structure. Real-time hardware-in-the-loop results show the feasibility of the proposed technique.