Active Synchronization of Islanded Microgrid using Droop-controlled Grid-forming Inverters

For smooth transition from islanded mode to grid-tied mode, synchronization to the incoming grid is required for any islanded microgrid. This paper is proposing a novel active synchronization method for such islanded microgrids. In this method, the proposed synchronization controller compensates the phase angle and voltage magnitude difference between islanded microgrid side and the incoming grid side to zero by adjusting the droop laws of all grid-forming (GFM) inverter systems of the microgrid dynamically via secondary control layer of microgrid hierarchical control architecture. It is analytically shown that compensation on only phase angle and voltage magnitude is sufficient for synchronization as frequency is compensated by this proposed method indirectly by the phase angle compensation. A systematic approach for tuning the proposed synchronization controllers are provided. For validation, a controller hardware-in-the-loop-based real-time simulation is conducted using OP5700 RT-simulator manufactured by OPAL-RT on a 3-phase, 480V, 500kVA, 55-bus urban microgrid system with 6 GFM inverters where control of 2 GFM inverters are realized on low-cost Texas-Instruments TMS28379D Delfino controller boards. The proposed method is relatively fast and enables a smooth re-connection of microgrid with grid with less transients.