Online Expansion Strategy for Islanded AC Microgrid Based on Short-term Harmonic Injection Without Communication Lines

Microgrid, integrated by distributed generation (DG), has attracted more and more attention. In islanded mode, the DGs may face power shortages under extreme conditions. To ensure a sufficient and uninterrupted power supply, the quest for online expansion strategy of microgrid assumes profound significance. Under the most of existing strategies, communication lines are required to achieve synchronization among multiple DGs during online expansion, which are unreliable in emergency. To achieve online expansion without communication lines, this paper proposes a short-term harmonic injection-based method. The harmonic injected by newly joined DG serves as a trigger signal to synchronize all DGs and realize secondary restoration, which is the key technique. Under the proposed method, online expansion is divided into system reset, inverter mode transition, and synchronous secondary restoration. With proposed strategy, online expansion can be achieved without communication lines. Simulation and experimental results have verified the feasibility and effectiveness of proposed strategy.