Seamless Grid-Connected/Islanded Transition in Fully Grid-Forming PV Powered Microgrids

This paper investigates the full-state virtual oscillator control (fVOC) method to realize the flexible operation of 100% PV-powered microgrid. In the grid-connected mode, the enhanced DC voltage stability of fVOC enables the grid-forming PVs to provide the dynamic support while being compatible with the maximum power point tracking function. This dual capability is critical to replace the grid-following operation under the very high penetration of renewable sources. In the islanded situation, the reconnection of the microgrid to the bulk grid leads to large phase jumps, which can trigger the overcurrent saturation and induce the DC voltage instability in grid-forming PVs. To address this issue, a phase adaption current limiting method in fVOC is proposed. The full-order system modeling is performed to guide the parameter design of the proposed method. The Lyapunov function is derived to reveal the enhanced transient stability of the grid-forming PV system during the reconnection of the islanded microgrid. The superiority of the proposed method is verified in both simulations and field tests in a 100% PV powered microgrid. The mode transition between grid-connected and islanded is seamless without islanding detection and pre-synchronization.