Seamless transition control strategy for three/single-phase multimicrogrids during unintentional islanding scenarios

Abstract One of the key features of a three/single-phase multimicrogrid (MMG) is the ability to transition between the grid-connected mode and the islanded mode. A seamless unintentional transition to the islanded mode is necessary for a three/single-phase MMG to achieve power balance and reliable operation. However, when the utility grid fails and the three/single-phase MMG unintentionally switches from the grid-connected mode to the islanded mode, the transition process can cause three-phase imbalance and power shortage problems. To solve these problems, this paper proposes a seamless unintentional transition control strategy for a three/single-phase MMG composed of a quick learning-to-rank combination method and a load shedding control strategy. In the proposed seamless transition control strategy, a quick learning-to-rank combination method that transforms a single-phase source-load-storage into a virtual three-phase load (VTL) is proposed to solve the three-phase imbalance problem. In addition, a load shedding control strategy based on a coordinated load shedding factor (CLSF) is proposed to solve the power shortage problem. The performance of the proposed strategy is verified for a three/single-phase MMG model based on a modified IEEE 37-bus system. During the unintentional islanding period, the proposed strategy reduces the three-phase imbalance by 0.36% and 0.13%, the range of frequency fluctuations by 26.42% and 17.02%, and the frequency recovery time by 36.59% and 29.73% compared with the implicit enumeration strategy (IES) and the compound implicit enumeration strategy (CIES), respectively. The test results show that the proposed strategy can reduce the three-phase imbalance, reduce the range of frequency fluctuations, and ensure the power supply reliability for critical loads during the unintentional islanding period.