13. Unified Grid-Connected and Islanded Operation

In order to ensure the uninterrupted power supply, it is necessary for microgrid to operate in both grid-connected mode and islanded mode. To address this concern, this chapter introduces a unified decentralized control strategy for both grid-connected and islanded operation of the series-type microgrid (CMG). It can realize the smooth mode switching without the need of changing controllers. In terms of the scheme, the system always holds a unique equilibrium point regardless of the grid-connected or islanded operation. Since the CMG is performed in decentralized manner without any communication, it is a reliable and cost-effective solution. Moreover, the self-synchronization of each DG is obtained under both the resistance-inductance (RL) and resistance-capacitance (RC) loads. The small-signal stability is proved and the design of control parameters is given. Finally, the feasibility of the method is verified by simulation and OPAL-RT based real-time simulation results.