Distributed Economic Control With Seamless Mode Switching for Networked-Microgrids

A networked-microgrids (NMG) system consists of multiple individual microgrids that are connected together for increased resilience, reliability, and economy. Focusing on off-grid AC NMGs subjected to various power disturbances, this paper proposes a hierarchical control scheme with seamless mode switching, which combines the frequency(f)/voltage(V) control and economic power-sharing among MGs and distributed generators (DGs). The lower-layer aims at rapid f/V response and achieves local and autonomous economic operation among DGs by tracking the upper-layer signals. The upper layer aims at f/V regulation and real-time economic dispatch among MGs. For practical application, local and global economic operation modes are specially designed by constructing equivalent MG cost functions in upper-layer tertiary coordination. The local mode enables the MG to eliminate the steady-state impact on other MGs when an inner power disturbance occurs, while the global mode aims to minimize the total operating cost of the NMG system. The seamless switching between two modes is enabled by adjusting only one cost function parameter, thus much smoother than complex controller switching. Besides, the mode switch has a negligible effect on system small-signal stability, which is proved by the small-signal sensitivity analytical results and time-domain simulation. The proposed control scheme is verified on the OPAL-RT platform.