An inversion-free sparse Zbus power flow algorithm for large-scale droop controlled islanded microgrids

Abstract

A main feature of microgrid systems is their ability to operate in isolation from the main grid, i.e., islanded microgrid (IMG) operation. The power flow (PF) problem of IMGs has been shown to exhibit several distinct features compared to that of traditional distribution systems; namely: (1) having no slack bus, (2) frequency being a PF variable, and (3) generators controlled as droop units. Several studies in the literature have focused on and addressed these particularities. Still, existing IMG PF algorithms are not suitable for the analysis of large-scale IMG systems comprising thousands of buses. On the other hand, practical distribution systems and the IMGs created within such systems may in many cases comprise thousands of electric nodes. As such, the implementation of the microgrid concept in real distribution systems requires PF tools suitable for the analysis of large-scale IMGs. This paper proposes a Zbus PF algorithm for droop-controlled IMG that is particularly suited for sparse implementation. The proposed algorithm is inversion-free, does not require bus renumbering and is independent of third party sparse linear algebra libraries. The accuracy and effectiveness of the proposed algorithm is validated through several case studies considering balanced and unbalanced test systems.