Control of a back-to-back VSC system from grid-connection to islanded mode in microgrids

VSC systems have been deployed in microgrids which have two operating modes: grid-connected and autonomous modes. The initial phase of VSC output voltage after a microgrid switches from the grid connected mode to the autonomous mode could significantly affect the microgrid performance if there is an induction machine. This paper focuses on the initial phase selection at the control mode transition point of VSC systems and its effect on microgrids. A simple and effective approach to determine the initial phase is developed. A microgrid consisting of a back-to-back VSC, passive loads and an induction machine is built in PSCAD/EMTDC. The impacts of initial phase of the VSC output voltage on both the induction machine and the VSC system are investigated. The comparison of with and without initial phase consideration demonstrates that both a better transient and steady-state performances are obtained if the initial phase is selected properly.