Control strategies for Multi-Microgrids islanding operation through Smart Transformers

•Development of a new control framework using Smart-Transformers (ST) for Multi-Microgrid (MMG) systems in islanding operation.•Smart Transformer with two configurations: a) with a storage unit in DC link and b) without storage at the DC link.•Smart Transformer with a storage unit in the DC link assumes a central role as the enabler of the MMG islanding operation.•Without storage, the Smart-Transformer emulates a grid-forming unit for the LV side and a grid-feeding inverter for the MV side.•A new vision on how to operate in islanding mode an MMG with STs, as wells as proper models to include those devices in the dynamic simulations. Smart Transformers (ST) are power-electronic based apparatuses that bring new opportunities for defining innovative operating strategies for Medium Voltage (MV) and Low Voltage (LV) distribution grids in future scenarios with increased shares of small-scale generation units. A distinctive advantage of ST is the possibility of connecting a storage device in the DC link when considering a three-stage configuration (AC/DC/AC). In this paper, ST is exploited within the context of Multi-Microgrids (MMG) in order to enhance the possibility of islanding operation through the identification of new control functionalities. The identification of robust control strategies to coordinate the flexibility of all the available resources (distributed resources at the LV and MV grids and storage units connected to the ST) is required to guarantee the successful operation of the MMG in the islanded mode. In order to address the need of specific control requirements, two different configurations are considered, being the proposed control strategies properly described: (1) ST with an energy storage unit in the DC link which fully decouples all the control possibilities for the MV and LV grid sides; (2) ST without an energy storage unit, requiring proper coordination between the MV and LV grid levels.