Supervisory control of microgrids in grid-connected and islanding mode — Investigations using a real-time digital simulation platform

A robust grid structure, which makes use of distributed renewable energy generation, should consist of grid sections, which are able to seamlessly switch between grid-connected and islanding mode of operation. This way, failures in the upper level part of the grid can be mitigated. This contribution discusses local and supervisory control strategies, and requirements for microgrids. A robust hierarchical control structure that allows smooth transitions of the microgrid between the grid-connected and islanded mode is proposed. The proposed local controllers for the distributed energy resources (DER) are robust against disturbances, decentralized and show a reliable reference-tracking capability, which is important since transitions between operation modes involve disturbances. The local controllers represent the primary level of the hierarchical control structure. To operate a microgrid in islanded mode, a centralized supervisory control structure is presented that is composed of a simple DER coordination logic, the aforementioned local controllers, a voltage controller, and an open-loop frequency controller. To perform studies on the proposed control structures, the simulation tool PSCAD/EMTDC was used to evaluate the performance of each controller. Verification in a real-time digital simulator confirms the findings. The proposed control strategies appear to be robust which suggests their applicability for a use in microgrid structures.