Control of utility interfaces in low voltage microgrids

The paper presents a general control technique for Utility Interfaces acting in low-voltage microgrids. The Utility Interface (UI) is a three-phase power conversion unit, equipped with energy storage, which governs the interaction between the utility and the microgrid. It is in charge of several functions: in grid-connected operation, UI performs as a voltage-supporting unit and compensates for reactive power, unbalance and distortion caused by loads; in islanded operation, UI performs as a voltage-forming unit and sets the voltage and frequency for the entire microgrid; moreover, it ensures soft transitions from on-grid to off-grid operation, soft start after a power outage (black start), and compliance with applicable grid codes. Finally, UI can perform as master controller for distributed energy resources. Due to this variety and complexity of functions, the UI control is a critical issue and must be analyzed carefully, to ensure safe and reliable operation of the microgrid. In this paper, we discuss a control approach that provides all required functionalities and ensures proper microgrid operation even in case of unintentional islanding or severe load transients.