A Flexible Energy Gateway for Hybrid Nanogrids

This paper presents the topology and control of a three-port energy gateway for hybrid ac/dc nanogrids. The simple hardware architecture allows to connect renewable energy generators, energy storage devices, like ultra-capacitors, and the utility grid through three different interface converters, which, altogether, define the three-port energy gateway of the nanogrid. The proposed energy gateway represents a trade-off between circuit complexity and control flexibility, allowing i) operation of the energy storage port over a wide voltage-range, ii) control of the local dc-bus voltage at a predefined set-point, iii) multi-directional power flow, iv) support of the local ac-bus voltage with possibility to transition into islanded operation. A hierarchical control strategy is presented that enables flexible power exchange between the ac and dc buses. At the top of the control hierarchy, a human-machine interface is dedicated to operation mode selection and parameter preset; then, a supervisory control layer is present for system-level monitoring and control functions; the lower layer of the hierarchy is constituted by converter control functions for power flow regulation, achieved leveraging on voltage and current controllers. The flexibility and effectiveness of the proposed energy gateway architecture, control, and implementation are demonstrated in the paper in a variety of operation modes by means of experimental results.