Cooperative Control in a Hybrid DC/AC Microgrid based on Hybrid DC/AC Virtual Generators

Abstract

This paper deals with the dynamic control of the DC bus voltage and the AC voltage magnitude and frequency in a hybrid DC/AC Microgrid (MG). In order to allow a high penetration of renewable energies, provide an increased system reliability during islanding and reduce the dependency on the mains, Energy Storage Systems (ESSs) are included in both the DC and AC grids. The MG is composed by a multiport solid-state-based transformation center, with connexion to the mains and a central battery energy storage system (BESS), and a flexible number of AC Nanogrids (NGs) coupled to a Low Voltage DC bus (LVDC) through DC/AC 3-phase Power Electronic Converters (PECs) operated as grid forming, hereinafter referred as Nanogrid Head Converters (NGHCs). The MG control presents two main characteristics: 1) A DC bus regulation scheme is proposed based on DC virtual generators and P/V DC droop that allows to adapt the participation in the power sharing for DC bus regulation and provides with an automatic transition between grid connected and islanding modes; 2) In order to provide a cooperative operation between the different AC NGs, allowing the automatic power sharing between them through the DC bus, a DC/AC virtual generator control scheme, based on the theory of Virtual Synchronous Machines (VSM), is proposed for its implementation in the NGHCs, thus coupling the control of the LVDC and the NGs. The theoretical discussion is supported with simulations.