Investigation of STATCOMs and Energy Storage Systems in Smart Microgrids

Abstract

This thesis presents a hybrid AC/DC microgrid (H-MG) topology that integrates bidirectional, four-quadrant, static synchronous compensators (STATCOMs), high voltage lithium-based energy storage and managed, high-frequency, step-up DC/DC solar converters. Using this topology, an actual H-MG was built and studied to find an efficient solution to peak-demand energy management, increase renewable/distributed energy penetration, and support electric vehicle (EV) integration with commercial buildings. An H-MG testing facility was constructed in a test building at Griffith University to assess and integrate the different elements involved in constructing distribution architectures that combine both AC- and DC-based electrical networks. The facility comprised a total of 15.5 kW of reconfigurable solar photo-voltaic (PV) modules and 80 kWh of reconfigurable lithium energy storage with a variable, 145 kVA, unbalanced, three-phase, commercial building load. The DC element in the H-MG was formed by a combination of managed DC/DC converters and a lithium battery module. The base parameters of the DC bus were dependent on the AC component of the network, as a single- or three-phase AC supply determined the base DC bus voltage. The base DC bus voltage then deviated based upon the state of charge (SoC) of the lithium battery. Managed solar PV DC/DC converters were placed in parallel and set to follow the SoC voltage characteristics of the lithium battery. The DC/DC converters utilised maximum power point tracking (MPPT) and were placed in parallel to provide direct distributed generation charging functionality for the battery. A STATCOM was directly connected to the DC bus, enabling four-quadrant operation to improve power quality and reduce the peak demand of the building. Utilising both high-power AC and DC, plug-in EVs (PEV) can be a burden or benefit to electricity networks. Using vehicle-to-grid (V2G) connectivity, a PEV can perform similar power quality and peak demand reduction functionality if a bi-directional AC or DC connection is present. This research successfully implemented and operated a managed DC/AC hybrid microgrid with a four-quadrant STATCOM and DC/DC coupled solar PV array on a commercial building at Griffith University. The system provided voltage regulation and a unity power factor in the connected phase during different modes of strategic operation. Power factor control of the building was also achieved during both discharging and charging of the connected energy storage system.