Study and design of a multiport converter Optimized for applications in renewable energy field using flow batteries

Achieving a sustainable future depends significantly on the transformative capability of the energy sector. The international energy agency presented a roadmap towards achieving net-zero emissions by 2050, in which a substantial upscaling of renewable energy resources is needed. Distributed Energy Resources (DERs), such as photovoltaic (PV) solar panels and energy storage systems (ESS), exhibits substantial potential to curtail carbon emissions. As the energy generation shares move toward cleaner sources, the electrification process emerge as crucial aspect for achieving broad-scale emissions reduction and safety and efficient control a vast number of DER. Microgrids, serving as decentralized entities, alleviate the control burden on the central grid. Central to the dynamic efficacy of microgrids is the role of power electronics converters. These converters manage the energy flow, allowing the reliable integration of various DERs within the microgrid framework. The application of ESS offers an array of benefits that extend far beyond simple energy storage. ESS's multifaceted nature underscores its significance within microgrids as a dynamic component that enhances operational flexibility, amplifying the efficacy of the microgrid concept. The main objective of this work is to design a power electronics converter and its control architecture for integrating Vanadium Redox Flow Batteries (VRFB) to a microgrid that comprises two distinct lines: one linked to the main grid and another connected to a PV array. Both lines operate as AC systems, with a nominal voltage of 230 Vrms and a frequency of 50 Hz. The VRFB is a dynamic element of the microgrid. To comply with the plant's requirements a double stage topology featuring a isolated DC-DC stage and a DC-AC stage is considered. A dual active bridge (DAB) is used as DC-DC stage, it provides galvanic isolation by means of an high frequency transformer, its output is connected to the dc-link and to the DC-AC stage, realized with an h-bridge with an LC-filter connected on the output AC port. The control architecture is responsible for managing the power flow and ensuring the reliable operation of the VRFB, loads, and PV array. The controller has to operate in two modes depending on the AC line to which the converter is connected. The methodology applied in this work is based on theoretical modeling of the power converter. Initially, the control architecture has been studied and simulated offline in continuous time mode in Matlab Simulink. After a discretization process, the control algorithms have been implemented in a real-time controller and tested with hardware in the loop simulator OPAL-RT. Finally, the control algorithms have been tested experimentally on a converter prototype. The offline simulation results prove the correct functioning and the stability of controller architecture in both operating forms. In addition to conventional PI and resonant controllers, this work introduces two model predictive control (MPC) algorithms for the control of double stage converter made of a DAB and a three phase two-level three-phase grid side converter (GSC). Several Hardware in the loop simulations are performed to replicate the off-line simulation results, and to design and test the start-up procedures of the converter which is a critical moment in particular for the DAB high-frequency transformer and dc-link capacitors. Finally, the control architecture is tested experimentally on a converter prototype built in the power electronics laboratory of the University of Pavia. The experimental results are presented for both operating modes, together with the results of the start-up procedure from the battery and from the grid.