Multi-Purpose Coordinated Control of Distributed Energy Resources in Transactive AC Microgrids

Pervasive penetration of distributed energy resources (DERs), usually constituted by renewable energy sources and/or storage systems along with their interfacing inverters, are pushing AC electrical grids toward a power electronics-based paradigm. Although the presence of DERs in power grids brings more flexibility of operation and the decentralization of energy generation allows us to obtain more efficient power dispatch, it is imperative to achieve proper control over the existing inverters to support the synergistic integration of multiple electric apparatuses. This is particularly true from the perspective of inverter-dominated AC microgrids (MGs), which rely on the implementation of coordination strategies to adequately exploit DERs to support controlled power dispatchability, power quality interventions, as well as accessibility to energy markets. Within such a context, this thesis presents a coordinated control strategy capable of supporting multiple operation modes for transactive AC MGs through a modelfree, plug-and-play and topology-independent steering of inverters. Such a control approach, namely Generalized Current-Based Control (GCBC), is capable of accommodating inverters of assorted operational natures, being of a dispatchable (d-DER) or non-dispatchable (nd-DER) nature, relying on a centralized unit and on low-bandwidth communication links. By flexibly coordinating DERs, the strategy supports the implementation of active current sharing among inverters, also endowing compensation of reactive currents, as well as offering distributed and selective harmonic mitigation. In addition, the control approach is capable of coping with intermittent energy generation profiles, which are typical of nd-DERs. As another feature, the proposed coordination strategy provides proportional current sharing without being affected by line impedance parameters, in contrast to the conventional droop control method. Above all, the GCBC strategy is capable of managing an interconnected MG to operate as a single controllable entity, providing full controllability over its power dispatch to an upstream grid, allowing it to trade energy services in transactive energy markets. The merits of the GCBC strategy are thoroughly assessed throughout this thesis by means of simulation and experimental results, based on multiple MG prototypes focusing on the low-voltage (LV) perspective, ensuring that the method is feasible for implementation in real-life applications. Numerous MG scenarios are evaluated, such as under limited power capabilities, considering the presence of non-ideal voltage waveforms, as well as upon communication issues, ensuring that the GCBC approach endures operation under adverse conditions. Moreover, it is experimentally demonstrated that the method is also capable of improving voltage quality in weak LV MGs of homogeneous features, as an indirect outcome of the proportional sharing of nonactive currents. Lastly, advanced control functionalities are devised by flexibly adapting the GCBC strategy, endowing LV MGs with the capacity to shape their operation to behave as a variable and selective resistor, which supports a more efficient operation of the distribution grid and favors the damping of harmonic resonances. As another advanced functionality, distributed compensation of active and reactive unbalanced currents is also possible, based on concepts from the Conservative Power Theory. Moreover, voltage regulation can be ensured for the MG by means of an automatic scheme incorporating the GCBC, allowing the possibility to concomitantly increase energy exploitation from nd-DERs. Finally, considerations on the integration of optimization methods highlight that further capabilities can be formulated upon the adoption of the GCBC strategy