Power Converters and Control for Grid Connected Microgrids under Unbalanced Operating Conditions

Abstract

Nowadays, Renewable Energy Sources (RESs) have become more attractive and affordable due to recent advances in power electronic devices and control systems. Micro-Grids (MGs) represent a new paradigm of electrical grids with Distributed Generation (DG) units: they are generally composed of power converters, RESs, Energy Storage Systems (ESSs), local loads with measurement and control systems. MGs have the potential to operate in both, grid connected and islanded mode. During islanded mode, ESSs are essential for MGs to enable grid forming control. On the other hand, during grid connected mode, the ESSs allow the MGs to provide different services to the main grid. For example, grid power supporting during peak periods; however, in case of unbalanced operational conditions for MGs, due to single phase loads and single phase generation units, the power exchange between MGs and the main grid will add negative effects to the main grid. Since the MG is supposed to be grid friendly when connected to the external grid, the unbalanced currents should preferably be handled within the MG. Therefore, the power converters of MGs have to provide the zero sequence and negative sequence currents. The main objective of this thesis is to obtain grid friendly MGs, in order to improve the functionality of MGs during the grid connected mode, under unbalanced operating conditions. Power converters with ESSs can be adopted to mitigate the negative effects of unbalanced grid connected MGs. However, suitable control strategies are required. In this thesis, a control strategy based on vector control and symmetrical components is proposed for three-phase four-legs power converter interfaces Energy Storage Batteries (ESBs) to obtain a Multi-Functional Power converter (MFPC), in order to resolve the problems of the unbalanced three phase currents, and to the reactive power compensation. Several working conditions have been analyzed, and solutions for some common and frequent critical conditions, such as the imbalance of the power system due to single-phase loads and single phase DGs have been presented. Discussions of technical issues like output filter design and four legs VSIs modulation techniques, synchronization, power converters topologies and control have also been discussed. The proposed control strategy of MFPC is able to mitigate the negative effects of grid connected MGs such as, unbalanced and reactive power compensation. This allows the MGs to become grid friendly , even it is working under highly unbalanced and poor power factor conditions, and also performs power management to optimize the MGs supporting services with smart grid functionality.