DEVELOPMENT OF CONTROL STRATEGIES FOR DC MICROGRID

Abstract

A new trend of small-scale power generation, integrated locally at the distribution voltage level, is emerging to solve the problems of climate change, energy security, and sustainable development. This power generation is known as distributed generation (DG), and the distribution system comprising DGs is called an active distribution system. Due to their intermittent power output, integrating renewable energy resources (RESs) into the utility grid and supplying local loads necessitates an energy storage system to enable a time shift between energy production and consumption. A microgrid (MG) is a cluster of DG sources, distributed storage devices, and distributed loads in which all components operate in a controlled manner to improve the reliability and quality of the local power supply and the power system as a whole. Based on their common link, MGs are classified as AC microgrid (ACMG), DC microgrid (DCMG), and AC/DC hybrid microgrid. The DCMG has gained more attention, acceptance, and popularity than the ACMG since the benefits include higher reliability, higher power quality, higher efficiency, low cost, absence of frequency and reactive power control, and more straightforward analysis and design of control loops. From an operation point of view, there are three main issues to be addressed in DCMG: (i) harnessing maximum DG/SPV power generation, (ii) effective use of energy storage system, and (iii) proper regulation of DC bus voltage. These are interrelated control issues. The MG can operate as a controllable coordinated module in an On-grid or Off-grid mode. In DCMG, DC voltage is the unique quantity controlled by injecting or absorbing power. Less research is reported for short-term source disturbance, fast load fluctuation, and nonlinear load in both Off-grid and On-grid operations. These situations impose a high-frequency transient current in the DCMG that is not shared appropriately among sources and storage by droop control. Thus, a circulating current flows among sources and storage. It becomes the cause of concern for power quality, reliability, stress on storage, and efficiency of converters. AC side fault and its impact on AC and DC side are also not reported in the literature in the presence of high-power density storage on DC bus. In this thesis, a novel control strategy is developed to address these problems with a composite energy storage system (CESS).