Investigation on non-isolated DC-DC converters for DC micro-grid

Abstract

A shift in the current trends of electric power consumption from AC to DC is seen due to increased use of DC in many end user equipments or appliances such as LED bulbs, computer, laptop and other electronic appliances used in residential buildings and offices. Power generation through renewable energy resources (RERs), such as photovoltaics (PV), is inherently intermittent in nature; therefore, an energy storage system (ESS) is employed to ensure a stable power supply. DC distribution eases the interfacing of RERs, ESS and DC loads. The DC power system with DC sources and loads reduces the AC-DC and DC-AC conversion stages losses. Therefore, DC microgrids were introduced to enhance the sustainability, efficiency and reliability of the mordern electric power system. Although AC and DC Microgrids both will exist, AC microgrid faces several challenges such as phase and frequency synchronization, power angle stability issue, reactive power regulation. On the contrary, DC microgrid offers several benefits such as no frequency control, fault ride-through capability, higher transmission capacity, immune to power quality issues. Thus, DC microgrid demonstrates significant potential for development in the power sector. The development in DC microgrid technology demands efficient and high performance DC-DC converters capable of providing stable and reliable power distribution. The DC microgrid is commonly classified into two types: unipolar and bipolar. In comparison to unipolar, bipolar DC (BPDC) microgrid is more versatile and advantageous for following reasons: multiple voltage levels, high-efficiency, high reliability and high-quality power supply, easy compatibility with existing AC system. Despite of the advantages, the BPDC microgrid faces some challenges such as voltage imbalance and the need for multiple DC-DC converters. To address these challenges, multiport DC-DC converters play an important role. Several advanced DC-DC converter topologies have been explored to address these challenges. Many multiport converters suffer from drawbacks such as high component count, increased losses, bulkiness, high cost, and unnecessarily large voltage gain ratios, making them unsuitable for low-voltage bipolar DC (BPDC) microgrid applications. To address these, a novel non-isolated boost-zeta single input dual output multiport converter is proposed. This converter features an input parallel output series configuration of switching cells, providing output lines at positive, negative, and neutral potentials tailored for low voltage BPDC microgrid applicai tions. Key benefits include continuous input current, absence of shoot-through issues, support for multiple output voltages, and non-pulsating current delivery to specific 24 V loads, ensuring costeffectiveness, reliability, and efficiency. The design incorporates small signal analysis using state space averaging for controller development, alongside comprehensive loss estimation and efficiency analysis. Experimental validation through a laboratory prototype verifies theoretical and simulation results across varied operational conditions. The operational characteristics of DC-DC converters under discontinuous conduction mode (DCM) are also examined in this thesis. Comparative analysis is conducted between single inductor-based converters like boost and two inductor-based converters such as zeta converters, emphasizing differences in DCM behavior. It asserts that inductor current need not be zero to enter DCM and highlights the roles of equivalent inductance and total inductor current in determining discontinuity. Analysis includes a detailed study of a Boost-Zeta combined converter under various DCM scenarios, demonstrating the advantages of introducing phase shifts between switch gate pulses.