Development of DC Nanogrid Devices

Abstract

The shift towards the increased use of Distributed Energy Resources (DERs) and the need for efficient energy use, has favored the development and deployment of smart grids. The different DERs that can be used, the sizes of the network, and the power capacities of smart grids lead to numerous research and development projects. Photovoltaic (PV) sources as DERs are more and more included in the grid architecture topologies, often together with storage units and small power electronic converters. Considering the most common Renewable Energy Sources (RES) and the increasing of Direct Current (DC) loads, DC connection lines are progressively integrated in the grid architectures. Even when Alternating Current (AC) motors have to be powered, for example, it is customary to drive them with an inverter, starting from a defined DC bus voltage. In this scenario, NanoGrids (NGs), and in particular DC NGs, have promising characteristics that can be investigated. The main research activities on NGs are: the reduction of the number of conversion stages, an increase on the efficiency (facilitated by the aforementioned objective), together with smart systems. Development and deployment of electrical grids is necessary not only where access to the Power Grid (PG) is possible and economically viable. For remote communities, especially in developing countries, NGs with storage systems can improve the reliability and availability of electricity. However, in literature, islanded DC NG architectures are less explored than AC, DC, or hybrid architectures with PG connection. The main objective of this PhD thesis has been to analyze, simulate and develop DC NG devices for a smart energy management and load control. An NG architecture with storage capabilities has been proposed for residential buildings and offices, both connected to the PG or islanded. Moreover, the proposed NG can be applied also to automotive, More Electric Aircrafts or naval applications. Beyond the impulse to the research given by the spreading of DERs, during the last decade, the technology evolution of semiconductors is influencing the development of power converters. Wide Band Gap (WBG) materials, such as Silicon Carbide (SiC) and Gallium Nitride (GaN), allow a boost in power electronics applications. The use of GaN transistor for the DC NG devices has been investigated, analyzing both electrical and thermal behaviors. The key element of the proposed NG is the Triple Active Bridge (TAB) converter with GaN devices, which controls the power flows between PV, storage and DC bus. Respect to the works that can be found in literature, the TAB interfaces directly PV, battery, and DC bus, avoiding intermediate power stages. Moreover, the control strategy applied can be implemented with a general-purpose microcontroller, avoiding complex matrix calculus. Starting from three DC ports, the converter uses three full-bridges and a high-frequency transformer for the power transfer between the ports. The control strategy applied to the converter is defined to have both a controlled 48 V voltage on the DC bus port, and a Maximum Power Point Tracking on the PV port side. The TAB has been analyzed, simulated and prototypes have been characterized for experimental tests. The proposed NG includes also smart point of load converters. A solution for the so-called smart plugs has been investigated in order to connect different loads, adjusting the voltage and current ratings. In order to absorb arbitrary power profiles to test the converters and to extract battery characteristics through charge and discharge profiles, programmable active load prototypes have been developed. Furthermore, smart solar PV module maintenance has been studied from the source point of view. An ad-hoc board with soiling sensor, irradiation measuring, and I-V characteristic extraction has been designed. The board works whenever the power output of the PV modules reaches low values. In these cases, the TAB excludes the PV port, and an I-V characteristic of the PV modules can be registered to observe if there is low irradiation, soiling or concentrated dirt on the modules, and PV cells malfunctions. Finally, thermal analysis on the different components and prototype boards have been done. Thermal issues have an important role for the reliability of the proposed NG architecture, where there are strict constrains on natural air convection and high power densities. Finite Element Method simulations and tests have been carried out the GaN transistor used in the smart plugs, and the snubber capacitors used in the TAB. Thermal maps and thermal dynamic behavior have been analyzed also on the different PCB prototypes: the TAB, the smart plug converter, and the active load.