Resumen de Highly efficient, full zvs, hybrid, multi-level dc/dc topology for two-stage grid-connected 1500-v solar string inverter
Solar energy has experienced an exponential growth in the contribution in the total global energy consumption in the last 10 years. The main reasons for this improvement are the fast technological development of both, photovoltaic and power electronics materials, and rapid progress in power processing techniques. In the last 10 years, the efficiency of average commercial wafer-based silicon photovoltaic modules increased from about 12% to 17% (super-mono 21%). Additionally, the price has dropped more than 5 times for the same period. This has led to both, decrease in the total photovoltaic system price and to decrease in the share of the total system price related to photovoltaic module. In 2015 module price represented 19.26% of total photovoltaic system costs. The rest was related to the installed non-module prices: hardware costs, such as inverters and racking equipment; and the wide assortment of soft costs, including such things as marketing and customer acquisition, system design, installation labor, permitting and inspection costs, and installer margins. According to this, the bottleneck for the further decrease in photovoltaic system cost and efficiency and, thus, for the further increase in the global photovoltaic installation and decrease in the price of the electricity provided from photovoltaic is related to the power conditioning unit, the part of the system between photovoltaic string/array and the load. The main goal of this dissertation is to investigate the contributions of the combined application of the growing technologies (650-V and 900-V wide bandgap devices and multi-level, partial power processing topologies) into the emerging 1500-V grid-connected photovoltaic systems. The main focus is placed on a dc/dc stage of a transformerless, string/multi-string, three-phase-grid-connected solar inverter for use in commercial/residential applications that is designed for operation in power range of 10 kW- 30 kW. In order to justify the use of a dc/dc stage in a grid-connected photovoltaic system, a very detailed analysis on the system level is conducted. The void in the comprehensively reviewed literature on the comparison of 1000-V and 1500-V photovoltaic systems is detected related to the subject of energy harvesting. Due to this, the investigation of different architectures of a grid-connected solar string inverter known from the literature and commercial market are compared in terms of the energy harvested during the year. The comparison is based on the real measurements of the environmental conditions in the sample location, detailed models of the commercially available photovoltaic panels and very precise loss models in the semiconductor devices and passive circuit elements. The analysis is conducted for different ac grid connections (400-V, 480-V, 600-V and 800-V) in combination with 1000-V and 1500-V PV systems. Additionally, the impact of a dc/dc stage on the compactness of the inverter part is also investigated. Output filter of the dc/ac stage is designed to comply with the several requirements for grid-connected systems and fully optimized for the case when dc bus is very well controlled to the minimum operating value and when it varies in the full operating range. Results for these two cases are compared in terms of losses and volume. Following the analysis and conclusions on the system level, hybrid, multi-level, partial power processing dc/dc topology is proposed as a candidate to be used in a 1500-V grid-connected solar string inverter. Operating principle and different variations of this topology are discussed in details and main design aspects that influence the efficiency and compactness are addressed. Full multivariable optimization is conducted and results are compared with the state-of-the-art topologies optimized for the same operating conditions. Finally, all the theoretical analysis conducted in this thesis is confirmed by the detailed simulations in PSIM package and comprehensive set of measurements of the several prototypes in the power range 7 kW- 10 kW.
