Convertidores CC/CC para inversores multinivel en cascada en instalaciones fotovoltaicas conectadas a red

Laburpena

In recent years, renewable energies have reached a significant level of development, especially photovoltaics, for which Gigawatt plants can already be found in operation. In this respect, it is necessary to provide suitable solutions that allow máximum use of the energy generated. Medium Voltage DC (MVDC) grids offer numerous advantages in this respect, with a growing trend towards direct connection to the grid, without the need for bulky and heavy low-frequency transformers. To achieve transformerless Medium Voltage levels, it is possible to take the traditional two-level converter approach; or the multilevel approach. The latter has been widely developed in recent years, with Cascaded H-Bridge (CHB) being the most prominent in photovoltaic applications. Initially, it was proposed to achieve the multilevel configuration by means of the single-phase association of cascaded inverters, however, this solution had the risk of injecting energy in an unbalanced way into the grid and heavily penalised the design of the decoupling capacitors, which came to represent an important part of the overall cost of the converter. This was because the capacitors had to filter a current twice the grid frequency and with an amplitude similar to the DC current provided by the photovoltaic panels. The three-phase association of inverters was proposed as a solution to both problems. However, the reduction of the capacitors will depend on the type of DC/DC converter used. This PhD thesis originates from the need to select the best solution for the DC/DC stage of a multilevel converter applied to a three-phase association of cascaded inverters, so that a significant reduction in the size of the converter capacitors can be achieved. To this end, a series of design characteristics for a grid-connected photovoltaic multilevel converter are proposed as a basis for the selection of this DC/DC stage. As part of this preliminary phase, several aspects relating to the operation of the multilevel converter are analysed, from the selection of components to the effect of inequalities in the inverter supply voltages on harmonic distortion. On this last aspect, the importance it has on the current shape of the multilevel is verified and a design method for the output filter is proposed so that harmonic cancellation still occurs even under conditions of unequal voltajes For the part concerning the selection of the best solution for the DC/DC stage, an initial comparison between two different solutions is proposed: one unidirectional and the other bidirectional. From this initial comparison it can be concluded that a solution based on a isolated unidirectional DC/DC topology working at its optimal point and a non-isolated DC/DC converter in charge of the MPPT tasks would be the most recommendable. Bidirectionality is no longer a prerequisite in the latter solution, so the effect of a unidirectional converter working with power factors other than unity is analysed. A calculation method for filter and decoupling capacitors is proposed for these cases, which demonstrates that bidirectionality is not necessary and that the reduction in capacitor size can be more than significant compared with that of a singlephase inverter for power factor not unity. Based on the above, a Series Resonant Converter (SRC) is proposed for the isolated unidirectional DC/DC topology working at its optimum point. From the application of the SRC to the three-phase association of inverters, it can be extracted that, when working with unity power factor, it is possible to eliminate the low frequency pulsating power from the grid and the conduction losses in the transistors are reduced by 33.33% compared to the single-phase association of inverters. On the other hand, it is possible to extend the soft switching range of the converter thanks to an auxiliary circuit external to the transformer, as well as being suitable for applications requiring large stray inductances. Some drawbacks of the SRCs are also analysed and several solutions are proposed. On the other hand, from the analysis and comparison of various non-isolated DC/DC converters for the stage in charge of MPPT tasks, a solution based on a three-level asymmetrical boost or 3L Flying-Capacitor (3L FC) is proposed, also including possible improvements. Finally, a scale prototype of the isolated DC/DC converter has been built, working in open loop, with which the most relevant aspects of this PhD thesis have been experimentally verified.