Juez de consultas SQL basado en postgresql

Este documento recoge el proceso de desarrollo de una aplicación de escritorio para la corrección de consultas SQL. El objetivo del proyecto es proporcionar una herramienta para el aprendizaje y la enseñanza en los fundamentos de bases de datos relacionales. La aplicación ha sido desarrollada en Python usando Postgresql como sistema gestor de bases de datos.Este proyecto continúa desarrollando la idea de los jueces de código y abre el camino para nuevos proyectos de aprendizaje interactivo en el ámbito de las bases de datos.This document includes the developmental process of a desktop application to SQL query correction. The goal of this project is to provide a tool to teach and learn about relational databases fundamentals. The application has been developed in Python using Postgresql as its database management system. This project continues to expand the code judge idea and opens a new path for new projects of interactive learning in the field of databases.Grado en Ingeniería Informátic

Estudio y evaluación de prestaciones estáticas y dinámicas de los integradores generalizados de segundo orden en las estructuras de control de inversores fotovoltaicos de dos etapas con conexión a red

[EN] The use of renewable energy sources has a positive impact on the environment and the economy, so that countries with a deficit of fossil fuels can reduce their energy dependence on other countries. Photovoltaic (PV) systems connected to the grid must comply with the current EMC regulations, which regulate the level of distortion of the current injected into the grid, THDi. Moreover, the PV market requires cheaper and more reliable systems. One of the major issues about the reliability of PV inverters is the presence of electrolytic capacitors at the DC-Link feeding the inverter. There is a trend to reduce the required capacitance in order to replace the electrolytic capacitors by film capacitors, which are more durable. Two major effects of the DC-Link capacitance reduction are the increase of the voltage ripple at the capacitors and higher transient variations of that voltage under small operating point changes of the inverter. These variations must be bounded for the proper operation of the inverter. In this Ph.D. thesis the implementation of control structures based on second order generalized integrators (SOGI) are studied. These structures are developed for two-stage PV inverters in order to achieve a higher dynamic response of the DC-Link voltage control. The frequency adaptability of SOGI structures helps to reduce the THDi of the grid injected current in the case of variations of the grid frequency, even under highly distorted grid voltage conditions. Recently, several research works have focused on reducing the number of sensors in the PV inverters, which has a positive impact on costs reduction. Maximum power point tracking (MPPT) algorithms are implemented to optimize the performance of PV systems. Classical MPPT algorithms use current and voltage sensors for estimating the power extracted from the PV panel. MPPT sensorless algorithms that do not require sensors in the PV panel have been developed in this thesis, based on the improvement of the PV inverter voltage control loop achieved by the use of SOGIs. The performance of the proposed MPPT algorithms is similar to that of classical ones, but at a lower cost. Two two-stage PV inverter prototypes have been built for performing experimental tests in order to validate the theoretical studies and proposals presented in this thesis. The specifications of the inverter have been defined to connect a single PV panel of 230W to the grid with high frequency galvanic isolation between the PV panel and the grid. One inverter is equipped with a standard size DC-Link and the other one is equipped with a reduced size DC-Link in order to compare the behavior of both DC-Link sizes. The inverter is operating with current and voltage sensors to measure the extracted power from the PV panel. This implementation allows comparing classical MPPT algorithms, which make use of those sensors, with sensorless MPPT algorithms, working at the same environmental conditions.[ES] El aprovechamiento de las fuentes de energía renovables tiene un impacto positivo tanto en el medio ambiente como en la economía, de manera que los países deficitarios en combustibles fósiles pueden disminuir su dependencia energética de otros países. Los sistemas fotovoltaicos (PV) conectados a la red eléctrica han de ajustarse al cumplimiento de la normativa vigente de compatibilidad electromagnética, que regula el nivel de distorsión de la corriente inyectada en la red. Además, el mercado demanda sistemas más económicos y más fiables. Uno de los puntos críticos en la fiabilidad de los inversores, es la presencia de condensadores electrolíticos en la DC-Link del convertidor. Se están realizado numerosos estudios para disminuir la capacidad de los condensadores necesaria, con el objetivo de poder sustituir los electrolíticos por condensadores de película, que son más duraderos. La disminución en el tamaño de la DC-Link tiene como consecuencias principales el aumento del rizado de tensión en los condensadores y mayores variaciones del valor medio de la tensión ante pequeños cambios en el punto de trabajo del inversor. Para el correcto funcionamiento del inversor estas variaciones deben estar acotadas. En la presente tesis se estudia la implementación de estructuras de control de la tensión de la DC-Link con mayor respuesta dinámica, orientada a inversores fotovoltaicos de dos etapas. Empleando estructuras basadas en integradores generalizados de segundo orden (SOGI) se ha conseguido inyectar una corriente de muy baja distorsión en la red, incluso funcionando con redes eléctricas con tensión altamente distorsionada. La adaptabilidad en frecuencia de las estructuras SOGI es un factor clave para mantener la eficacia del control de la tensión. Numerosas investigaciones se centran en la reducción del número de sensores de los inversores. Esto tiene un impacto positivo en la reducción de costes. Los algoritmos de seguimiento de máxima potencia (MPPT) clásicos, que se implementan en los inversores para optimizar el rendimiento de los paneles fotovoltaicos, hacen uso de sensores de tensión y corriente para estimar la potencia extraída del panel. En los últimos tiempos se ha desarrollado diversos algoritmos MPPT sensorless que no requieren sensores en el panel fotovoltaico. Las mejoras obtenidas en la presente tesis para el control de la tensión en la DC-Link, han permitido la implementación de un algoritmo MPPT sensorless con prestaciones similares a los algoritmos clásicos, pero con un coste inferior. Se ha realizado ensayos experimentales para validar los estudios y las propuestas realizadas. Para ello se han construido dos prototipos de un inversor fotovoltaico de dos etapas. Las especificaciones del inversor se han definido para conectar a la red un panel de 230W con aislamiento galvánico en alta frecuencia entre el panel y la red. Cada inversor tiene una DC-Link diferente para comparar el efecto entre una DC-Link de tamaño reducido con una DC-Link de tamaño estándar. El inversor está equipado con sensores de tensión y corriente para medir la potencia extraída del panel fotovoltaico. Esto permite comparar, en las mismas condiciones, algoritmos MPPT clásicos que emplean sensores de las magnitudes eléctricas del panel y algoritmos sensorless que no hacen uso de esos sensores.[CA] L'aprofitament de les fonts d'energia renovables té un impacte positiu, tant en el medi ambient com en l'economia, de manera que els països deficitaris en combustibles fòssils poden disminuir la seua dependència energètica d'altres països. Els sistemes fotovoltaics (PV) connectats a la xarxa elèctrica han d'ajustar-se al compliment de la normativa vigent de compatibilitat electromagnètica, que regula el nivell de distorsió del corrent injectat a la xarxa. A més, el mercat demana sistemes més econòmics i més fiables. Un dels punts crítics de la fiabilitat dels inversors és la presència de condensadors electrolítics en la DC-link dels convertidors. S'estan realitzant nombrosos estudis per disminuir la capacitat dels condensadors necessària amb l'objectiu de substituir els electrolítics per condensadors de pel·lícula, que son més duradors. La disminució en la grandària de la DC-link té com a conseqüència principal l'augment de l'arrissament de la tensió en els condensadors, davant de canvis menuts en el punt de treball de l'inversor. Per al correcte funcionament de l'inversor, aquestes variacions han d'estar acotades. En la present tesi s'estudia la implementació d'estructures de control de la tensió en la DC-link amb major resposta dinàmica, orientada a inversors fotovoltaics de dues etapes. Emprant estructures basades en integradors generalitzats de segon ordre (SOGI) s'ha aconseguit injectar un corrent amb molt baixa distorsió a la xarxa, fins i tot, funcionant amb xarxes elèctriques amb tensió altament distorsionada. L'adaptabilitat en freqüència de les estructures SOGI és un factor clau per mantenir l'eficàcia del control de l'inversor. Nombroses investigacions es centren en la reducció del nombre de sensors dels inversors. Això te un impacte positiu en la reducció de costos. Els algorismes de seguiment de màxima potencia (MPPT) clàssics, que s'implementen en els inversors per optimar el rendiment dels panells fotovoltaics, fan ús de sensors de tensió i corrent per estimar la potència treta del panell. En els últims temps, s'han desenvolupat diversos algoritmes MPPT sensorless, que no requereixen sensors en el panell fotovoltaic. Les millores obtingudes en la present tesi per al control de la tensió en la DC-link han permès la implementació d'un algorisme MPPT sensorless amb prestacions similars als algorismes clàssics, però amb un cost inferior. S'han realitzat assajos experimentals per validar els estudis i les propostes realitzades. Per a això, s'han construït dos prototips d'un inversor fotovoltaic de dues etapes. Les especificacions de l'inversor s'han definit per connectar a la xarxa un panell de 230W amb aïllament galvànic d'alta freqüència entre el panell i la xarxa. Cada inversor té una DC-link diferent per comparar l'efecte entre una DC-link de grandària reduïda amb una DC-link de grandària estàndard. L'inversor està equipat amb sensors de tensió i corrent per mesurar la potència treta del panell fotovoltaic. Això permet comparar, en les mateixes condicions, algorismes MPPT clàssics que empren ambdós sensors i algorismes sensorless que no fan ús d'aquests.González Medina, R. (2015). Estudio y evaluación de prestaciones estáticas y dinámicas de los integradores generalizados de segundo orden en las estructuras de control de inversores fotovoltaicos de dos etapas con conexión a red [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/57487TESISPremios Extraordinarios de tesis doctorale

Effects of the PWM carrier signals synchronization on the DC-link current in back-to-back converters

[EN] This paper presents a study about the synchronization effects of the PWM carrier signals of a back-to-back converter for grid connection of Wind Energy Conversion Systems based on Permanent Magnet Synchronous Generators. It is demonstrated by means of a spectral analysis that, with the proper synchronization of the carrier signals of both the rectifier and inverter stages, the rms value of the current through the DC-link capacitors is greatly reduced. As a result, the number of capacitors needed to build the back-to-back converter decreases, whereas its life cycle is expanded, so that the Wind Energy Conversion System becomes more cost effective. It is shown that the worst case occurs when the phase difference between both carrier signals is ±¿/2, yielding the highest rms value of the DC-link capacitors current. In that case the harmonic with the highest rms value is located at twice the switching frequency. The theoretical analysis is compared with experimental results from a 10 kW back-to-back converter in order to validate the effects of the carrier signals phase shift on the DC-link capacitors current.Luis Gerardo González Morales; Garcerá, G.; Figueres Amorós, E.; González-Medina, R. (2010). Effects of the PWM carrier signals synchronization on the DC-link current in back-to-back converters. Applied Energy. 87(8):2491-2499. doi:10.1016/j.apenergy.2010.02.023S2491249987

Grid-tie inverter topology with maximum power extraction from two photovoltaic arrays

This study presents a transformerless topology for a grid-tied single-phase inverter capable of performing the simultaneous maximum power point tracking of two independent and series connected photovoltaic sources. This topology is derived from the neutral point clamped multilevel inverter in half-bridge configuration. The use of a half-bridge topology reduces the leakage current to very low values, whereas the multilevel topology presents an output voltage quality similar to that of a full-bridge inverter. To simultaneously track the maximum power of both photovoltaic sources, a generation control circuit is used. With this topology, it is possible to improve the performance of the converter under partial shadowing conditions, very common in photovoltaic facilities operating in residential areas. A 5 kW prototype of this topology has been implemented and tested in the laboratory.This work is supported by the Spanish Ministry of Science and Innovation under grants ENE2009-13998-C02-02 and ENE2012-37667-C02-01.Patrao Herrero, I.; Gabriel Garcerá; Figueres Amorós, E.; González Medina, R. (2014). Grid-tie inverter topology with maximum power extraction from two photovoltaic arrays. Renewable Power Generation, IET. 8(6):638-648. doi:10.1049/iet-rpg.2013.0143S63864886Bevrani, H., Ghosh, A., & Ledwich, G. (2010). Renewable energy sources and frequency regulation: survey and new perspectives. IET Renewable Power Generation, 4(5), 438. doi:10.1049/iet-rpg.2009.0049Zhu, J., Bründlinger, R., Mühlberger, T., Betts, T. R., & Gottschalg, R. (2011). Optimised inverter sizing for photovoltaic systems in high-latitude maritime climates. IET Renewable Power Generation, 5(1), 58. doi:10.1049/iet-rpg.2009.0162Amoiralis, E. I., Tsili, M. A., & Kladas, A. G. (2012). Power Transformer Economic Evaluation in Decentralized Electricity Markets. IEEE Transactions on Industrial Electronics, 59(5), 2329-2341. doi:10.1109/tie.2011.2157291Bowtell, L., & Ahfock, A. (2010). Direct current offset controller for transformerless single-phase photovoltaic grid-connected inverters. IET Renewable Power Generation, 4(5), 428. doi:10.1049/iet-rpg.2009.0043Patrao, I., Figueres, E., González-Espín, F., & Garcerá, G. (2011). Transformerless topologies for grid-connected single-phase photovoltaic inverters. Renewable and Sustainable Energy Reviews, 15(7), 3423-3431. doi:10.1016/j.rser.2011.03.034Kjaer, S. B., Pedersen, J. K., & Blaabjerg, F. (2005). A Review of Single-Phase Grid-Connected Inverters for Photovoltaic Modules. IEEE Transactions on Industry Applications, 41(5), 1292-1306. doi:10.1109/tia.2005.853371Puttgen, H. B., MacGregor, P. R., & Lambert, F. C. (2003). Distributed generation: Semantic hype or the dawn of a new era? IEEE Power and Energy Magazine, 1(1), 22-29. doi:10.1109/mpae.2003.1180357Nian, H., & Zeng, R. (2011). Improved control strategy for stand-alone distributed generation system under unbalanced and non-linear loads. IET Renewable Power Generation, 5(5), 323. doi:10.1049/iet-rpg.2010.0216Thomson, M., & Infield, D. G. (2007). Impact of widespread photovoltaics generation on distribution systems. IET Renewable Power Generation, 1(1), 33. doi:10.1049/iet-rpg:20060009Karatepe, E., Syafaruddin, & Hiyama, T. (2010). Simple and high-efficiency photovoltaic system under non-uniform operating conditions. IET Renewable Power Generation, 4(4), 354. doi:10.1049/iet-rpg.2009.0150Wang, Y.-J., & Hsu, P.-C. (2010). Analytical modelling of partial shading and different orientation of photovoltaic modules. IET Renewable Power Generation, 4(3), 272. doi:10.1049/iet-rpg.2009.0157López, Ó., Freijedo, F. D., Yepes, A. G., Fernández-Comesaña, P., Malvar, J., Teodorescu, R., & Doval-Gandoy, J. (2010). Eliminating Ground Current in a Transformerless Photovoltaic Application. IEEE Transactions on Energy Conversion, 25(1), 140-147. doi:10.1109/tec.2009.2037810Cavalcanti, M. C., Farias, A. M., Oliveira, K. C., Neves, F. A. S., & Afonso, J. L. (2012). Eliminating Leakage Currents in Neutral Point Clamped Inverters for Photovoltaic Systems. IEEE Transactions on Industrial Electronics, 59(1), 435-443. doi:10.1109/tie.2011.2138671Wu, T.-F., Chang, C.-H., Lin, L.-C., & Kuo, C.-L. (2011). Power Loss Comparison of Single- and Two-Stage Grid-Connected Photovoltaic Systems. IEEE Transactions on Energy Conversion, 26(2), 707-715. doi:10.1109/tec.2011.2123897Shimizu, T., Hirakata, M., Kamezawa, T., & Watanabe, H. (2001). Generation control circuit for photovoltaic modules. IEEE Transactions on Power Electronics, 16(3), 293-300. doi:10.1109/63.923760Gonzalez-Espin, F., Figueres, E., & Garcera, G. (2012). An Adaptive Synchronous-Reference-Frame Phase-Locked Loop for Power Quality Improvement in a Polluted Utility Grid. IEEE Transactions on Industrial Electronics, 59(6), 2718-2731. doi:10.1109/tie.2011.2166236Mastromauro, R. A., Liserre, M., & Dell’Aquila, A. (2012). Control Issues in Single-Stage Photovoltaic Systems: MPPT, Current and Voltage Control. IEEE Transactions on Industrial Informatics, 8(2), 241-254. doi:10.1109/tii.2012.2186973Delfino, F., Denegri, G. B., Procopio, R., & Invernizzi, M. (2012). Feedback linearisation oriented approach to Q–V control of grid connected photovoltaic units. IET Renewable Power Generation, 6(5), 324-339. doi:10.1049/iet-rpg.2011.0075Ishaque, K., & Salam, Z. (2013). A review of maximum power point tracking techniques of PV system for uniform insolation and partial shading condition. Renewable and Sustainable Energy Reviews, 19, 475-488. doi:10.1016/j.rser.2012.11.03

Modelling and Control of Parallel-Connected Transformerless Inverters for Large Photovoltaic Farms

[EN] This paper presents a control structure for transformerless photovoltaic inverters connected in parallel to manage photovoltaic fields in the MW range. Large photovoltaic farms are usually divided into several photovoltaic fields, each one of them managed by a centralized high power inverter. The current tendency to build up centralized inverters in the MW range is the use of several transformerless inverters connected in parallel, a topology that provokes the appearance of significant zero-sequence circulating currents among inverters. To eliminate this inconvenience, this paper proposes a control structure that avoids the appearance of circulating currents by controlling the zero-sequence component of the inverters. A second contribution of the paper is the development of a model of n parallel-connected inverters. To validate the concept, the proposed control structure has been applied to a photovoltaic field of 2 MW managed by four 500 kW photovoltaic inverters connected in parallel.This work is supported by the Spanish Ministry of Economy and Competitiveness (MINECO), the European Regional Development Fund (ERDF) under Grant ENE2015-64087-C2-2-R and the Spanish Ministry of Education (FPU15/01274).Liberos-Mascarell, MA.; González-Medina, R.; Garcerá, G.; Figueres Amorós, E. (2017). Modelling and Control of Parallel-Connected Transformerless Inverters for Large Photovoltaic Farms. Energies. 10(8):1-25. https://doi.org/10.3390/en10081242S125108Pazheri, F. R., Othman, M. F., & Malik, N. H. (2014). A review on global renewable electricity scenario. Renewable and Sustainable Energy Reviews, 31, 835-845. doi:10.1016/j.rser.2013.12.020Subudhi, B., & Pradhan, R. (2013). A Comparative Study on Maximum Power Point Tracking Techniques for Photovoltaic Power Systems. IEEE Transactions on Sustainable Energy, 4(1), 89-98. doi:10.1109/tste.2012.2202294Borrega, M., Marroyo, L., Gonzalez, R., Balda, J., & Agorreta, J. L. (2013). Modeling and Control of a Master–Slave PV Inverter With N-Paralleled Inverters and Three-Phase Three-Limb Inductors. IEEE Transactions on Power Electronics, 28(6), 2842-2855. doi:10.1109/tpel.2012.2220859Araujo, S. V., Zacharias, P., & Mallwitz, R. (2010). Highly Efficient Single-Phase Transformerless Inverters for Grid-Connected Photovoltaic Systems. IEEE Transactions on Industrial Electronics, 57(9), 3118-3128. doi:10.1109/tie.2009.2037654PowerGate Plus 500 kWhttp://www.satcon.comAgorreta, J. L., Borrega, M., López, J., & Marroyo, L. (2011). Modeling and Control of $N$ -Paralleled Grid-Connected Inverters With LCL Filter Coupled Due to Grid Impedance in PV Plants. IEEE Transactions on Power Electronics, 26(3), 770-785. doi:10.1109/tpel.2010.2095429Power Electronicshttp://www.power-electronics.comPVS980—1818 to 2091 kVAhttp://new.abb.comInfineon, Central Inverter Solutionshttps://www.infineon.com/cms/en/applications/solar-energy-systems/central-inverter-solutions/Xiao, H., Xie, S., Chen, Y., & Huang, R. (2011). An Optimized Transformerless Photovoltaic Grid-Connected Inverter. IEEE Transactions on Industrial Electronics, 58(5), 1887-1895. doi:10.1109/tie.2010.2054056Mazumder, S. K. (2003). A novel discrete control strategy for independent stabilization of parallel three-phase boost converters by combining space-vector modulation with variable-structure control. IEEE Transactions on Power Electronics, 18(4), 1070-1083. doi:10.1109/tpel.2003.813770Ching-Tsai Pan, & Yi-Hung Liao. (2008). Modeling and Control of Circulating Currents for Parallel Three-Phase Boost Rectifiers With Different Load Sharing. IEEE Transactions on Industrial Electronics, 55(7), 2776-2785. doi:10.1109/tie.2008.925647Ogasawara, S., Takagaki, J., Akagi, H., & Nabae, A. (1992). A novel control scheme of a parallel current-controlled PWM inverter. IEEE Transactions on Industry Applications, 28(5), 1023-1030. doi:10.1109/28.158825Figueres, E., Garcera, G., Sandia, J., Gonzalez-Espin, F., & Rubio, J. C. (2009). Sensitivity Study of the Dynamics of Three-Phase Photovoltaic Inverters With an LCL Grid Filter. IEEE Transactions on Industrial Electronics, 56(3), 706-717. doi:10.1109/tie.2008.2010175Mohd, A., Ortjohann, E., Hamsic, N., Sinsukthavorn, W., Lingemann, M., Schmelter, A., & Morton, D. (2010). Control strategy and space vector modulation for three-leg four-wire voltage source inverters under unbalanced load conditions. IET Power Electronics, 3(3), 323. doi:10.1049/iet-pel.2008.0281Albatran, S., Fu, Y., Albanna, A., Schrader, R., & Mazzola, M. (2013). Hybrid 2D-3D Space Vector Modulation Voltage Control Algorithm for Three Phase Inverters. IEEE Transactions on Sustainable Energy, 4(3), 734-744. doi:10.1109/tste.2013.224568

A contribution to the selection of tsunami human vulnerability indicators: conclusions from tsunami impacts in Sri Lanka and Thailand (2004), Samoa (2009), Chile (2010) and Japan (2011)

After several tsunami events with disastrous consequences around the world, coastal countries have realized the need to be prepared to minimize human mortality and damage to coastal infrastructures, livelihoods and resources. The international scientific community is striving to develop and validate methodologies for tsunami hazard and vulnerability and risk assessments. The vulnerability of coastal communities is usually assessed through the definition of sets of indicators based on previous literature and/or post-tsunami reports, as well as on the available data for the study site. The aim of this work is to validate, in light of past tsunami events, the indicators currently proposed by the scientific community to measure human vulnerability, to improve their definition and selection as well as to analyse their validity for different country development profiles. The events analysed are the 2011 Great Tohoku tsunami, the 2010 Chilean tsunami, the 2009 Samoan tsunami and the 2004 Indian Ocean tsunami. The results obtained highlight the need for considering both permanent and temporal human exposure, the former requiring some hazard numerical modelling, while the latter is related to site-specific livelihoods, cultural traditions and gender roles. The most vulnerable age groups are the elderly and children, the former having much higher mortality rates. Female mortality is not always higher than male mortality and not always related to dependency issues. Higher numbers of disabled people do not always translate into higher numbers of victims. Besides, it is clear that mortality is not only related to the characteristics of the population but also of the buildings. A high correlation has been found between the affected buildings and the number of victims, being very high for completely damaged buildings. Distance to the sea, building materials and expected water depths are important determining factors regarding the type of damage to buildings

Measurement of the Loop Gain Frequency Response of Digitally Controlled Power Converters

[EN] The study of the loop gain frequency response in a power converter is a powerful tool commonly used for the design of the controllers used in the control stage. As the control of medium- and high-power electronic converters is usually performed digitally, it is useful to find a method to measure the digital loop gains. The purpose of this paper is to present a method for properly measuring the loop gain frequency response of digitally controlled power converters by means of an analog frequency response analyzer (FRA). An analog sinusoidal reference signal generated by the FRA is injected through an analog-to-digital converter into the digital controller, and added to the discrete feedback signal. To obtain the frequency response of the open-loop gain, both feedback and disturbed feedback signals are sent back to the FRA by using the pulsewidth modulation peripherals of the controller.This work was supported by the Spanish Ministry of Science and Innovation under Grants ENE2006-15521-C03-02 and ENE2009-13998-C02-02.González Espín, FJ.; Figueres Amorós, E.; Garcerá, G.; González-Medina, R.; Pascual Molto, M. (2010). Measurement of the Loop Gain Frequency Response of Digitally Controlled Power Converters. IEEE Transactions on Industrial Electronics. 57(8):2785-2796. https://doi.org/10.1109/TIE.2010.2056610S2785279657

Analysis of the control structure of wind energy generation systems based on a permanent magnet synchronous generator

This paper presents the analysis of the two usual control structures for variable speed and fixed pitch wind energy generation systems, namely speed and torque control, to determine the most appropriate structure to improve both robustness and reliability of this kind of distributed generators. The study considers all the elements of a typical wind power generation system and it has been carried out in a general way, so that conclusions are independent of the kind of the AC/DC converter that it is used to process the energy at the output of the generator. Particular emphasis was placed on developing a model of the turbine where the mechanical torque is considered as a system variable and not an exogenous disturbance for the system, as in other previous studies. After showing that speed control presents several advantages in terms of stability and reliability, an experimental study of this technique was carried out by using a grid connected wind generation system, which is composed by a three-phase boost rectifier feeding the grid connected inverter. Other practical issues for the design of high efficient wind generation systems, like the use of a Kalman speed estimator to avoid the need of mechanical sensors, are also implemented in the prototype and discussed in the paper. (c) 2012 Elsevier Ltd. All rights reserved.The first author thanks the support of the Instituto Politecnico Nacional (IPN) and of the Comision de Operacion y Fomento de Actividades Academicas (COFAA). This work was supported by the Spanish Ministry of Science and Innovation under Grant ENE2009-13998-C02-02.Carranza Castillo, O.; Figueres Amorós, E.; Garcerá Sanfeliú, G.; González Medina, R. (2013). Analysis of the control structure of wind energy generation systems based on a permanent magnet synchronous generator. Applied Energy. 103:522-538. https://doi.org/10.1016/j.apenergy.2012.10.015S52253810

A Method to Enhance the Global Efficiency of High-Power Photovoltaic Inverters Connected in Parallel

[EN] Central inverters are usually employed in large photovoltaic farms because they offer a good compromise between costs and efficiency. However, inverters based on a single power stage have poor efficiency in the low power range, when the irradiation conditions are low. For that reason, an extended solution has been the parallel connection of several inverter modules that manage a fraction of the full power. Besides other benefits, this power architecture can improve the efficiency of the whole system by connecting or disconnecting the modules depending on the amount of managed power. In this work, a control technique is proposed that maximizes the global efficiency of this kind of systems. The developed algorithm uses a functional model of the inverters¿ efficiency to decide the number of modules on stream. This model takes into account both the power that is instantaneously processed and the maximum power point tracking (MPPT) voltage that is applied to the photovoltaic field. A comparative study of several models of efficiency for photovoltaic inverters is carried out, showing that bidimensional models are the best choice for this kind of systems. The proposed algorithm has been evaluated by considering the real characteristics of commercial inverters, showing that a significant improvement of the global efficiency is obtained at the low power range in the case of sunny days. Moreover, the proposed technique dramatically improves the global efficiency in cloudy days.This work is supported by the Spanish Ministry of Economy and Competitiveness (MINECO), the European Regional Development Fund (ERDF) under Grants ENE2015-64087-C2-2-R and RTI2018-100732-B-C21, and the Spanish Ministry of Education (FPU15/01274).Liberos-Mascarell, MA.; González-Medina, R.; Garcerá, G.; Figueres Amorós, E. (2019). A Method to Enhance the Global Efficiency of High-Power Photovoltaic Inverters Connected in Parallel. Energies. 12(11):1-19. https://doi.org/10.3390/en12112219S1191211Wu, H., Locment, F., & Sechilariu, M. (2019). Experimental Implementation of a Flexible PV Power Control Mechanism in a DC Microgrid. Energies, 12(7), 1233. doi:10.3390/en12071233Strzalka, A., Alam, N., Duminil, E., Coors, V., & Eicker, U. (2012). Large scale integration of photovoltaics in cities. Applied Energy, 93, 413-421. doi:10.1016/j.apenergy.2011.12.033Zhang, P., Li, W., Li, S., Wang, Y., & Xiao, W. (2013). Reliability assessment of photovoltaic power systems: Review of current status and future perspectives. Applied Energy, 104, 822-833. doi:10.1016/j.apenergy.2012.12.010Kim, Y. S., Kang, S.-M., & Winston, R. (2011). Modeling of a concentrating photovoltaic system for optimum land use. Progress in Photovoltaics: Research and Applications, 21(2), 240-249. doi:10.1002/pip.1176Müller, B., Hardt, L., Armbruster, A., Kiefer, K., & Reise, C. (2015). Yield predictions for photovoltaic power plants: empirical validation, recent advances and remaining uncertainties. Progress in Photovoltaics: Research and Applications, 24(4), 570-583. doi:10.1002/pip.2616Borrega, M., Marroyo, L., Gonzalez, R., Balda, J., & Agorreta, J. L. (2013). Modeling and Control of a Master–Slave PV Inverter With N-Paralleled Inverters and Three-Phase Three-Limb Inductors. IEEE Transactions on Power Electronics, 28(6), 2842-2855. doi:10.1109/tpel.2012.2220859Araujo, S. V., Zacharias, P., & Mallwitz, R. (2010). Highly Efficient Single-Phase Transformerless Inverters for Grid-Connected Photovoltaic Systems. IEEE Transactions on Industrial Electronics, 57(9), 3118-3128. doi:10.1109/tie.2009.2037654Mohd, A., Ortjohann, E., Morton, D., & Omari, O. (2010). Review of control techniques for inverters parallel operation. Electric Power Systems Research, 80(12), 1477-1487. doi:10.1016/j.epsr.2010.06.009Su, J.-T., & Liu, C.-W. (2013). A Novel Phase-Shedding Control Scheme for Improved Light Load Efficiency of Multiphase Interleaved DC–DC Converters. IEEE Transactions on Power Electronics, 28(10), 4742-4752. doi:10.1109/tpel.2012.2233220Ahn, Y., Jeon, I., & Roh, J. (2014). A Multiphase Buck Converter With a Rotating Phase-Shedding Scheme For Efficient Light-Load Control. IEEE Journal of Solid-State Circuits, 49(11), 2673-2683. doi:10.1109/jssc.2014.2360400Peng, H., Anderson, D. I., & Hella, M. M. (2013). A 100 MHz Two-Phase Four-Segment DC-DC Converter With Light Load Efficiency Enhancement in 0.18/spl mu/m CMOS. IEEE Transactions on Circuits and Systems I: Regular Papers, 60(8), 2213-2224. doi:10.1109/tcsi.2013.2239157Costabeber, A., Mattavelli, P., & Saggini, S. (2010). Digital Time-Optimal Phase Shedding in Multiphase Buck Converters. IEEE Transactions on Power Electronics, 25(9), 2242-2247. doi:10.1109/tpel.2010.2049374Sánchez Reinoso, C. R., Milone, D. H., & Buitrago, R. H. (2013). Simulation of photovoltaic centrals with dynamic shading. Applied Energy, 103, 278-289. doi:10.1016/j.apenergy.2012.09.040Muñoz, J., Martínez-Moreno, F., & Lorenzo, E. (2010). On-site characterisation and energy efficiency of grid-connected PV inverters. Progress in Photovoltaics: Research and Applications, 19(2), 192-201. doi:10.1002/pip.997Davila-Gomez, L., Colmenar-Santos, A., Tawfik, M., & Castro-Gil, M. (2014). An accurate model for simulating energetic behavior of PV grid connected inverters. Simulation Modelling Practice and Theory, 49, 57-72. doi:10.1016/j.simpat.2014.08.001Rampinelli, G. A., Krenzinger, A., & Chenlo Romero, F. (2014). Mathematical models for efficiency of inverters used in grid connected photovoltaic systems. Renewable and Sustainable Energy Reviews, 34, 578-587. doi:10.1016/j.rser.2014.03.047MathWorks Statistics and Machine Learning Toolboxhttps://www.mathworks.co

A Control Scheme to Suppress Circulating Currents in Parallel-Connected Three-Phase Inverters

[EN] The parallel operation of inverters has many benefits, such as modularity and redundancy. However, the parallel connection of inverters produces circulating currents that may result in malfunctions of the system. In this work, a control technique for the elimination of the low-frequency components of the circulating currents in grid-connected inverters is presented. The proposed control structure contains n - 1 zero-sequence control loops, with n being the number of inverters connected in parallel. Simulation and experimental results have been carried out on a prototype composed of two 5 kW inverters connected in parallel. The results have been obtained by considering the following mismatches between both inverters: inductance values of the grid filters, unbalance of the delivered power, and the use of different modulation techniques.This research was funded by the Spanish "Ministerio de Asuntos Economicos y Transformacion Digital" and the European Regional Development Fund (ERDF), under grants RTI2018100732-B-C21 and PID2021-122835OB-C22.Liberos, M.; González-Medina, R.; Patrao Herrero, I.; Garcerá, G.; Figueres Amorós, E. (2022). A Control Scheme to Suppress Circulating Currents in Parallel-Connected Three-Phase Inverters. Electronics. 11(22):1-23. https://doi.org/10.3390/electronics11223720123112