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When no transformer is used in a grid-connected photovoltaic (PV) system, a galvanic connection between the grid and the PV array exists. In these conditions, dangerous leakage currents (common-mode currents) can appear through the stray capacitance between the PV array and the ground. In order to avoid these leakage currents, different inverter topologies that generate no varying common-mode voltages, such as the half-bridge and the bipolar pulsewidth modulation (PWM) full-bridge topologies, have been proposed. The need of a high-input voltage represents an important drawback of the half-bridge. The bipolar PWM full bridge requires a lower input voltage but exhibits a low efficiency. This letter proposes a new high-efficiency topology that generates no varying common-mode voltage and requires the same low-input voltage as the bipolar PWM full bridge. The proposed topology has been verified in a 5-kW prototype with satisfactory results.Index Terms-DC-AC power conversion, photovoltaic (PV) systems, transformerless inverter.

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Abstract--This paper presents the design of a low complexity Fuzzy Logic Controller of only 25-rules to be embedded in an Energy Management System for a residential grid-connected microgrid including Renewable Energy Sources and storage capability. The system assumes that neither the renewable generation nor the load demand is controllable. The main goal of the design is to minimize the grid power profile fluctuations while keeping the Battery State of Charge within secure limits. Instead of using forecasting-based methods, the proposed approach uses both the microgrid energy rate-of-change and the battery SOC to increase, decrease or maintain the power delivered/absorbed by the mains. The controller design parameters (membership functions and rule-base) are adjusted to optimize a pre-defined set of quality criteria of the microgrid behavior. A comparison with other proposals seeking the same goal is presented at simulation level, whereas the features of the proposed design are experimentally tested on a real residential microgrid implemented at the Public University of Navarre.Index Terms--Distributed power generation, energy management, fuzzy control, microgrid, renewable energy sources, smart grid. I. INTRODUCTION HE ENVIRONMENTAL and economic benefits related to the reduction of both carbon dioxide emission andThis work was partially supported by the Secretaría Nacional de Educación Superior, Ciencia, Tecnología e Innovación SENESCYT and the Instituto de Fomento al Talento Humano del Ecuador under the grant No. 2013-AR2Q4081. Government of Navarra and the FEDER funds under project "Microgrids in Navarra: design, development and implementation". Spanish Ministry of Economy and Competitiveness under grant DPI2013-42853-R; European Union under the project FP7-308468, "PVCROPS-Photovoltaic Cost reduction, reliability, operational performance, prediction and simulation"; grant RUE CSD2009-00046, Consolider-Ingenio 2010 Programme, Spanish Ministry of Science and Innovation, and the grants DPI2013-41224-P, DPI2012-31580 from the Spanish Ministry of Economy and Knowledge.D. Arcos-Aviles is with the Departamento de Ingeniería Eléctrica y Electrónica, Propagation, Electronic Control and Networking (PROCONET) research group, Universidad de las Fuerzas Armadas ESPE, 171-5-231B Sangolquí, Ecuador (e-mail: dgarcos@espe.edu.ec).J. Pascual, L. Marroyo and P. Sanchis are with the Department of Electrical and Electronic Engineering, Public University of Navarre, Pamplona, Spain (e-mail: juliomaria.pascual@unavarra.es; luisma@unavarra.es; pablo.sanchis@unavarra.es.).F. Guinjoan is with the Department of Electronics Engineering, Escuela Técnica Superior de Ingenieros de Telecomunicación de Barcelona, Universitat Politècnica de Catalunya, 08034 Barcelona, Spain (e-mail: francesc.guinjoan@upc.edu).transmission losses have made distributed renewable generation systems became a competitive solution for future smart grids [1]. In this context, microgrids are considered as the key building blocks of smart grids [2] and have aroused g...

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The relative weight of the energy generated by means of renewable sources is constantly increasing. Among all these sources, the photovoltaic (PV) systems present the higher and more stable relative growth. However, the PV system is still too expensive and a significant effort is being done to increase the efficiency and reduce the cost. Concerning the PV inverters, this has lead to the elimination of the low frequency (LF) transformer that has been traditionally included. The LF transformer provides isolation from the grid but reduces the PV inverter efficiency and increases its size and cost. However, the elimination of the transformer might generate strong ground currents, which become now an important design parameter for the PV inverter. The ground currents are a function of the system stray elements. However, there is no simple model and procedure to study the common mode behavior of a PV system, which is required to analyze the ground currents. In this paper, a comprehensible model is proposed which provides a better understanding of the common mode issue in single-phase transformerless PV systems. In addition, a procedure is developed to analyze the global performance, efficiency, grid current quality, and common mode behavior of a PV inverter as a function of its particular structure and modulation technique.

This work has been performed within a Research and Development contract on Hydrogen Energy granted by Acciona Biocombustibles S. A., a branch of Acciona Energía, to the Public University of Navarra and deals with the production of sustainable hydrogen from renewable primary energy sources. In this work, the results obtained in several experiments carried out with a commercial alkaline water electrolyzer of 5 kW working under the dynamic conditions typical of wind energy systems are presented. Actual wind speed data from a wind farm were considered and allowed us to establish the power profile that would give a wind turbine of a rated power equivalent to that of the electrolyzer under the considered wind conditions. This power profile was supplied to the electrolyzer by means of a power supply (wind emulator) designed and built by our group. The evolution of the stack voltage, efficiency, and temperature, as well as the purity of the produced hydrogen and oxygen, was monitored. The results showed that the electrolyzer behavior was reliable with high efficiencies, satisfactory gas purities, and fast dynamic response to the varying conditions imposed.