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Grid-connected photovoltaic (PV) power systems are one of the most promising technologies to address growing energy demand and ecological challenges. This paper proposes smart switching to mitigate inrush currents during the connection of single-phase transformers used in PV systems. An effective inrush current mitigation contributes to the reliability of PV systems. The inrush current severity is influenced by the pseudorandom residual flux at the transformer core and the energization point-on-wave. The most common approach to avoid inrush currents is controlled connection, which requires prior knowledge of the residual flux. However, the residual flux can differ in each case, and its measurement or estimation can be impractical. The proposed smart switching is based on a comprehensive analysis of the residual flux and the de-energization trajectories, and only requires two pieces of data (ϕRM and ϕ0, flux values of the static and dynamic loops when the respective currents are null), calculated from two simple no-load tests. It has a clear advantage over common approaches: no need to estimate or measure the residual flux before each connection, avoiding the need for expensive equipment or complex setups. Smart switching can be easily implemented in practical settings, as it considers different circuit breakers with distinctive aperture features, making it cost-effective for PV systems. [ABSTRACT FROM AUTHOR]

Students' learning can be improved by using online quizzes with a valuable feedback, especially in large groups, making it very interesting for engineering subjects which are based on mathematics. This paper presents the evaluation of an online quiz experience based on a case study in two university subjects. Quizzes were created by using Wiris Quizzes (WQ) that can register the complete solution with all the steps involved. WQ has a calculator that allows us to solve the mathematical part of the quiz. The first aim of the online WQ experience is to provide feedback to students through automatic grading and clues to find the correct answers. The second aim is to promote autonomous student learning. The teachers also have excellent feedback about faulty interpretations or weaknesses in the students' understanding. Consequently, teachers can provide corrective information to clarify ideas about what they want to be understood and what is understood by students. WQ allows teachers to develop algorithms for the quizzes by using random data, figures realized through programming, and multiple solutions that depend on the values of a table. The teaching experience of using WQ has shown that this tool can be useful in university technological subjects with large groups that require advanced and customized quizzes. [ABSTRACT FROM AUTHOR]

To ensure the safe operation of the grid, there are some requirements to be taken into consideration to connect power converters. During abnormal conditions -- e.g. during voltage sags --, the control of the converters is a very important key to guarantee power quality and good behaviour of the distributed generation system. The aim of this study is to employ two possible control strategies for a grid-connected inverter according to the Spanish grid code, and to analyse the behaviour of the output voltages during both symmetrical and unsymmetrical voltage sags. The analytical development shows the sag influence on currents, voltages, active and reactive powers. These influences are explained through Ku transformation in the synchronous reference frame, thus giving a representation for electrical variables easiest to analyse. The results show how control strategies affect the converter behaviour and how they can support the main grid during faults through the control of active and reactive power injection. Sags with different durations and depths have been taken into account, which can provoke critical values for electrical magnitudes and can lead to the violation of the grid code. The proposed control strategies study has been validated by means of both simulations in MATLABTM--Simulink and experimental results. [ABSTRACT FROM AUTHOR]