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The modular multilevel converter (MMC) based on half-bridge modules is a power converter topology suitable for high-power medium-voltage variable-speed drives. However, the voltage of its flying capacitors is negatively affected when low frequencies appear at the AC port. This paper analyzes the influence of using a variable DC port voltage in a machine-side MMC by implementing a closed-loop approach, ensuring a constant voltage fluctuation in the capacitors of the MMC during the whole operating range. The effectiveness of the proposed control scheme is demonstrated through simulation studies and experimental validation tests conducted using a 7.5 kW experimental prototype composed of an induction machine fed by an MMC with 18 half-bridge cells.

The Modular Multilevel Matrix Converter is an AC-AC power converter proposed for high power applications such as motor drive and wind energy conversion systems. The M 3C has 9 clusters, allowing 4 circulating currents for converter energy management. Control of the M 3C is frequently divided into Different Frequency Mode (DFM) and Equal Frequency Mode (EFM). EFM is more challenging, because of the larger capacitor voltage oscillations that can be produced. Conventional energy management control strategies for EFM/DFM are usually based on 8 energy control loops used to define four circulating current references composed of several predefined frequencies and positive/negative sequences. The control schemes are typically different for EFM/DFM operation increasing the complexity. In this paper, a Continuous-Control-Set Model Predictive Control (CCS-MPC) for energy management of the M 3C is proposed. The control scheme is based on solving an equality constrained quadratic programming problem, using a state variable model of the M3C, where the optimal solution is analytically obtained. The result is a single and simple control law to obtain circulating current references during EFM/DFM, ensuring a good performance with optimal current specifications. The proposed strategy is experimentally validated using a down-scaled M3C prototype composed of 27 power cells.

This article proposes a new scalar control scheme for variable speed drives of induction motors with an adaptive starting control. Besides centrifugal pumps, blowers, and fans, it expands the scalar control application field to move, for instance, a class of conveyor belts with nominal torque loading. An adaptive passivity-based controller designed for a class of nonlinear systems encompassing the dynamical machine model keeps a constant stator starting current. The proposed methodology uses information from the motor nameplate. Hence, it keeps a simple control scheme, not needing parameter estimates, neither adjusting controllers nor variable observers depending on them. Experimental results with a 10 HP downscaled laboratory prototype validate the proposed control strategy effectiveness. It shows rated-starting torque capabilities and fast speed response. Moreover, it shows lower stator current consumption than previous scalar control schemes

The main application of off-grid solar photovoltaic (SPV) systems is water extraction in rural areas where access to the grid is restricted. In this application, photovoltaic (PV) and pump system regulation are crucial to increase its overall efficiency. In this context, this work presents a simple and efficient off-grid SPV water pumping system (SPVWPS). The designed system is based on a DC-DC boost converter, a three-phase DC-AC inverter, and a three-phase induction motor (IM) coupled to the centrifugal pump. The proposed solution is operated using a control strategy that associates an improved fractional open-circuit voltage (FOCV) method for maximum power point tracking (MPPT) and closed-loop scalar control. This association avoids the use of a speed sensor/encoder and a current sensor for the IM. Finally, the effectiveness of the proposed off-grid SPVWPS and its control system for both steady-state and dynamic conditions of insolation change is verified using a 1KVA rated prototype. The relevance of the drive is also checked in various operating conditions and is found to be adequate for pumping water. Moreover, the proposed method guarantees a fast response, less oscillations around the MPP, a system efficiency of 99%, and a high flow rate due to the extraction of maximum power.

Renewable energy sources have become a significant alternative energy source due to the continuing depletion of conventional energy sources and fluctuation in fuel costs. Currently, wind energy is the foremost among all other renewable energy sources. However, modeling and analyzing industrial wind turbines is complex as the wind turbine power ratio and size have steadily increased. Undoubtedly, industrial wind turbines are huge and challenging to keep in research labs; simultaneously, exploring the controller/power converter performance is practically impossible. Therefore, to overcome the above drawbacks, wind turbine emulators have been developed to achieve the static and dynamic characteristics of wind energy conversion systems. This paper aims to present a comprehensive review of the different wind turbine emulators available in the literature. In addition, the implementation of real-time emulators is classified according to the structure and approaches. Furthermore, an extensive analysis of the emulators was presented based on the significant parameters utilized for the real-time wind turbine emulators. Finally, this review analyzes the different emulator topologies according to cost, accuracy, complexity, and hardware implementation.

Wind energy has received considerable attention during the past two decades, owing to its vital role in battling both the energy crisis and global climate change. Wind energy has grown to become the most significant nonconventional renewable energy source. Today, modern wind turbines (WTs) are controlled to offer several functionalities that improve their flexibility at a level similar to conventional power sources. However, until now, there have been few reports in the literature summarizing and listing WT control concepts to encourage industry and academia to pursue improvements in developing more efficient WTs and bridge the gap between businesses and universities. Hence, this article focuses on a thoughtful investigation into developing better and more efficient WTs, detailing different issues involved in wind energy generation, such as WT components, control systems, operational modes, and, especially, new research and development trends.

The closet exoplanets to the Sun provide opportunities for detailed characterization of planets outside the Solar System. We report the discovery, using radial velocity measurements, of a compact multiplanet system of super-Earth exoplanets orbiting the nearby red dwarf star GJ 887. The two planets have orbital periods of 9.3 and 21.8 days. Assuming an Earth-like albedo, the equilibrium temperature of the 21.8-day planet is ~350 kelvin. The planets are interior to, but close to the inner edge of, the liquid-water habitable zone. We also detect an unconfirmed signal with a period of ~50 days, which could correspond to a third super-Earth in a more temperate orbit. Our observations show that GJ 887 has photometric variability below 500 parts per million, which is unusually quiet for a red dwarf. © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works, S.V.J. acknowledges the support of the German Science Foundation (DFG) Research Unit FOR2544 "Blue Planets around Red Stars" project (JE 701/3-1) and DFG priority program SPP 1992 "Exploring the Diversity of Extrasolar Planets" JE 701/5-1. J.R.Ba.nd CA.H. acknowledge support from STFC Consolidated Grants ST/P000584/1 and ST/T000295/1. R.PN. was supported by STFC Consolidated Grant ST/P000592/1. E.R. M.J.L.-G. N,M., and P.J.A. acknowledge support from the Spanish Agencia Estatal de Investigacion through projects AYA2017-89637R, AYA2016-79425-C3-3-S, ESP2017-87676-C5-2-R, and ESP2017-87143R and the Centre of Excellence "Severo Ochoa" Instituto de Astrofisica de Andalucia (SEV-2017-0709). E.P. acknowledges support from the Spanish Agencia Estatal de Investigacion PGC2018-098153-B-C31 and ESP2016-80435-C2-2-R. Z.M.B. acknowledges funds from CONICYT/FONDECYT Postdoctorado 3180405. G.A.L.C. acknowledges support from the Swiss National Science Foundation. M.R.D.acknowledges support of CONICYT/PFCHA-Doctorado Nacional 21140646, Chile. I.R. acknowledges support from the Spanish Ministry of Science and Innovation and the European Regional Development Fund through grants ESP2016-80435-C2-1-R and PGC2018-098153-B-C33 and the Generalitat de Catalunya/CERCA program. H.R.A.J. acknowledges support from the U.K.Science and Technology Facilities Council grant ST/M001008/1. C.G.T. is supported by Australian Research Council grants DP0774000, DP130102695, and DP170103491. J.T. wassupported by NASA through Hubble Fellowship grant HST-HF2-51399.001 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. G.A.-E is supported by the Ministerio de Ciencia, Innovacion y Universidades Ramon y Cajal fellowship RYC-2017-22489 and by the Science and Technology Facilities Council grant number ST/P000592/1.

We report the detection of a transiting, dense Neptune planet candidate orbiting the bright ($V=8.6$) K0.5V star HD 95338. Detection of the 55-day periodic signal comes from the analysis of precision radial velocities from the Planet Finder Spectrograph on the Magellan II Telescope. Follow-up observations with HARPS also confirm the presence of the periodic signal in the combined data. HD 95338 was also observed by the Transiting Exoplanet Survey Satellite ({\it TESS}) where we identify a clear single transit in the photometry. A Markov Chain Monte Carlo period search on the velocities allows strong constraints on the expected transit time, matching well the epoch calculated from \tess{} data, confirming both signals describe the same companion. A joint fit model yields an absolute mass of 42.44$^{+2.22}_{-2.08} M_{\oplus}$ and a radius of 3.89$^{+0.19}_{-0.20}$ $R_{\oplus}$ which translates to a density of 3.98$^{+0.62}_{-0.64}$ \gcm\, for the planet. Given the planet mass and radius, structure models suggest it is composed of a mixture of ammonia, water, and methane. HD 95338\,b is one of the most dense Neptune planets yet detected, indicating a heavy element enrichment of $\sim$90\% ($\sim38\, M_{\oplus}$). This system presents a unique opportunity for future follow-up observations that can further constrain structure models of cool gas giant planets., 13 pages, 11 figures, MNRAS accepted

The problems associated with active power oscillations (APOs) in grid-connected converters are well-known. Imbalances in the grid usually produce double-frequency oscillations in the dc-link voltage and current which could reduce the useful life of solar panels, batteries, and capacitors connected to this point. Moreover, as reported in the literature, double-frequency reactive power oscillations (RPOs) also produce adverse effects in distribution systems, and it is desirable to eliminate or mitigate them. When a four-leg power converter is connected to an unbalanced grid, the zero-sequence current provides extra degrees of freedom to compensate or even eliminate the power oscillations at the converter dc-link side. In this paper, a new methodology to regulate these double-frequency power components is proposed. It is based on a closed-loop vector control approach, where the active power oscillations (APOs) at converter side are transformed into a synchronous frame rotating at twice the grid frequency and regulated using the zero-sequence current. To avoid overcurrent produced by the circulation of positive-, negative-, and zero-sequence components a current limiter is also proposed in this paper. Experimental results obtained with a 4-kW four-leg power converter prototype are presented and discussed in this paper.

The Technische Universitat Dresden (TU Dresden), Faculty of Education and the Universidad de Santiago de Chile (USACH) Faculty of Engineering, collaborate for the period from01.01.2017 to 31.12.2020 in the research and development project " Strengthening engineering training at Chilean universities through practice partnerships, STING Project (in german “Starkung derIngenieurausbildung an chilenischenHochschulen durch Praxispartnerschaften"). This paper presents goals, activities and results of the STING Project, focused on the needs of academic training related to the different pedagogical and technological aspects that influence the training of engineers in the engineering faculty of the USACH. The STING-Project is oriented to the development and testing of training modules for students (either for electrical or mechanical engineers) and teaching qualifications of teaching staff in academic engineering education based in demands and employment- requirements of German and Chilean companies. For this reason, was developed a questionnaire by the TU Dresden and the USACH as part of a stage of information gathering to obtain the strategic positioning and future development of the participant enterprises. The goal of the application of this questionnaire is to know the opinion of strategic staff of the enterprises regarding the actual needs and the projected future scenario for engineers and the type of “technology transferences” between university-company. The results of this survey were used in the development of two training modules for students at the USACH. This project is financially supported by the German Academic Exchange Service (DAAD) as part ofthe program "PraxispartnerschaftenZwischenHochschulen und Unternehmen in Deutschlandund in Entwicklungslandern ab 2017“

Revisión sistemática. Objetivo: Evaluar los valores de las presiones bucales estáticas máximas inspiratorias y espiratorias en adultos jóvenes sanos latinoamericanos, así como los instrumentos y las características técnicas para su medición. Materiales y método: Se incluyeron estudios observacionales que reportaron los valores de las mediciones de presiones bucales estáticas máximas inspiratorias y/o espiratorias (Pimax y/o Pemax) en jóvenes, adultos jóvenes, adultos y estudiantes latinoamericanos. Se desestimaron los artículos que no lograron recuperarse para la lectura completa, publicados por tiempo superior a diez años y en idiomas distintos al inglés, español o portugués. La búsqueda fue realizada en las bases de datos Medline/Pubmed, Biblioteca Virtual en Salud y SciELO. El análisis del riesgo de sesgo se realizó a través de la escala de Newcastle Ottawa. Se realizó una síntesis cualitativa de los resultados. Registro OSF: https://osf.io/32jqp/ Resultados: Se incluyeron 9 artículos observacionales. El rango etario de los sujetos fue de entre 15 y 35 años. Los valores medios de Pimax y/o Pemax fueron mayores en el sexo masculino. Las mediciones fueron realizadas con manovacuómetros digitales y aneroides, y los tipos de boquilla fueron rígidos y de tipo buceo, en posición supina, sentada o semisentada. Conclusión: La mayoría de los estudios sobre presiones bucales estáticas máximas de jóvenes adultos latinoamericanos sanos fueron realizados en sujetos de entre 15 a 35 años. Se observaron valores mayores de presión respiratoria estática máxima en el sexo masculino, y las mediciones fueron realizadas en posición sentada, mediante el uso de manovacuómetro analógico. Estudios futuros son necesarios a fin de ayudar a la implementación de estos hallazgos para una práctica real basada en la evidencia.