In the last 15 years, the use of doubly fed induction machines in modern variable-speed wind turbines has increased rapidly. This development has been driven by the cost reduction as well as the low-loss generation of Insulated Gate Bipolar Transistors (IGBT). According to new grid code requirements, wind turbines must remain connected to the grid during grid disturbances. Moreover, they must also contribute to voltage support during and after grid faults. The crowbar system is essential to avoid the disconnection of the doubly fed induction wind generators from the network during faults. The insertion of the crowbar in the rotor circuits for a short period of time enables a more efficient terminal voltage control. As a general rule, the activation and the deactivation of the crowbar system is based only on the DC-link voltage level of the back-to-back converters. In this context, the authors discuss the critical rotor speed to analyze the instability of doubly fed induction generators during grid faults.

In a railway line, trains are constantly changing their positions and loads. At each specific time, trains, substations, contact line and earthing system establish a different DC circuit configuration. To solve those circuits, several approaches were tested. These approaches may be divided into linear systems and nonlinear systems. The ICCG (incomplete cholesky conjugate gradient) solver for DC traction load flow is proposed in the paper. This method is described and applied to a test circuit. To verify the ICCG effectiveness applied to DC traction load flow, a comparison with other commonly used methods is made.

The teaching-learning process in engineering aims to meet current societal demands and address real challenges faced by businesses and the job market. Challenge-based learning (CBL) has gained traction as an active and innovative approach in engineering education, introducing real challenges and open questions to the classroom regarding environmental sustainability and issues faced by Industry 4.0. These challenges require resources and technology that turn the teaching-learning process into an open system, demanding partnerships beyond academia for validating deliverables and projects. Following the PRISMA 2020 guidelines, this study aimed to systematically analyze the implementation and implications of CBL for competencies development. Following the inclusion and exclusion criteria of the PRISMA method, 62 articles were used for abstract analyses to identify methods, workload, resources, structure, technology, and stakeholder integration, as well as to answer the research questions. The studies were categorized into three types of applied challenges: 1) social and environmental sustainability; 2) Industry 4.0; and 3) those related to Educational Institutions. A total of 46 articles were analyzed in their entirety, and summarized in three tables. All analyzed studies showed that CBL is effective according to summative and formative assessments, leading to sociotechnical competencies development through experience with real-world challenges, teamwork, and interaction with external partners. As most studies are qualitative, there is room for quantitative investigations to better justify the relevance of CBL, especially in terms of adaptive and personalized learning. Moreover, the workload and complexity imposed by CBL, particularly on teachers, warrant further study to facilitate implementation and engagement.

In the first part some information and characterisation about an AC distribution network that feeds traction substations and their possible influences on the DC traction load flow are presented. Those influences are investigated and mathematically modelled. To corroborate the mathematical model, an example is presented and their results are confronted with real measurements.

The use of wind power has been increasing very fast in the last 10 years. Many new projects for the next 10 years including offshore and onshore wind farms are been developed and planned. The fast growing of the use of wind power has brought new challenges to the Transmission System Operators (TSO) in regions where wind power has reached significant penetration levels like Denmark, United Kingdom, Spain and Germany. According to new grid code requirements wind turbines must remain connected to the grid during grid disturbances and, moreover, they must also contribute to voltage support during and after grid faults. Dynamic models of doubly fed induction generator (DFIG) were developed to investigate the behavior off different converter control and protection strategies of the back-to-back IGBT-based converters during grid fault. The results have showed that reactive power injection by DFIG-based wind farms is limited when the rotor side converter is blocked.