A Review on Multi-Terminal High Voltage Direct Current Networks for Wind Power Integration

With the growing pressure to substitute fossil fuel-based generation, Renewable Energy Sources (RES) have become one of the main solutions from the power sector in the fight against climate change. Offshore wind farms, for example, are an interesting alternative to increase renewable power production, but they represent a challenge when being interconnected to the grid, since new installations are being pushed further off the coast due to noise and visual pollution restrictions. In this context, Multi-Terminal High Voltage Direct Current (MT-HVDC) networks are the most preferred technology for this purpose and for onshore grid reinforcements. They also enable the delivery of power from the shore to offshore Oil and Gas (O&G) production platforms, which can help lower the emissions in the transition away from fossil fuels. In this work, we review relevant aspects of the operation and control of MT-HVDC networks for wind power integration. The review approaches topics such as the main characteristics of MT-HVDC projects under discussion/commissioned around the world, rising challenges in the control and the operation of MT-HVDC networks and the modeling and the control of the Modular Multilevel Converter (MMC) stations. To illustrate the challenges on designing the control system of a MT-HVDC network and to corroborate the technical discussions, a simulation of a three-terminal MT-HVDC network integrating wind power generation and offshore O&G production units to the onshore grid is performed in Matlab's Simscape Electrical toolbox. The results highlight the main differences between two alternatives to design the control system for an MT-HVDC network

Tolerance of combined drought and heat stress is associated with transpiration maintenance and water soluble carbohydrates in wheat grains

Published 15 October 2020Wheat (Triticum aestivum L.) production is increasingly challenged by simultaneous drought and heatwaves. We assessed the effect of both stresses combined on whole plant water use and carbohydrate partitioning in eight bread wheat genotypes that showed contrasting tolerance. Plant water use was monitored throughout growth, and water-soluble carbohydrates (WSC) and starch were measured following a 3-day heat treatment during drought. Final grain yield was increasingly associated with aboveground biomass and total water use with increasing stress intensity. Combined drought and heat stress immediately reduced daily water use in some genotypes and altered transpiration response to vapor pressure deficit during grain filling, compared to drought only. In grains, glucose and fructose concentrations measured 12 days after anthesis explained 43 and 40% of variation in final grain weight in the main spike, respectively. Starch concentrations in grains offset the reduction in WSC following drought or combined drought and heat stress in some genotypes, while in other genotypes both stresses altered the balance between WSC and starch concentrations. WSC were predominantly allocated to the spike in modern Australian varieties (28–50% of total WSC in the main stem), whereas the stem contained most WSC in older genotypes (67–87%). Drought and combined drought and heat stress increased WSC partitioning to the spike in older genotypes but not in the modern varieties. Ability to maintain transpiration, especially following combined drought and heat stress, appears essential for maintaining wheat productivity.Abdeljalil El Habti, Delphine Fleury, Nathaniel Jewell, Trevor Garnett and Penny J. Tricke

Detection by acoustic emission of damage in cable anchorage

La sécurité des ponts suspendus et à haubans dépend fortement de la durabilité de leurs câbles, ces derniers peuvent présenter un aspect sain sur leurs longueurs visibles mais être en fait endommagés dans les parties non accessibles (ancrages notamment). Dans cette étude, l'émission acoustique (EA) a été utilisée afin de détecter et de localiser des fils rompus dans les zones d'ancrages. L'activité acoustique sur laquelle se base cette méthode provient du frottement du fil rompu avec ses voisins. Lorsque le câble présentant une rupture artificielle est mis en vibration, plusieurs paramètres des signaux acoustiques peuvent être utilisés pour mettre en évidence une différence entre un câble endommagé et sain. Les principaux paramètres sont le nombre total de coups et l'énergie cumulée. Afin de mieux comprendre ce mécanisme de frottement entre fils, une analyse du comportement d'un fil rompu dans un câble tendu soumis à une flexion dynamique a été faite. Cela a permis de définir les conditions de sollicitation du câble favorables à la détection. Ce mécanisme et sa détection par EA sont influencés par le changement de la rugosité, de la lubrification, des forces de contact entre les fils

Numerical Simulation of Aging by Water-Trees of XPLE Insulator Used in a Single Hi-Voltage Phase of Smart Composite Power Cables for Offshore Farms

Submarine power cables are expected to last 20 years without maintenance to be considered technologically reliable enough and economically beneficial. One of the main issues facing this target is the development of what is called commonly water-trees (nanometer-sized flaws filled with residual humidity), that form within XLPE (cross-linked Polyethylene) insulators and then migrate towards copper, thus leading to its corrosion and further to possible shut-down. Water trees are resulting from the coalescence of nanovoids filled with residual humidity that migrate towards copper under the combined effects of electrical forces and plastic deformation. The nanovoids are originated during manufacturing, shipping, handling and embedding in deep seas. The formation of these nanovoids leads to the degradation of the service lifetime of submarine power cables. Current research is intended to come up with a way to go a little further towards the generalization of coalescence of n nanovoids. In the perspective of multi-physics modeling, a preliminary 3D finite element model was built. Although water voids are distributed randomly inside XLPE, in this study, two extreme cases where the voids are present parallel and perpendicular to the copper surface, were considered for simplification. This will enable checking the electric field effect on neighbouring voids, in both cases as well as the influence of the proximity of the conductor on the plasticity of voids, that further leads to their coalescence. It is worthwhile to note that assessing water-trees formation and propagation through an experimental campaign of ageing tests may extend over decades. It would therefore be an exceptional opportunity to be able to get insight into this mechanism through numerical modeling that needs a much shorter time. The premilinary model suggested is expected to be extended in the future so that to include more variables (distribution and shapes of nano-voids, water pressure, molecular modeling, electric discharge

Detection by acoustic emission of damage in cable anchorage

La sécurité des ponts suspendus et à haubans dépend fortement de la durabilité de leurs câbles, ces derniers peuvent présenter un aspect sain sur leurs longueurs visibles mais être en fait endommagés dans les parties non accessibles (ancrages notamment). Dans cette étude, l'émission acoustique (EA) a été utilisée afin de détecter et de localiser des fils rompus dans les zones d'ancrages. L'activité acoustique sur laquelle se base cette méthode provient du frottement du fil rompu avec ses voisins. Lorsque le câble présentant une rupture artificielle est mis en vibration, plusieurs paramètres des signaux acoustiques peuvent être utilisés pour mettre en évidence une différence entre un câble endommagé et sain. Les principaux paramètres sont le nombre total de coups et l'énergie cumulée. Afin de mieux comprendre ce mécanisme de frottement entre fils, une analyse du comportement d'un fil rompu dans un câble tendu soumis à une flexion dynamique a été faite. Cela a permis de définir les conditions de sollicitation du câble favorables à la détection. Ce mécanisme et sa détection par EA sont influencés par le changement de la rugosité, de la lubrification, des forces de contact entre les fils

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Chronic granulomatous disease in Morocco: Genetic, immunological, and clinical features of 12 patients from 10 kindreds

Purpose: Chronic granulomatous disease (CGD) is characterized by an inability of phagocytes to produce reactive oxygen species (ROS), which are required to kill some microorganisms. CGD patients are known to suffer from recurrent bacterial and/or fungal infections from the first year of life onwards. From 2009 to 2013, 12 cases of CGD were diagnosed in Morocco. We describe here these Moroccan cases of CGD. Methods: We investigated the genetic, immunological and clinical features of 12 Moroccan patients with CGD from 10 unrelated kindreds. Results: All patients were children suffering from recurrent bacterial and/or fungal infections. All cases displayed impaired NADPH oxidase activity in nitroblue tetrazolium (NBT), dihydrorhodamine (DHR) or 2′,7′ dichlorofluorescein diacetate (DCFH-DA) assays. Mutation analysis revealed the presence of four different mutations of CYBB in four kindreds, a recurrent mutation of NCF1 in three kindreds, and a new mutation of NCF2 in three patients from a single kindred. A large deletion of CYBB gene has detected in a patient. The causal mutation in the remaining one kindred was not identified. Conclusion: The clinical features and infectious agents found in these patients were similar to those in CGD patients from elsewhere. The results of mutation analysis differed between kindreds, revealing a high level of genetic and allelic heterogeneity among Moroccan CGD patients. The small number of patients in our cohort probably reflects a lack of awareness of physicians. Further studies on a large cohort are required to determine the incidence and prevalence of the disease, and to improve the description of the genetic and clinical features of CGD patients in Morocco. © 2014 Springer Science+Business Media