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

Comportement d’aciers inoxydables à hautes caractéristiques mécaniques vis-à-vis de la corrosion par piqûre et de la fragilisation par hydrogène

Cette étude présente le comportement vis-à-vis de la corrosion par piqûre et de la fragilisation par hydrogène de deux nuances d’aciers inoxydables à hautes caractéristiques mécaniques. Ceci a pour objectif de vérifier si, pour ces aciers, un écrouissage important associé à un détensionnement thermomécanique engendre une baisse notable de leur comportement en corrosion. L’étude électrochimique « à contrainte œ nulle » montre que le potentiel de piqûre Epit de la nuance austéno-ferritique (duplex) est plus faible que celui de la nuance austénitique, contrairement à ce que prévoit le Pitting Resistant Equivalent Number (PREN). La présence de précipités de nitrure de chrome semble pouvoir expliquer ce résultat. Les observations réalisées après les essais de fragilisation par hydrogène montrent que la fissuration s’amorce sur des piqûres. La nuance duplex présente cependant une durée de vie plus importante que celle de la nuance austénitique. Le piégeage de l’hydrogène aux joints de grains α/γ pourrait expliquer la grande résistance de la nuance duplex

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

Numerical simulation of high-voltage composite cable for offshore wind

Conference of 17th European Conference on Composite Materials, ECCM 2016 ; Conference Date: 26 June 2016 Through 30 June 2016; Conference Code:126913International audienceIn submarine power cables, conductors are subjected to variety of mechanical loads, that affect the material properties and eventually lead to failure. In this study, a simple straight "1+6" strand is modelled as a copper conductor and the relationship between mechanical loads to which conductors are subjected to and its mechanical responses is addressed. Finite Element Analysis (FEA) of the model is carried out to study localized non-linear effects such as contact stress, friction and plastic deformation

Modélisation du martelage à haute fréquence des assemblages soudés

Les joints soudés présentent bien souvent une durée de vie en fatigue relativement faible comparée à celle des matériaux de base et d’apport qui les composent. Plusieurs techniques d’amélioration post-soudage ont par conséquent été proposées et mises en œuvre dans ce domaine dont le martelage à haute fréquence ou « MHF ». Ce dernier constitue l’un des procédés les plus prometteurs de par son influence significative sur la tenue en fatigue de ce type d’assemblages. Toutefois, il est encore délicat de pouvoir prédire la durée de vie en fatigue des joints soudés traités par MHF. Nous proposons ici une démarche pour l’estimation de la durée de vie des assemblages traités par MHF. Cette démarche est en cours de validation mais permet d’ores et déjà d’identifier l’influence relative des principaux paramètres du traitement

Experimental characterization of interfacial adhesion of an optical fiber embedded in a composite material

International audienceThe efficiency of an optical sensor embedded in a composite structure strongly depends on the interfacial adhesion between the optical fiber coating and the surrounding solid material. The present paper reports on the study of the interfacial adhesion of an optical fiber embedded in a composite material. A simple system composed of optical fibers embedded in an epoxy vinylester resin was first studied to evaluate the influence of embedded length, curing temperature and curing time. Pull-out tests on optical fibers bonded in epoxy vinylester/glass fiber composite material were carried out to measure the effect of glass concentration on the fiber bonding. The pull-out results showed no effect of both embedded length and curing temperature. However, an increase of the interfacial debonding stress is reported with increased curing time. For the optical fiber/composite system, a linear evolution of interfacial debonding stress with increasing glass fiber concentration is reported. (C) 2012 Elsevier Ltd. All rights reserved

Mechanical properties of a TAS fiber: a preliminary study

International audienceThe mechanical properties, including elastic moduli, hardness, fracture toughness and tensile strength of a glass fiber in the Te-As-Se system (TAS) were studied. The values for the hardness (1.4 GPa) and the fracture toughness (0.18 MParootm) show that this glass is both soft and brittle in comparison to glasses from other systems. However, indentation measurements should be interpreted with caution due to an indentation creep phenomenon and to a delayed fracture process. In addition, the effect of treatments in air (relative humidity about 60%) at different temperatures below T-g were investigated. The main result of this study is that the studied TAS glass is sensitive to the presence of humidity, and aging treatments have a pronounced detrimental effect on the strength of the uncoated fibers. (C) 2003 Elsevier Science B.V. All rights reserved

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