Nouvelle alimentation pour les fours à arc à courant alternatif

Destinés au recyclage des ferrailles, les fours à arc à courant alternatif sont des grands pollueurs du réseau électrique. Ils génèrent notamment des harmoniques de courant en basse fréquence et provoquent du flicker. Compte tenu des puissances mises en jeu (jusqu’à 100 MW pour les plus gros fours), la mise en place d’un dispositif de compensation des perturbations est obligatoire. Celui-ci est généralement basé sur un compensateur statique de puissance réactive associé à des éléments de filtrage passif. Cet équipement, qui est coûteux, lourd et volumineux, n’apporte dans la plupart des cas que peu d’amélioration dans le transfert d’énergie entre le réseau et le four. Le travail présenté dans cette thèse concerne une nouvelle alimentation électronique de puissance pour les fours à arc à courant alternatif. L’idée n’est plus de dépolluer le réseau mais de proposer une alimentation naturellement peu polluante qui garantit un transfert d’énergie optimal entre le réseau et le four. La topologie proposée est modulaire, elle repose sur une association série et parallèle de gradateurs à découpage et peut s’adapter à des niveaux de tension et de puissance différents en fonction des caractéristiques du four. La commande est basée sur une régulation cascade, elle permet un fonctionnement du four à puissance active constante et limite la consommation de puissance réactive, ce qui réduit de manière importante le dimensionnement de la compensation statique. Afin de définir un cahier des charges et d’établir un modèle électrique d’un four et de son alimentation, une campagne de mesures a été effectuée sur un site industriel équipé de deux fours de 75 MVA. Une étude par simulation permet ensuite de démontrer que grâce à sa commande, la nouvelle alimentation augmente l’énergie électrique transmise à la ferraille et réduit le flicker. Finalement, il est mis en évidence que le dimensionnement de l’électronique utilisée dans cette nouvelle alimentation est nettement inférieur à celui d’un compensateur de puissance réactive classiquement associé à un four à courant alternatif. Dans le but de valider expérimentalement cette nouvelle alimentation, un dimensionnement de la topologie à gradateur MLI est proposé dans le cas d’un four d’essais de 3 MW. Une maquette de faible puissance (qq kW) réalisé en laboratoire permet de valider le fonctionnement du convertisseur et de sa commande. ABSTRACT: Electrical arcs Furnaces (EAFs), supplied with alternating current (AC EAF) or direct current (DC EAF), provide a relatively simple means of melting scrap and are widely used in steel industry. The first EAFs were supplied with AC current. The DC EAFs appeared at the beginning of 1980s in order to reduce electrical disturbances on the power network such as flicker effect. Today 90% of the electrical steel in the world is produced by the AC EAFs. Nowadays, arc furnaces are designed for very large power input rating (up to 100 MVA), and due to the nature of both, the electrical arc and the meltdown process, various electrical characteristics such as the arc voltage and current, the active and reactive power exhibit large variations especially at the beginning of the scrap melting. These variations cause many power quality problems on the high voltage network such as flicker effect, unbalance and harmonics current, and affect the power system performances. To reduce these disturbances several solutions are available and differ in performance, flexibility and cost. For flicker mitigation, the Static Var Compensation (SVC) or the Static Synchronous Compensator (STATCOM), connected at the PCC in parallel with the load, are the most used structures. However, the compensating current injected by the SVC or the STATCOM has no effect on the arc current. During the meltdown, the mechanical control (electrode position) of the electrical power, which has very low bandwidth, affects the furnace performances by limiting the energy transferred to the furnace. This thesis presents a new electrical power supply for AC arc furnaces. This power supply uses AC chopper to control the arc current. A constant power control on the AC chopper is used. Compared to a classical supply, without electronic control, the active constant power operations leads to a higher average power level to the furnace reducing the melting time and increasing the furnace productivity. Furthermore, the reactive constant power operation limits the arc current especially during short circuit, decreasing the flicker effect. Owing to lower active and reactive power fluctuations the compensator has the smallest power rating. Simulation results, which take into account measurements carried out on a 75 MVA arc furnace, show that the furnace productivity could be increased by 11% and also that a flicker reduction of 55% could be expected from this new power supply. Finally design criteria of the proposed supply are presented for a 3 MVA arc furnace. The operation of PWM AC chopper is validated in laboratory (LEEI) by experimental test on a 3kVA prototype

Nouvelle alimentation pour les fours à arc à courant alternatif

L objet du travail présenté dans cette thèse et de proposer une nouvelle alimentation électronique de puissance pour les fours à arc à courant alternatif. L'idée n'est plus de dépolluer le réseau mais de proposer une alimentation naturellement peu polluante qui garantit un transfert d'énergie optimale entre le réseau d'alimentation et le four. Ainsi le fonctionnement du four à arc peut être amélioré et la puissance peut être prélevée sur le réseau sans le polluer.La structure du convertisseur est adaptée à une configuration très forte puissance, le convertisseur est constitué d'un assemblage série parallèle de convertisseurs élémentaires, ce qui permet de pouvoir aisément adapter l'alimentation proposée à des niveaux de tension et puissance différents. Une étude par simulation nous a permis de démontrer qu'une augmentation de l'énergie électrique transmise à la ferraille de 11% etet une réduction du flicker de 55% peuvent être attendus de cette nouvelle alimentation.Nowadays, arc furnaces are designed for very large power input ratings (up to 100 MW), and due to the arc voltage fluctuations the active and reactive power exhibit large variations in time.These variations cause many power quality problems on the high voltage network such as flicker effet and harmonics current To reduce these disturbances a reactive power compensator is added. This thesis presents a new electrical power supply for AC arc furnaces. This power supply uses AC chopper to control the arc current. A constant power control on the AC chopper is used. Compared to a classical supply, the active constant power operations leads to a higher average power level to the furnace increasing the furnace productivity and the reactive constant power operation limits the arc current especially during short circuit, decreasing the flicker effect. Owing to lower active and reactive power fluctuations the reactive power compensator has the smallest power rating.TOULOUSE-ENSEEIHT (315552331) / SudocSudocFranceF

Multilevel Converter for 4.16- and 6.6-kV Variable Speed Drives

International audienc

Appropriateness of antiplatelet therapy for primary and secondary cardio- and cerebrovascular prevention in acutely hospitalized older people

Aims: Antiplatelet therapy is recommended for the secondary prevention of cardio- and cerebrovascular disease, but for primary prevention it is advised only in patients at very high risk. With this background, this study aims to assess the appropriateness of antiplatelet therapy in acutely hospitalized older people according to their risk profile. Methods: Data were obtained from the REPOSI register held in Italian and Spanish internal medicine and geriatric wards in 2012 and 2014. Hospitalized patients aged ≥65 assessable at discharge were selected. Appropriateness of the antiplatelet therapy was evaluated according to their primary or secondary cardiovascular prevention profiles. Results: Of 2535 enrolled patients, 2199 were assessable at discharge. Overall 959 (43.6%, 95% CI 41.5–45.7) were prescribed an antiplatelet drug, aspirin being the most frequently chosen. Among patients prescribed for primary prevention, just over half were inappropriately prescribed (52.1%), being mainly overprescribed (155/209 patients, 74.2%). On the other hand, there was also a high rate of inappropriate underprescription in the context of secondary prevention (222/726 patients, 30.6%, 95% CI 27.3–34.0%). Conclusions: This study carried out in acutely hospitalized older people shows a high degree of inappropriate prescription among patients prescribed with antiplatelets for primary prevention, mainly due to overprescription. Further, a large proportion of patients who had had overt cardio- or cerebrovascular disease were underprescribed, in spite of the established benefits of antiplatelet drugs in the context of secondary prevention

Current practice on the use of prophylactic drain after gastrectomy in Italy: the Abdominal Drain in Gastrectomy ({ADiGe}) survey

Evidence against the use of prophylactic drain after gastrectomy are increasing and ERAS guidelines suggest the benefit of drain avoidance. Nevertheless, it is unclear whether this practice is still widespread. We conducted a survey among Italian surgeons through the Italian Gastric Cancer Research Group and the Polispecialistic Society of Young Surgeons, aiming to understand the current use of prophylactic drain. A 28-item questionnaire-based survey was developed to analyze the current practice and the individual opinion about the use of prophylactic drain after gastrectomy. Groups based on age, experience and unit volume were separately analyzed. Response of 104 surgeons from 73 surgical units were collected. A standardized ERAS protocol for gastrectomy was applied by 42% of the respondents. Most of the surgeons, regardless of age, experience, or unit volume, declared to routinely place one or more drain after gastrectomy. Only 2 (1.9%) and 7 surgeons (6.7%) belonging to high volume units, do not routinely place drains after total and subtotal gastrectomy, respectively. More than 60% of the participants remove the drain on postoperative day 4-6 after performing an assessment of the anastomosis integrity. Interestingly, less than half of the surgeons believe that drain is the main tool for leak management, and this percentage further drops among younger surgeons. On the other hand, drain's role seems to be more defined for duodenal stump leak treatment, with almost 50% of the surgeons recognizing its importance. Routine use of prophylactic drain after gastrectomy is still a widespread practice even if younger surgeons are more persuaded that it could not be advantageous