Quelques données préliminaires sur l'environnement et la qualité des apports du Niger au Sahel

La présente étude fait partie d'un programme global de recherche sur la désertification liée aux changements climatiques au Mali. Le paramètre climatique considéré ici est la température moyenne relevée sur 30 ans. On procède au suivi des décades les plus représentatives de la saison sèche et de la saison humide pour identifier une tendance quelconque. (...)Des résultats de l'analyse des températures moyennes il apparaît une tendance à la hausse pendant les 3 dernières décennies. (Résumé d'auteur

Modified Peek formula for calculating positive DC corona inception voltage on polluted insulator

In this contribution, a mathematical model is proposed to predict the discharge inception voltage on polluted insulators under a non-uniform field of a high-voltage DC system. The present study is derived from the modified Peek’s law used for predicting the threshold voltage of a typical corona discharge. An onset criterion useful for practical electrode geometries was experimentally validated by considering insulators’ level of pollution along with the electro-geometrical parameters including inter-electrode distance, HV electrode radius, and width of the ground electrode. It was observed that the proposed mathematical model is much precise for smaller electrode radii and inter-electrode distances with higher conductivity of the pollution layer. Since prediction of the electrical onset on a polluted surface is of general interest for designing HV equipment suitable for heavily polluted regions in the world, it is hoped that the results presented can be viewed as a benchmark and a challenge for further research

On discharge inception voltage for insulators under non-uniform field with AC voltage

In this paper, an analytical expression has been exploited for computing the discharge inception voltage of polluted insulators under a high voltage AC system. This expression was established for DC high voltage. The present contribution is inspired from the modified Peek's law used for calculating the inception voltage of a typical streamer discharge. An experimental investigation of the electric arc initiation on laboratory physical model using a photomultiplier has been also reported. It was observed that, the threshold voltage rises with increasing inter-electrodes gap and high voltage electrode radius. It decreases with increasing conductivity

Condition monitoring of in-service oil-filled transformers: Case studies and experience

Transformers are one of the most strategic components in balancing the voltage levels and hence a high priority is given to their performance [1]. It is established that, insulation technology plays a critical role in judging the performance and service life in oil filled apparatus [2]. Performance of the insulation system depends mainly on the deterioration behavior of insulation oil and paper. The mechanisms that are responsible for premature aging of oil/paper insulation are almost the same in all the oil filled apparatus. Yet, there will be a significant difference in the intensity of the aging mechanisms in different apparatus. This intensity is attributable to rating, design, and duration of operation for different machines. The detailed discussions on these mechanisms are presented in the subsequent sections of this paper. However, aging of service insulants is unavoidable and is to be maintained at a lower rate or arrested to the greatest possible extent, such that, catastrophic failures and unscheduled outages may be mitigated [3]. Normally, utilities follow scheduled condition monitoring activities to avoid the consequences of premature aging. Hence, knowledge on these in-service condition monitoring activities will be helpful in understanding the exact deterioration rate of the insulation system. Real time in-service experience of several transformer fleets that belong to United Kingdom utilities are reported in [4]. An early degradation of insulation is noticed through increase in acidity and furan concentration in oil for several transformers in the fleet. Authors investigated this early degradation in different perspectives including loading conditions, manufacturers, and oil chemistry changes. It is inferred that changes in oil chemistry is an important attribute for early degradation and hence utilities are advised to adopt different asset management strategies for affected and unaffected transformers in a fleet. Recently, failure rate data of service aged transformer fleets of an Australian utility were analyzed to establish the relationship between aging and different failure types [5]. It is noticed that degradation of the insulation system will be rapid after 20 years of service aging thus entailing the frequent condition monitoring activities after 20 years to identify premature aging failures