Contribution à l'amélioration de l'interprétation des techniques de diagnostic des transformateurs de puissance par spectroscopie diélectrique

Les transformateurs de puissance sont les équipements les plus importants et les plus coûteux utilisés dans le transport et la distribution de l'énergie électrique. Malgré de grands progrès dans la conception des équipements de puissance ces dernières années, le maillon faible de la chaîne reste encore le système d'isolation. Lorsque les transformateurs de puissance tombent en panne, la défaillance est généralement due à un défaut d'isolement. Les défaillances imprévues provoquent d'importantes perturbations des systèmes d'exploitation. Ce qui entraîne des pannes subites et des problèmes de livraison d'énergie. Une fiabilité extrême est exigée de la distribution dans la mesure où en cas de panne, ils conduisent inévitablement à des coûts de réparation élevés, les temps d'arrêt à long et éventuels risques de sécurité du personnel. De plus, les aspects environnementaux tels que les dommages indirects, les incendies et la pollution sont à haut risques. Les besoins croissants en outils appropriés pour le diagnostique de l'isolation des systèmes de puissance, de manière non destructive et fiable, ont conduit au développement d'outils de diagnostic tels que la mesure dans le domaine des fréquences (FDS), au cours de ces dernières années. Cette technique s'est révélée être sensible à l'humidité et l'isolation du vieillissement, c'est-à-dire qu'avec cette technique, il est difficile de prédire l'état réel de l'isolation solide d'un transformateur de puissance et partant d'estimer sa durée de vie résiduelle. Le présent projet de recherche vise à améliorer cette situation. À ce titre, l'objectif principal visé est la séparation de l'effet de l'humidité et du vieillissement sur l'isolation solide des transformateurs de puissance à partir des mesures diélectriques réalisés dans le domaine fréquentiel. À cet objectif principal, a été ajouté un objectif secondaire relatif à la transformation des mesures effectuées dans le domaine temporel en fréquentiel. Le but ultime est de réduire considérablement le temps des tests effectués par la technique FDS. Pour mener à bien cette étude, nous avons essayé de reproduire les conditions d'utilisation du papier dans les transformateurs en effectuant des vieillissements accélérés sur des échantillons de papiers imprégnés d'huile dans un four à convection à 115°C pendant des durées de 250, 500, 750 et 1000 heures. L'analyse des résultats, a permis de constater qu'il est possible de trouver une corrélation entre le degré de polymérisation (DP) qui indique l'état de dégradation réel du papier et les mesures du facteur de dissipation (tanô) indiquant l'état global de l'isolation des transformateurs de puissance. En effet, l'analyse des résultats a permis de montrer qu'il est possible de séparer l'effet du vieillissement et de l'humidité de l'isolation solide pour des teneurs en humidité inférieurs ou égales à 2%, en analysant les valeurs du facteur de dissipation aux basses fréquences de l mHz à 0,l mHz. Afin de faciliter et surtout d'automatiser l'analyse des données relatives aux mesures effectuées dans le domaine fréquentiel par le IDA 200, nous avons utilisé les réseaux de neurones en tant qu'outil d'aide à la prise de décision qui permettra d'obtenir rapidement les résultats concernant l'état de l'isolation solide (papier). Ainsi, après le prétraitement réalisé par l'analyse statistique, puis l'apprentissage et la validation du traitement d'un réseau de neurones, nous avons pu obtenir en sortie les informations concernant le classement des données des mesures en fonction de l'humidité et du vieillissement/Cette analyse, réalisée par l'utilisation des réseaux de neurones a également permis d'obtenir la séparation de l'humidité et du vieillissement pour des taux d'humidité inférieurs à 2%. Concernant le deuxième objectif, nous avons montré qu'il est possible de réduire le temps des mesures effectuées dans le domaine fréquentiel, en effectuant les mesures dans le domaine temporel par la mesure des courants de polarisation et de dépolarisation (PDC) convertis dans le domaine fréquentiel en utilisant la méthode de la décomposition en somme de fonctions exponentielles. Par cette technique, nous avons réduit le temps des tests de 15 heures à environ 2 heures 30 minutes

Modeling the insulation paper drying process from thermogravimetric analyses

It is now well-established that moisture in the oil paper insulation used in power and instrument transformers significantly reduces the transformers’ lifetimes, and can eventually lead to premature failure. This moisture should, therefore, always be removed, not only during production but also after repairs. At the final stage of manufacturing, the drying process should be carried out to remove water and air vacuoles contained in the cellulose-based paper before impregnation. Successful drying helps increase the residual life of transformers, because the presence of moisture and air vacuoles accelerates the aging/degradation process of the oil paper insulation. Proper estimation of residual moisture before impregnation and the determination of the time required for drying play key roles in the time-consuming process of drying. In this paper, the disadvantages of inadequate drying are addressed, followed by a mathematical approach to model the paper drying process. A mathematical model describing the kinetics of drying according to temperature, initial moisture, paper weight, final moisture, and extraction rate is proposed. This model also estimated the amount of moisture removed at the end of the drying process

Lessons to learn from post-installation pollution levels assessment of some distribution insulators

Among the main causes of outdoor insulation failures is their poor specifications in terms of leakage distances. This happens when the selected criteria are unable to cope with all the stresses imposed by the changes in environmental pollutions. Therefore, it is important for utilities to fully understand the actual pollution characteristics of the service environment in which the insulators are operating. In this paper, the pollution severity and performance of some 13.2 kV ceramic insulators, sampled in different areas of a Canadian aluminum factory, are assessed. The investigations were performed taking into account the influence of air humidity. Various characteristics were investigated to assess the pollution levels of the insulators, such as equivalent salt deposit density (ESDD) and non-soluble deposit density (NSDD), surface resistance, and leakage current characteristics (density, 3rd harmonic amplitude, and phase). It was witnessed that the insulators, collected around the factory, were much more polluted in comparison to the initial expectation. The pollution level should not be considered static due to the environmental parameters’ dynamics. Lessons to learn: the reliability of an electrical grid is dependent on components whose own reliability is strongly affected by external factors, of which there is often a poor awareness. If care is not taken to re-evaluate the post-installation pollution levels of the insulators, the light may simply turn out

Influence of cellulose paper on gassing tendency of transformer oil under electrical discharge

The intent of this work is to evaluate the influence of cellulose insulation paper on gassing tendency of transformer oil under electrical discharge conditions. For comprehensive enumeration, insulation oil was thermally stressed at 115°C as per ASTM D 1934 with a controlled thermal aging history. The thermally aged oils were considered for gassing tendency evaluation as per ASTM D 6180 (5 hours, 10 kV AC) in the presence of cellulose. Different cellulose Kraft paper proportions including 0, 10, 20, and 30% of aged oil were introduced in to discharge cell. Different cellulose proportions and aging conditions were considered to understand the impact of cellulose while understanding the decomposition aspects of insulation. Diagnostic measurements with those including dissolved gas analysis, moisture, interfacial tension, turbidity, dissolved decay contents, acidity, and dissipation factor were performed before and after gassing tendency evaluation. It was found that, cellulose paper aids significantly in increasing dissolved gases and moisture in oil during electrical discharge. Importantly, diagnostic measurements have shown the ability of paper to absorb certain decomposition products during the discharge process

Neural network approach to separate aging and moisture from the dielectric response of oil impregnated paper insulation

This paper presents a study of the impact of two important parameters, moisture and aging of the oil/paper dielectric used as insulation in power transformers.The way in which these two parameters influence different parameters of the Frequency Domain Spectroscopy (FDS) measurements, is emphasized.Different FDS parameters were measured by varying the moisturecontent and the aging degree of the oil impregnated paper.The use of two types of neural networks for analysis of the results was necessary in order to help discriminating the impact of moisture and aging on the FDS measurements and, in some cases, to estimate the aging duration of the paper impregnated with oil

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

Comparative study between conventional and hybrid solid insulation systems

A conventional mineral oil/cellulose insulated transformer's functional life is limited by the life of the cellulose paper. Since cellulose paper aging is accelerated by temperature, moisture and oxygen, poly-aramid-based synthetic insulation paper, with better thermal stability, has emerged for use in transformers. Two ASTM tests methods: DPV 'degree of polymerization' as a direct diagnostic tool according to ASTM D4243 and DGA 'Dissolved Gas Analysis' as an indirect diagnostic tool to quantify carbon oxides (CO and CO2) according to ASTM D3612 were used to monitor the thermal aging of the cellulose paper within solid hybrid insulation aged with mineral oil. It was then submitted to local overheating, with a comparison to cellulose paper aged with mineral oil to provide a benchmark for evaluation. The results indicate that the cellulose paper within solid hybrid insulation is slightly less degraded compared to the cellulose paper of conventional insulation

Monitoring power transformers oils deterioration using structured laser illumination planar imaging

Reliable quality assessments of oils in power transformers are important as they provide valuable information regarding the proper functioning of transformers. Thus, an early and accurate diagnostic of power transformers oils can prevent potential failures of transformers. In this paper, an imaging technique known as Structured Laser Illumination Planar Imaging (SLIPI) was used to monitor the extinction coefficient in various oil samples. The proposed technique offers the advantage of extracting the light intensity contribution from singly scattered photons and rejecting most of the light intensity from photons that have been scattered many times. This leads to more accurate and reliable measurement of the extinction coefficient , in optically dense oil samples. The variation of the extinction coefficient was therefore determined as a function of oil aging. The results demonstrate that SLIPI is reliable as a practical measurement method for the diagnosis of power transformer oils and present an attractive solution, alternative to the conventional methods such as Dissolved Decay Products, Interfacial Tension and Turbidity