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With growing environmental concerns, biodegradable insulating fluids are being investigated as alternatives to petroleum-based mineral insulating oil in power transformers. In order to optimize their use, decomposition studies of these alternative fluids are therefore of ultimate importance. This paper investigates the thermal decomposition kinetics of mineral oil and two vegetable-based ester fluids, using the thermogravimetric analyses. The initial and maximal decomposition temperatures are determined using thermal analyses. In addition, the Arrhenius activation energy is calculated for the fluids under study using the Ozawa-Flynn-Wall iso-conversional method. In addition, TGA studies are extended to mineral and natural ester impregnated paper, with the aim of assessing their impact on the thermal decomposition of insulation paper. It is found that the level of mono-unsaturated acids in esters is important for enhancing the thermal stability. The enhanced thermal stability of the esters has advantageously contributed to the improvement of the thermal performance of the insulation paper., IEEE Conference on Electrical Insulation and Dielectric Phenomena (IEEE CEIDP 2021), December 12-15, 2021, Vancouver, Canada

This paper investigates the residue formed on silicone rubber filled with natural silica in the inclined plane tracking and erosion test under AC and DC voltages. Simultaneous thermogravimetric and differential thermal analyses reveal residue formation dependent on volatilization and crosslinking in silicone rubber. Thermo-oxidation of volatiles yields silica-based residue protecting the surface during dry-band arcing; whereas, crosslinking leaves carbonaceous residue leading to more severe dry-band arcing. Volume fraction of natural silica replacing silicone rubber is shown to be a main source of silica residue suppressing carbonaceous residue, thus preventing failure in the inclined plane test. More severe erosion under the equivalent +DC voltages as compared to standard AC and equivalent - DC voltages of the inclined plane test is attributed to more severe dry-band arcing heat promoted by carbonaceous residue. Silicone rubber filled with lower particle sizes of natural silica is demonstrated to deteriorate erosion performance, possibly as result of filler-polymer interactions or lower particle size reducing the integrity of the protective silica residue. The following paper highlights the importance of using critical AC and DC voltages during the inclined plane tracking and erosion tests.

The presented paper aims to highlight the role of filler-polymer interface on the DC erosion resistance of silicone rubber (SiR) outdoor insulators. SiR composites filled with nano-sized boron nitride and fumed silica have been prepared in this study. Electrical and thermal experimental tools of the inclined plane tracking-erosion test, simultaneous thermogravimetric- differential thermal analyses, and thermal conductivity measurements are used to analyze the prepared nanocomposites. The analysis primarily investigates whether the composite’s enhancement of thermal conductivity or filler interface would be more influential on erosion suppression. Literature reports that the boron nitride filler improves SiR’s thermal conductivity, whereas fumed silica provides an exceptional interaction with SiR at the filler interface. SiR was filled with boron nitride at 5wt% (percent by weight) and 10wt%, while fumed silica was loaded with 5wt% and used as a reference composite in the study. The obtained results clearly indicate a superior erosion resistance of the silica filled SiR over the boron nitride filled one. An observation which seemingly could give more significant role of the filler-polymer interface on the erosion resistance of SiR used for high voltage DC outdoor insulators., IEEE 2020 International Conference on Dielectrics, 6-31 July, Virtual Edition, Valencia, Spain

This paper investigates the erosion suppression effects of the water of hydration in alumina tri-hydrate and magnesium hydroxide fillers in silicone rubber during the AC and DC dry-bad arcing. Simultaneous thermogravimetric and differential thermal analyses show the hydrated filler type, level and size affecting erosion. Water of hydration in alumina tri-hydrate is shown to be released before the depolymerization of silicone rubber; whereas, the water of hydration in magnesium hydroxide is released during the depolymerization. As such, alumina tri-hydrate could promote a stronger shield of residue against dry-band arcing as compared to magnesium hydroxide, which could further suppress erosion. Increasing the hydrated filler loading raises the amount of the water of hydration released, which could dilute the silicone rubber in the gas phase and suppress combustion. However, insignificant effect on the suppression of depolymerization is obtained at higher levels of the hydrated filler, possibly due to the lack of filler-polymer interactions tethering silicone rubber chains. Internal oxidation promoted with lower sizes of alumina tri-hydrate could promote a radical-based crosslinking, leaving a stronger shield of residue against dry-band arcing. However, hydrolysis may take place and insignificant improvement is shown for the erosion performance with internal oxidation.

Moisture in transformer paper insulation is one of the most important parameters to be monitored in order to assess the useful life of the transformer. Time domain spectroscopy based on polarization and depolarization current measurements is one of the methods that can be used for detection of moisture content in the transformer paper insulation. However, these measurements could be influenced by thermal transient during the application of voltage, which may lead to an inaccurate interpretation of the insulation condition. In this paper, a novel method is proposed in order to avoid this thermal transient effect by using the high frequency component of the of depolarization current acquired during the measurement. Preliminary results show a correlation between the high frequency component of the depolarization current waveform and the moisture content in the paper insulation within a moisture content range of about 3 %. At higher moisture content, a resistive leakage current seems to have a negative effect on the measured depolarization current., 2019 IEEE 20th International Conference on Dielectric Liquids (ICDL), June 23-27, 2019, Roma, Italy

The Effect of the type, size and level of organoclay nanoparticles on the dielectric properties of polypropylene nanocomposites is investigated at different temperatures. Two types of nanoclay, synthetic and natural, are added to polypropylene at different levels. The incorporation of nanoclay into polypropylene is shown to increase the dielectric losses. Unlike the unfilled polypropylene, both real and imaginary part of the permittivity increases at low frequencies with the increase in the clay level. Such a finding can be related to the effect of the interface between the nanoclays and the host PP matrix. The effect of compatibilizer added, i.e maleic anhydride, on the losses at very low frequencies is more recognizable at 80 °C. This could be due the conductive nature of the maleic anhydride at higher temperatures. Lower losses were measured for the polypropylene filled with 2 wt% synthetic nanoclay as compared to polypropylene filled with 2 wt% of natural nano clay. This finding can be attributed to the effect of the size (aspect ratio) of the platelets., IEEE Conference on Electrical Insulation and Dielectric Phenomena, October 22-25, 2017, Worth, TX, USA

This paper discusses the lessons learned about fillers that are added to silicone rubber insulation housing composites, in order to improve the erosion resistance against the dry-band arcing are presented and common perceptions. Common practices that employ alumina tri-hydrate are reviewed, including the additional influential effect of the water of hydration in the suppression of the dry-band arcing. The effect of the water of hydration is shown to be dependent on various factors, such as the hydrated filler level and the type of the hydrated filler. More recent paradigms in which hydrated fillers have not been employed are also reviewed. Volume and shield actions of fillers are essential aspects that need to be understood in the design of silicone rubber insulation housing composites for new applications such as HVDC. In addition, the thermal degradation mechanisms of silicone rubber and the corresponding suppression effects of the added fillers are summarized.

Transformer oils are vulnerable to moisture, which can have adverse effects including a dramatic reduction in dielectric strength, accelerating the aging of the solid insulation (cellulose depolymerization) and the formation of decompositional gasses at high temperatures. Moisture may be generated inside the transformer due to degradation of the oil-paper insulation or there may be ingress of moisture due to free-breathing arrangements. A moisture content exceeding 50% of oil saturation greatly reduces the breakdown voltage, resulting in catastrophic failures as well as the potential for transformer fires [1]. The detrimental effect of water within a transformer is not only limited to the breakdown voltage of the oil. The rate of degradation of paper insulation increases in direct proportion to the water content [2], [3]. Moisture also has a decisive effect on the maximum temperature that the transformer can run at before moisture-induced failure occurs. Consequently, transformer life expectancy can be calculated from water content and temperature [3]. A failure of transformer accessories such as condenser bushings sometimes leads to a transformer failure and long-term outage. Often, moisture is a cause of explosion for service-aged bushings [4].

The following topics are dealt with: electric breakdown; permittivity; filled polymers; partial discharges; space charge; transformer oil; ageing; power cable insulation; nanocomposites; power transformer insulation., 2018 IEEE Conference on Electrical Insulation and Dielectric Phenomena (CEIDP), October 21-24, 2018, Cancun, Mexico

This paper investigates the erosion of silicone rubber composites filled with various fillers. The fillers under study are alumina tri-hydrate, magnesium hydroxide and silica in a liquid silicone ru bber. The inclined plane tracking and erosion test was used to compare the erosion of the composite materials at 50 wt% filler level and at 4.5 kV. Simultaneous thermogravimetric and differential thermal analyses are reported which shows various mechanisms by which the fillers can su ppress the dry­ band arcing erosion. The effect of water of hydration in the alumina tri-hyd rate and magnesium hydroxide is investigated and its effect on the suppression of the dry-band arcing erosion is reported for the composites filled with alumina tri-hydrate and magnesiu m hydroxide., 2017 IEEE Conference on Electrical Insulation and Dielectric Phenomenon (CEIDP), 22-25 October 2017, Fort Worth, TX, USA

Oil is a vital part of the transformer body and (similarly to blood in a human being body) keeps responsibility for the condition of the entire organism. Oil is particularly responsible for functional serviceability of the entire insulation system. The insulating oil must be kept in pristine condition, since its condition can be a decisive factor, which determines the life span of the transformer. Fields and laboratory experiences have shown that transformer oil contains a vast amount of information. Oil analyses can be extremely useful in monitoring the condition of power transformers. To meet pressing needs of power industries, fast, inexpensive and reliable laboratory testing procedures are necessary. To ensure long-term reliability of oil filled power transformers, it is important to identify early sign of degradation of the insulating oil. In this paper, oil degradation was monitored with various ASTM test methods. Investigations were performed on service-aged oil samples as well as on oil samples aged in laboratory conditions. Many key parameters actually used to monitor the condition of transformer oil relative to oxidation/degradation were investigated. From the obtained results, correlations were found between some of them. The results indicate that Dissolved Decay Products (DDP) and turbidity, which change with a higher rate than interfacial tension (IFT) and Acid Number (AN) values, can be possibly used as an effective index for insulating oil degradation assessment. Limits are suggested which provide a “picture” of the fluid condition.

International audience; A power transformer outage has a dramatic financial consequence not only for electric power systems utilities but also for interconnected customers. The service reliability of this important asset largely depends upon the condition of the oil-paper insulation. Therefore, by keeping the qualities of oil-paper insulation system in pristine condition, the maintenance planners can reduce the decline rate of internal faults. Accurate diagnostic methods for analyzing the condition of transformers are therefore essential. Currently, there are various electrical and physicochemical diagnostic techniques available for insulation condition monitoring of power transformers. This paper is aimed at the description, analysis and interpretation of modern physicochemical diagnostics techniques for assessing insulation condition in aged transformers. Since fields and laboratory experiences have shown that transformer oil contains about 70% of diagnostic information, the physicochemical analyses of oil samples can therefore be extremely useful in monitoring the condition of power transformers.

Aging of a power transformer&rsquo, s insulation system produces carboxylic acids. These acids&mdash, acetic, formic and levulinic&mdash, are absorbed by the paper insulating material, thus accelerating the degradation of the whole insulation system. In this contribution, the effect of these acids on the aging of oil-impregnated paper insulation used in power transformer is reported. A laboratory aging experiment considering different concentrations of these three acids was performed to assess their effect on the insulation system&rsquo, s degradation. Each acid was individually mixed with virgin oil, and a mixture of acids was also blended with oil. The paper&rsquo, s degradation was assessed by the degree of polymerization (DPv). It was found that the DPv of paper aged with formic acid decreased much faster in comparison to the other acids.

This article presents the results of an industrial experiment, which demonstrate the possibility of arresting or reducing the oxidative decay of oil-paper insulation in freely breathing power transformers by using a new type of nitrogen blanketing system.

The most easily accessible and efficient way to determine the transformer condition is to use the insulating oil as diagnostic medium. Practical experiences have shown that more than 70% of incipient failures can be detected through oil tests. To meet pressing needs of power industries, fast, inexpensive and reliable laboratory testing procedures are necessary. In this contribution, oil samples aged in laboratory condition were monitored with traditional diagnostic methods - i.e., interfacial tension (IFT) and total acid number (TAN) - and alternative techniques based on spectral analysis, using Fourier Transform Infrared (FTIR) and Dissolved decay products (DDP) assessed by spectrophotometer. From the obtained results, the sensitivity and reliability of each method in monitoring step-by-step the decay products formation are analyzed and discussed. It is shown that ratio of DDP to FTIR transmittance measurements at 1710 cm-1 might be possibly be used as index for assessing the degradation of transformer oil.

Frequency Domain Spectroscopy (FDS) is an effective tool allowing assessing the condition of oil-paper insulation system in power equipment. However, results from these measurements are known to be greatly influenced by various parameters, including insulation aging, moisture content, and insulation geometry/volume, together with environmental condition such as temperature. In this contribution, a series of experiments have been performed under controlled laboratory conditions. The dielectric response of the oil impregnated paper, along with the degree of polymerization and moisture content, were monitored. Since dielectric parameters are geometry dependent, poles (independent of the geometry) which depends on resistivity and permittivity, were considered to assess the condition of the insulation. From the investigations performed on new and aged samples, it is shown that poles (P) can be regarded as insulation aging indicator. It is also shown that a per unit value based on the Dielectric Dissipation Factor (DDF), measured in the frequency range from 1 to 1000 Hz can be correlated to moisture content in the insulation paper.

In this contribution, the dielectric dissipation factor of oil impregnated paper were monitored along with the degree of polymerization. A series of experiments have been performed under controlled laboratory conditions. Since the dielectric parameters values are geometry dependent, poles which are independent of the geometry, calculated from resistances and capacitances, were considered to evaluate the condition of oil impregnated paper insulation. A correlation has been observed between the degree of polymerisation of the paper insulation and the poles values outputted from the frequency response. From the investigations performed on new and aged oil paper samples, it was shown that poles can be regarded as insulation aging indicator.

International audience; It is an established fact that regular oil analysis is extremely useful in monitoring the condition of power transformers. To ensure long-term reliability of oil filled power transformers, it is important to identify the degradation characteristics of insulating oil in long-term operations, which are the dominant factors of the transformer dielectric strength. In this contribution, oil decaying was monitored with various ASTM test methods. Oil samples were submitted to thermal aging with oxygen inlet in laboratory conditions. Many key parameters providing a "picture" of the fluid have been determined. From the obtained results, some correlations were found between some of them. Dissolved decay products, which change with a higher rate than interfacial tension and total acid values, can be possibly used as an effective index for insulating oil degradation.

L'indisponibilite d'un transformateur de puissance a generalement de lourdes consequences financieres pour les exploitants des reseaux electriques. Le bon fonctionnement des transformateurs de puissance repose en grande partie sur leur isolation electrique (huile/ papier) et sur le controle de leur temperature. Les isolants utilises dans la fabrication des transformateurs; qu'ils soient solides (papier, carton) ou liquides (huile), subissent avec le temps une alteration chimique sous l'influence de la chaleur et d'autres agents tels que l'oxygene et l'humidite. Le present memoire, entrepris dans le cadre des travaux de recherches de la chaire sur les isolants liquides et mixtes en electrotechnologie (ISOLIME) de l'universite du Quebec a Chicoutimi, a pour but d'etudier de nouveaux systemes de preservation d'huile. Il a egalement comme objectif d'explorer et de developper de nouvelles perspectives qui pourront resoudre les problemes d'analyse de vieillissement du papier. A l'interieur d'un transformateur, l'huile se dilate et se contracte en fonction de la temperature, ce changement de volume de l'huile se produit dans le conservateur d'huile. Concernant le circuit d'huile, on trouve deux grandes familles: les transformateurs dits hermetiques et les transformateurs dits respirants. Dans un transformateur hermetique, l'air du conservateur est emprisonne dans une membrane et comprime par l'huile en cas de dilatation. L'air et l'huile ne sont donc jamais en contact avec l'exterieur. Mais le probleme est que ce type de preservation ne tient pas compte de l'effet du champ electrique sur la decomposition de l'huile qui genere des gaz qui restent emprisonnes dans le conservateur et peuvent se dissoudre dans l'huile. Dans un transformateur respirant, le volume de l'air dans le conservateur est ajuste en fonction de l'expansion de l'huile au travers d'un dessiccateur. Dans ce type de transformateur, l'huile est exhibee au contact avec l'air ou l'impact de l'oxygene et de l'humidite degrade la qualite du systeme d'isolation. Pour palier aux inconvenients majeurs des systemes de preservations presentes precedemment, deux nouveaux systemes de preservation d'huile dans les transformateurs de puissance ont ete proposes et etudier dans ce memoire. Le premier utilise un systeme de preservation a gaz inerte (l'azote pur) et le second un nouveau systeme de preservation a diaphragme. L'analyse de l'huile pour ces deux systemes a ete effectuee avec differentes techniques de diagnostic et les resultats y sont discutes. Des essais in situ et en laboratoire ont ete realises. Les resultats obtenus ont montres que Pinertage par l'azote permet de reduire de facon significative le taux de l'oxygene dans l'huile. Ainsi que le nombre d'acidite (TAN), la turbidite et la surface en dessous des courbes d'absorption (DDP) varier tres peu avec le temps d'inertage avec l'azote. Les resultats pour le deuxieme systeme montrent que les produits dissous et la turbidite dans le conservateur varient legerement avec le temps dans le conservateur a diaphragme alors qu'elle est multipliee par un facteur 4 dans le cas du systeme sans diaphragme. Comme nous l'avons mentionne plus haut, la duree de vie d'un transformateur depend etroitement de son etat d'isolation, en particulier du papier cellulose qui le compose. L'evaluation de l'etat de l'isolation solide se fait generalement d'une facon indirecte via l'huile, par l'analyse des derives fiiraniques ou encore a travers des diagrammes d'equilibre pour estimer la teneur en eau dans le papier. Dans ce contexte, l'etude de l'utilisation d'un capteur capacitif avec comme isolant le meme papier cellulose qu'on utilise comme dielectrique solide dans le transformateur a ete realise. Cette etude se basera sur la mesure des parametres dielectriques : permittivite et facteur de dissipation (tan o). Les principaux elements du capteur ainsi que sa mise en oeuvre y sont decrits. Une procedure de vieillissement accelere de l'isolation papier/ huile incorporant le capteur est egalement decrit. L'etude des poles de la reponse frequentielle comme nouveau parametre d'evaluation de l'etat de l'isolation solide est aussi effectuee. Cette technique semble tout a fait prometteuse a basse frequences.