In a long 130 years of its journey in service to mankind, the power transformer has grown in many aspects; ratings, performance, technology, aesthetics and eco-friendliness. Today, the transformer industry is witnessing the era of shifting from petroleum based mineral oils that are depleting, environmentally unfavorable, and proven dangerous to natural esters (vegetable oil based) that are renewable, environmentally friendly, and non-hazardous. The market and regulatory pressures to reduce liability risk exposure of mineral-oil-filled distribution and power transformers are increasing. In addition, there are demands to improve equipment efficiency and adopt more "earth-friendly" options in our power systems. In the last two decades there has been a resurgence of usage of natural ester insulating liquids for their "GREEN" and Safety credentials. These renewable resources have good dielectric properties and are compatible for use in power transformers. This paper evaluates and relates the new natural ester oil with existing mineral oil technology in terms of environmental concern, fire safety, physio-chemical properties, thermal performance, and dielectric performance. It also covers key modifications in design, manufacturing and operation to help assure excellent functionality and long life. In brief, this paper provides transformer designers and users about the advantages and challenges of "Green Insulating Liquid". Due to the voluminous properties that are compared and discussed, this paper is presented in two parts: Part 1 discusses on key properties and standards for the two liquids. Part 2 emphasizes on additional key properties and material compatibility related to electrical design. Based on evidence discussed in both parts, it is concluded that natural esters are acceptable for both new transformers and for retro filling existing units.

A review of critical evaluation of natural ester vis-a-vis mineral oil insulating liquid for use in transformers: Part II

As a non-renewable resource, mineral oil has been re-evaluated for its value and suitability as the coolant and insulator in transformers because of its environmental unfriendliness, fire hazards, and operating efficiency dissatisfaction. As a result, alternative dielectric liquids meeting the demands for more reliable, safer, and cleaner energy has been a critical need. Plant-based insulating fluids, non-toxic to the environment and ultimately biodegradable, have been flowing into the mainstream for achieving cost efficiencies, performance advantages, and enhancing safety. With more than 20 years of field experience, plant-based oils are now employed in more than 600,000 transformers worldwide, and have been established an enviable performance track record. Far from being just a mineral oil replacement, but more significantly, plant-based oils have filled a gap in some specific application scenarios where mineral oils fail to satisfy the fire-safety and environmental standards. This article reviews the research status of plant-based insulating fluids for transformer, including both challenges and outlook circa 2020. Plant-based oils have many inherent advantages over mineral oil, but also have numerous and unique challenges.

A critical review of plant-based insulating fluids for transformer: 30-year development

Several methods have been proposed for dissolved gas analysis (DGA) of mineral oil in power transformers. One of the simple and most widely used methods is Duval triangle. However, Duval triangle does not consider the concentrations of two combustible gases, namely ethane (C2H6) and hydrogen (H2). As a result, Duval triangle exhibits a lower accuracy in diagnosing certain fault types, for which these gases are the key gases, such as low overheating and corona discharge. Accordingly, this paper proposes a novel graphical technique for DGA based on all the five combustible gases. The proposed graphical technique is developed in the form of a pentagon shape. The pentagon heads represents the percentage concentration of each individual gas to the total combustible gases. The corresponding point for a certain faulty case is determined by the center of mass of all pentagon heads. The knowledge extracted from previous DGA methods and field experiences are used to estimate the preliminary fault regions within the pentagon. The exact boundaries between fault regions are then specified using actual DGA data with corresponding fault types collected from the Egyptian Electricity Network and published cases. The overall performance of the proposed pentagon has been evaluated using a set of fault cases and it is revealed that the proposed pentagon has higher diagnostic accuracy compared to other methods including Duval triangles and IEC standard 60599.

Development of a new graphical technique for dissolved gas analysis in power transformers based on the five combustible gases

In this study, three different insulating liquids, natural ester, mineral oil which is currently used by the Public Power Corporation of Greece and a nanofluid of surface coated Fe3O4 nanoparticles into a natural ester matrix, were subjected to AC voltage stress and their statistical breakdown voltage was measured and compared. Three different electrode configurations were used during the measurements in order to study the effect of non-uniform fields on the breakdown voltage distribution. Four different statistical distributions were used for the statistical processing of the experimental results. The two of them were the frequently used normal and Weibull distributions; in addition, Gumbel and generalised extreme value (GEV) function distributions were studied. It was shown that, in most cases, the experimental data followed GEV and Weibull functions rather than normal or Gumbel. Therefore, GEV statistics fits indiscriminately better to the experimental results concerning breakdown voltage, for the three different electrodes configuration and the three understudied insulating oils. The experimental results indicated that nanofluid has better performance, in terms of dielectric breakdown voltage, compared with natural ester oil and mineral oil, hence nanofluid is considered as a potential substitute of the conventional dielectric liquids, with enhanced properties and prospects.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/iet-smt.2016.0031

Statistical investigation of AC breakdown voltage of nanofluids compared with mineral and natural ester oil

Transformer mineral oil has been replaced by alternative oils such as synthetic oil and natural esters due to their biodegradability and environmentally friendly nature. This paper presents performance of coconut oil as such an alternative. Generally, as the other alternative oils, coconut oil has high conductivity due to the presence of free fatty acids. In this study, three different types of coconut oil samples consisting of virgin, copra and RBD (refined, bleached and deodorized) were initially tested by frequency dielectric spectroscopy (FDS) measurements to see how the conductivity was improved by dehydration and neutralization. The FDS results were fitted by inverse power dependence and Cole-Cole models to estimate the conductivity and response functions. Afterwards, a set of new coconut oil samples extracted from copra were thermally aged at 120°C under sealed conditions and compared with that of mineral oil. The performances of oil samples were evaluated using the test results of breakdown voltage, acidity, interfacial tension and FDS measurements under different aging periods such as 2, 5 and 7 weeks. Another 4 sets of new coconut and mineral oil samples were subjected to simulated thermal faults and electrical faults which include aging for 12 hours at 160°C, one hour at 200°C, exposing to partial discharges for four hours and subjecting to 20 low energy breakdowns. The performance comparisons were done by FDS measurements and dissolved gas analysis. In parallel, field-aged coconut oil samples collected from a sealed distribution transformer with 11 years of service were also tested by FDS measurements. In general, coconut oil shows its suitability as an alternative to the mineral oils for transformers, despite limitations found in some of their physical properties. It was found that the FDS results were in good agreement with chemical test results and with the estimated conductivity values.

Performance of coconut oil as an alternative transformer liquid insulation

This paper reports on a comparative study between mineral and various ester oils based on Dissolved Gas Analysis (DGA). Electrical (breakdown of low energy) and thermal (stray gassing) faults were realized at laboratory scale, then gas formation as well as diagnosis methods were compared. Two extraction methods, described in IEC 60567 Ed.3 and independent from partition/solubility coefficients were used: multi-cycle vacuum degassing and stripping. Three different diagnosis methods were applied: IEC gas ratios, Duval's triangle and key gases. For electrical faults and especially for low energy discharges, the same main gases (Hydrogen and Acetylene) are created in relatively same ratios, thus confirming the efficiency of key gases method for ester oils. It is also shown that classic Duval's triangle (triangle 1) is well applicable for this type of fault confirming the results reported in literature. For thermal faults and especially for stray gassing issue (low thermal faults), Ethane (associated with Hydrogen) constitutes the key gas of natural esters. The new Duval?s triangles can be applied: Triangle 4 for mineral oil and Triangle 6 for ester oils. For extraction methods and in both electrical and thermal faults cases, it can be noted that results are comparable for mineral oil whereas they are slightly underestimated with stripping method for ester oils. The higher viscosity of ester could interfere with the extraction.

DGA comparison between ester and mineral oils

This paper is aimed at the study of aging behavior of cellulosic materials in different dielectric liquids under air and nitrogen atmospheres. Paper for copper conductor and high density transformer board were aged at 130°C up to 1128 h in sealed vessels in presence of mineral oil (MO), natural ester (VO) and synthetic ester (SE). The influence of thermal aging on cellulose and oil characteristics are investigated and compared. It's observed that the paper degradation is associated with the generation of CO2 and CO gases. Whatever the type of oil, this generation is more important with vessel under air atmosphere than that under nitrogen gap indicating a protective effect of nitrogen. In mineral oil, the degree of polymerization (DPv) reaches the end of life criteria at around 200 whereas it remains around 400 with ester oils. This is mainly due to the higher water solubility of esters compared to mineral oil. The measured degree of polymerization in MO is comparable to that calculated through 2-FAL (Furanic compounds) content using Vuarchex equation whereas for esters oil, there is no agreement between the measured and calculated DPv through 2-FAL content. This can constitute a limitation in the use of 2-FAL for the diagnosis of ester-filled transformers. Overall, the insulation cellulose/oil behavior seems better under nitrogen atmosphere.

Aging behavior of cellulosic materials in presence of mineral oil and ester liquids under various conditions

This paper reports on a comparative study between mineral and ester oils basing on DGA (Dissolved Gas Analysis). Electrical (breakdown of low energy) and thermal (stray gassing) faults are realized at laboratory scale and gas formation as well as diagnosis methods are compared. Two extraction methods described in IEC 60567 were used: total vacuum degassing and stripping method. Three different diagnosis methods were used: IEC 60599 gas ratios, Duval's triangle and key gases. It appears that for electrical faults and especially for low energy discharges, the same gases (Hydrogen and Acetylene), in relatively same ratios, are created. It is shown that classic Duval's triangle is well applicable for this type of fault confirming thus the results reported in literature. For thermal faults and especially for stray gassing issue (low thermal faults), Ethane (associated with Hydrogen) constitutes the key gas of natural esters and the new Duval's triangle can be applied for mineral oil whereas some arrangements need to be introduced for ester oils.

The purpose of this paper is to simulate an accelerated ageing of paper and high density pressboard type B.3.1 in three different liquids (mineral oil MO, natural ester VO and synthetic ester SE) for different periods of ageing at 130°C. The materials were aged in sealed vessels under two different atmospheres: air and nitrogen gaps. The effects of aging on physicochemical properties of oil and cellulose are analyzed and compared. It is shown that ageing rate of pressboard and paper are comparable in each type of oil. Nevertheless, it can be underlined that cellulosic materials are less affected after aging in ester oils evidencing a higher degree of polymerization (DPv). The use of 2-FAL in ester oils remains questionable. The reduction of oxygen content in mineral oil minimizes ageing of oil more than solid insulating materials.

M. Marugan

Papers

DGA comparison between ester and mineral oils

Aging behavior of cellulosic materials in presence of mineral oil and ester liquids under various conditions

DGA comparison between ester and mineral oils

Thermal aging of paper and pressboard in mineral and ester oils under air and nitrogen atmospheres