Decay Particles and Regeneration of Ester Dielectric Liquids A Challenge!

Ester dielectric fluids are no longer restricted to laboratory investigations and serve several transformers connected to the electric power network. Along with other technical aspects, it is also essential to understand the regeneration aspects of these alternative insulating liquids. Fuller’s earth is a popular and widely accepted adsorbent for regenerating mineral oils. The feasibility of using Fuller's earth for ester dielectric liquids is reported in this article

Gassing tendency of fresh and aged mineral oil and ester fluids under electrical and thermal fault conditions

Operational factors are known to affect the health of an in-service power transformer and to reduce the capabilities and readiness for energy transmission and distribution. Hence, it is important to understand the degradation rate and corresponding behavioral aspects of different insulating fluids under various fault conditions. In this article, the behavior of mineral oil and two environmentally friendly fluids (a synthetic and a natural ester) are reported under arcing, partial discharges, and thermal fault conditions. Arcing, partial discharges and thermal faults are simulated by 100 repeated breakdowns, top oil electrical discharge of 9 kV for five hours, and local hotspots respectively by using different laboratory-based setups. Some physicochemical properties along with the gassing tendency of fresh and aged insulating liquids are investigated after the different fault conditions. UV spectroscopy and turbidity measurements are used to report the degradation behavior and dissolved gas analysis is used to understand the gassing tendency. The changes in the degradation rate of oil under the influence of various faults and the corresponding dissolved gasses generated are analyzed. The fault gas generations are diagnosed by Duval’s triangle and pentagon methods for mineral and non-mineral oils. It is inferred that; the gassing tendency of the dielectric fluids evolve with respect to the degradation rate and is dependent on the intensity and type of fault

Influence of Ester Liquids on Dielectric Strength of Cellulose Kraft Paper

Generally, impregnation of solid insulation is performed to increase the dielectric strength and reduce the dielectric losses of solid insulants. This increase in dielectric strength depends on the oil’s diffusion and dielectric properties. This paper investigates the diffusion behavior of mineral oil and ester fluids (synthetic, natural, and mixed) to understand the influence of oil diffusion on paper breakdown voltage. To better understand this phenomenal influence, cellulose insulation paper of different thicknesses has been considered. Wetting characteristics of various oil-paper insulation systems were investigated with and without thermal stressing. Thermal aging was carried out as per modified ASTM D1934 at 110 °C, 140 °C, 160 °C, and 185 °C respectively for 2 weeks. The wetting characteristics and influence of different oils on paper breakdown voltage were explicitly reported. It is inferred that paper wetting characteristics are attributable to the type of oil, the thickness of paper, and the aging factor of oil-paper insulation. Importantly, the increase in paper dielectric strength and diffusion behavior for ester fluids is found to be superior to that of the mineral insulating oils

On some imperative IEEE Standards for usage of natural ester liquids in transformers

From the recent past, biodegradable dielectric fluids are becoming popular across the global utilities for use in oil-filled transformers. To date, several utilities have started using biodegradable insulating fluids for new and retro-filled power and distribution transformers. The intent of this article is to compare and analyze various aspects that are involved in the successful operation and maintenance of mineral oil and ester filled transformers. The comparative analysis in this article is aimed at properties, condition monitoring, diagnostic, and reclamation aspects of ester filled transformers. The imperative and pertinent standards, including IEEE Std C57.106, IEEE Std C57.104, IEEE Std 637, IEEE Std C57.147, and IEEE Std C57.155 remain the target objectives for potential analysis. It is hoped that this critical analysis will be useful for utility and condition monitoring engineers in understanding several key aspects of ester fluids vis-à-vis traditional insulating fluids. The present analysis might also be useful for researchers and industry interested in alternative dielectric fluids for transformer insulation technology

Mixed Insulating Liquids with Mineral Oil for High Voltage Transformer Applications: A Review

Due to the growing interest in environmental concerns, synthetic, and natural esters have become the key focus of the picture in the last two decades as insulating fluids for high voltage equipment. This is because, unlike mineral oil, ester liquids are biodegradable, non-toxic, and safe for the environment and human health. These fluids are derived from renewable sources and have high fire resistance. However, synthetic and natural ester fluids are still used in a rather limited number of electrical equipment for several reasons (high cost, high viscosity and density, increased tendency towards electrostatic charge, faster propagation of streamers in an inhomogeneous electric field, etc.). Besides, a huge amount of equipment is filled with mineral oils. The massive replacement of oil-filled equipment with natural or synthetic esters may be extremely expensive to utilities and transformer owners. In addition, in the event of partial or complete retrofilling of mineral oil with an ester fluid, the equipment may evidently contain a mixture of two insulating fluids; therefore, demanding the scope for research on mixed insulating liquids. The intent of this article is to present a comprehensive review of the literature on the blend of mineral oil and other alternative dielectric fluids. The critical research progresses, highlights, and challenges related to mixed liquids along with significant tutorial elements as well as some analyses are discussed. This review should be useful for researchers, utilities, and transformer owners concerned with ester liquids and retro filling aspects

Regeneration of transformer insulating fluids using membrane separation technology

Oxidation of oil/paper insulation initiates premature aging and introduces carboxylic acids with eventual increase in oil acidity, which hampers the properties of the oil. In this paper, a membrane separation technology-based purification process for aged insulation oil has been evaluated and reported. The intent of the present study is to eliminate carboxylic acids, dissolved decay contents and other colloidal contamination present in aged oil and enhance the useful life of oil. The potential of the membrane treatment process has been demonstrated using Ultraviolet Visible Infrared Spectroscopy and Fourier Transform Infrared Spectroscopy diagnostic measurements for oil and membrane. Additionally, membrane retention properties like membrane flux, retention coefficient, sorption time and membrane mass have been analyzed to understand the treatment process. To further evaluate the performance of the membrane and effectiveness of the treatment process, acidity, dielectric dissipation factor, relative permittivity, and resistivity measurements of the oil before and after filtration have been also reported. The proposed membrane purification method has been tested for Algerian utility in-service oil samples. It is inferred that, membrane filtration method is a simple and effective method for treatment of aged oils and aids in extending the remnant life of the oil. The procedure is economically attractive because of increasing prices for transformer liquids, cost effective and environmentally sounds

Mineral oil and ester based oil/paper insulation decaying assessment by FTIR measurements

Esters based dielectric fluids have been widely researched since 1990’s for application in high voltage insulation applications. Since then, researchers are affirmative towards usage of ester based insulating fluids as a replicated to mineral insulation oils. The operating properties and aging performance of ester oils proved to be potential candidates for high voltage applications. In view of high temperatures and longevity of insulation systems, there also is a need to understand the chemical perspectives along with aging behaviour of ester oils. Accelerated aging of oil/paper insulation associated with mineral oil and synthetic ester with cellulose insulant has been experimentally simulated as per ASTM D 1934 at 115 ℃. Fourier Transform Infrared spectroscopy analysis of oils and cellulose papers is carried out at different aging factors. The compositional changes in oils and cellulose kraft paper with aging have been enumerated. The changes in the absorbance area for appropriate functional groups have been also reported. It is found that, the chemical stability of synthetic esters is superior to that of the mineral oil

Temperature dependence of the pre-breakdown and breakdown phenomena in natural esters under AC stress

Natural ester dielectric fluids are being used as alternatives for mineral insulating oils in oil-filled power apparatuses. Although partial discharges (PDs) are recognised as one of the main causes of degradation of dielectrics used in power equipment, their characteristic voltages appear to be temperature-dependent. In this contribution, the pre-breakdown and breakdown behaviour of two natural ester fluids, extracted from canola oil (NE1) and soybean oil (NE2) is reported. To assess the effectiveness of the temperature dependence, −5, 0, and + 20°C have been identified as test temperatures. It was found that the different pre-breakdown and breakdown characteristic voltages increase with increasing temperature for both types of esters. The delay time is always longer for canola oil when compared to soybean oil regardless of the test temperature. The analysis of the average propagation velocity of NE1 and NE2 indicates that PDs propagate faster in NE2 than in NE1. The field strength at the tip of the needle electrode during the inception of the streamers leading to breakdown indicates that streamers in both oils are propagating in the slow mode. It is found that the temperature has a significant influence on the PDs’ parameters for both fluids

Prebreakdown and Breakdown Behaviour of Low Pour Point Dielectric Liquids Under Negative Lightning Impulse Voltage

In this article, some investigations on the prebreakdown and breakdown phenomena of low pour point insulating liquids under negative lightning impulse (LI) voltage are reported. The tested liquids include mineral oil (MO), a typical synthetic ester (TSE), and two low-pour point synthetic esters. These liquids underwent accelerated thermal aging. The nonaged and aged samples were subjected to LIs using a point-plane electrode arrangement. The discussions are focused on the initiation of partial discharges, propagation of streamers, and breakdown behavior in the nonaged and aged liquids. The investigated parameters include inception voltage, LI breakdown voltage, streamer acceleration voltage, and streamer velocity. The results are supported by the oscillographs of the light activity that is recorded during the discharge process. The prebreakdown phenomenon noticed in the TSE vis-à-vis mineral insulating oil is in line with the existing literature. Importantly, it is noticed that the inception and breakdown voltages of the nonaged low pour point synthetic esters are similar to nonaged MO. In addition, the inception and breakdown voltages of the aged low pour point synthetic esters are noticed to be higher than that of the aged MO. These results add to the arguments in favor of replacing MOs in power transformers