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The challenge of bio-based insulating oil research is the development of a new alternative insulating liquid to mineral oil (MO) for power transformers with identical and even better characteristics in the long run. Under operating conditions, the concentration of dissolved decay contents in oil increases with time and could alter its dielectric properties. This paper presents an experimental study comparing the long-term dielectric properties between castor oil methyl ester (COME), palm kernel oil methyl ester (PKOME) and MO. Six samples of each insulating liquid were submitted to accelerated thermal ageing at 115°C for 432 hours according to ASTM D1934. Every 72 hours, a sample of each insulating liquid is collected from the laboratory oven. Then the breakdown voltage (BDV), the partial discharge inception voltage (PDIV) and dissolved decay products (DDP) of these samples are measured and analysed. The BDV was investigated according to IEC 60156. The PDIV experiment was performed in conformity with a protocol modified from IEC 61294 at ambient temperature. The DDP were evaluated according to ASTM D6802. The experimental findings show natural monoesters have better long-term dielectric performance compared to MO.

Transformer oil is widely used in high‐voltage transformers as a cooling and insulation medium. Electrical and thermal stresses expose transformer oil to degradation, which may produce dissolved decay products (DDPs) that are partially adsorbed on the insulation paper and accelerate the degradation of oil and paper. An evanescent field D‐shaped plastic optical fibre (POF) sensor was fabricated using the side‐polishing technique to detect DDPs in transformer oil. A laser light source and power meter were used in the prototype to measure the output power of the POF sensor. The measurement of DDPs was conducted on seven samples of insulating oil collected from the field. The sensor performance was evaluated based on different sensing areas, and the sensor sensitivity was measured. Results show an excellent correlation between the output power and DDPs. A correlation analysis was conducted amongst sensor output power, transformer oil refractive index and area under the absorbance curve. The performance of the sensor demonstrates improved linearity and sensitivity of 97.49% and 26.6779 µW/RIU, respectively. The POF sensor is easily fabricated, low cost and can be used for real‐time monitoring of high‐voltage transformers. [ABSTRACT FROM AUTHOR]

This study reports experimental study of cellulose performance in mixture of mineral and synthetic ester (SE) oils. Solid insulation impregnated with mineral oil (MO) and mixture oil has been subjected to accelerated thermal stresses to understand the deterioration rate of cellulose. Diffusion of oil–moisture mixture into paper and corresponding variation in dielectric strength of paper has been examined. Compatibility of solid insulation with MO and mixture oil is analysed by thermal expansion coefficients, X‐ray diffraction analysis and dissolved decay products. Furthermore, chemical functional groups present in aged oils are identified by Fourier transform infrared spectroscopy analysis. It is observed that the performance of cellulose insulation is improved with addition of SE to MO. This is achieved by higher water solubility of esters, which enables higher moisture consumption during hydrolysis and keeps cellulose dry. [ABSTRACT FROM AUTHOR]

In this contribution the influence of various stresses and their combined impact on the electrostatic charging tendency of oil is studied. Various physicochemical properties were measured according to ASTM Standards to detect changes in oil quality. A free radical reagent, 2.2 diphenyl-1-picrylhydrazyl (DPPH), was added to oil before and after the application of stresses to determine free radical concentration. The results obtained show that the application of stresses contributes to an increase in the electrification current. These results also demonstrate that electrification current is affected by the quantity of the free radicals. [ABSTRACT FROM AUTHOR]

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.

Highlights: [•] Electrostatic charging tendency of fresh and aged oils were investigated in a spinning disk system. [•] The electrification currents were related to classical aging indexes. [•] Two testing procedures developed by ASTM were used to monitor decay products as trace impurities. [•] The electrification currents increase with temperature, fluid velocity and disk properties. [•] The electrification is definitely related to turbidity and spectrophotometry measurements. [ABSTRACT FROM AUTHOR]

The growing demands for improved fire safety, source material sustainability, environment friendliness, and asset life extension have driven the research and development efforts of synthetic esters, less-flammable fluids. However the relatively limited number of work on these esters and their cost still limit their uses to specific applications. This contribution reports some investigations onto the Electrostatic Charging Tendency (ECT) of a commercially available ester fluid. Comparison is made with mineral oil, as this is a fluid we are familiar with. Fresh (unaged) and aged fluid samples were investigated in a spinning disk system under laboratory controlled conditions. Changes in static electrification were compared to some aging indexes (interfacial tension, turbidity, relative dissolved decay products). This contribution is not only intended to provide a fresh review in this domain of research, but also contains a substantial amount of new material with a view of closing some gaps in the present state of knowledge of oil streaming electrification. The obtained results show that static electrification currents are affected by fluid motion velocity, type of paper, moisture content and the aging byproducts. It was also found that the removal of dissolved oxygen by nitrogen blanketing contributed to the reduction in static electrification. [ABSTRACT FROM AUTHOR]