Dissolved Decay Products Research Articles

Pre-breakdown Phenomena and Influence of Aging Byproducts in Thermally Aged Low Pour Point Ester Fluids Under AC Stress

Application of low pour point esters in transformers insulation serving in cold climatic regions is still a challenge. To address this concern, the pre-breakdown and breakdown analyses of thermally aged low pour point ester fluids are investigated. The breakdown measurements are performed at different aging conditions under AC stress. Needle-plane electrode geometry (with varying radii of the tip) is employed since this configuration is necessary for any practical diagnostic based on partial discharge (PD). This allows studying the effect of the chemical impurities (aging by-products) as assessed by some physicochemical properties (acidity, particles count, turbidity, and dissolved decay products) on the partial discharge and breakdown characteristics. The complete analysis is carried out for liquids having the pour points less than -50 °C, including a mineral oil (MO) and two synthetic ester liquids (SE1 and SE2). Changes in the streamer properties, such as partial discharge inception voltage (PDIV), breakdown-streamer inception voltage, and breakdown voltage are reported as a function of the needle tip radius and aging condition. It is observed that the breakdown properties of SE1 are better than that of SE2 and are comparable to MO. To understand the influence of aging on the PD, correlations between decay products, acidity, BSIV, and PDIV are reported. It is observed that the acidity and concentration of decay products directly impact the PD characteristics.

Comparative analysis of the impact of dissolved decay products on the dielectric performances of natural monoesters and mineral oil for power transformers

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.

Measurement of dissolved decay products of transformer oil using D-shaped plastic optical fibre as a sensor

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.

Comparative study on the accelerated thermal aging behavior between palm and rapeseed natural ester oils

The suitability of natural ester oils as an insulating medium in power transformers is discussed in this paper. Owing to environmental concerns, natural ester oils have great potential as mineral oil substitutes in power transformers. In this paper, the aging behaviors of palm and rapeseed natural ester oils were compared with that for mineral oil. The performance of these natural ester oils was assessed based on their properties (moisture content, acidity, and relative content of dissolved decay products) after accelerated thermal aging. The results showed that the palm oil has better performance compared to the rapeseed oil after accelerated thermal aging for 1500 h because of its lower acidity. This was further supported by the presence of sludge in the rapeseed oil after 1500 h of aging.

Investigations on reclaimed oil performance through measurement of the relative free radical content

The accelerated thermal ageing process was organized to compare the performance of oils before and after being subjected to oil reclamation. New mineral oil of the same brand was used as a base line measurement for evaluating the performance of the reclaimed oil with and without inhibitor. Samples consisting of oil, paper and metallic catalysts including zinc, copper, aluminum, and iron were aged at 115 °C in an air circulating oven for 250, 500 and 750 hours. A free radical reagent, 2.2 diphenyl-1-picrylhydrazyl (DPPH), was used to determine the relative concentration of free radicals in the oil samples. Key ageing indicators, namely free radical scavenging activity (RSA), relative content of dissolved decay products (DDP), turbidity, total acid number (TAN), interfacial tension (IFT), dielectric dissipation factor (DDF), and absolute water content for oil, and low molecular weight acids (LMA), and tensile index (TI) for paper, were measured regularly. It was found that the performance of the reclaimed oil is definitely better than the non-reclaimed oil, and is highly likely competitive with new mineral oil with the same brand. The results show that after a long run of the ageing process, RSA and DDP of the reclaimed oil without inhibitor are respectively more and less than that of the new oil. In addition, slower reduction of TI of paper aged in the reclaimed oils corroborates the theory that the paper degradation in a transformer can be reduced through oil reclamation.

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 μe 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 μe, in optically dense oil samples. The variation of the extinction coefficient was therefore determined asa function of oil aging. The results demonstrate that SLIPI is reliable asa 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.

Stability of mineral oil and oil–ester mixtures under thermal ageing and electrical discharges

This study summarises the results of an experimental investigation on the stability of mineral-based oil mixtures when they are confronted with different ratios of synthetic ester under conditions of electrical discharge, electrical breakdown and a combination of both. The condition of the oil samples was assessed using diagnostic techniques such as gassing tendency, turbidity, dissolved decay products (DDP) and dielectric dissipation factor (DDF), according to ASTM standards. A comparison is made between the performances of fresh and aged samples. The results provide experimental evidence that the chemical composition of hydrocarbon blend is contributing factors to oil gassing. It was observed that aged oils release more gases than new one. It was also observed that the gassing tendency increased with increasing amount of ester for the mixed fluids. The pressure and the absorbance of gases vary proportionally with ester content. Under thermal stress, an increase in pressure is observed especially for the mineral oil sample. The turbidity, DDP and DDF measurements revealed higher values for mineral oil. Importantly, the stability improved with increasing ester content in the blends. Mixed mineral oil/ester therefore offers many advantages with concomitant cost reductions compared with pure synthetic esters.

Physiometric and Fourier transform infrared spectroscopy analysis of cellulose insulation in blend of mineral and synthetic ester oils for transformers

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.

Analysis of Aged Oil on the Cooling of Power Transformers from Computational Fluid Dynamics and Experimental Measurements

In this paper, experimental and numerical investigations were conducted to study the aging impact on the cooling capacity of mineral oils in power transformers. The experimental investigations had three objectives. Firstly the study of the impact of oil aging on its physicochemical properties using two diagnostic techniques which are: DDP (Dissolved Decay Products) according to ASTM (American Society for Testing and Materials)-D6802 standard and Interfacial Tension (IFT) according to ASTM-D971 standard. Secondly,the study of the impact of oil aging on its viscosity according to ASTM D445. Finally the development of empirical equations depicting oil viscosity changes, to be implemented in the numerical model. To achieve this, accelerated thermal aging tests of mineral oil samples were conducted in laboratory conditions according to ASTM D-1934standard. Data from experimental investigations were used to conduct the numerical investigations. A 2D axisymmetric numerical model was developed with COMSOL Multiphysics 4.3a to study the cooling capability of oil aged at different levels. The results indicate that the oil physicochemical properties are affected by aging. Furthermore the results show that the oil viscosity increase with aging and give a good correlation between viscosity and aging indicators. Finally it was found that sludge from oil oxidation byproduct contributes to the formation of hotspots and this leads to a significant increase of hotspot temperature in the power transformers.

Assessing insulating oil degradation by means of turbidity and UV/VIS spectrophotometry measurements

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.

Impact of local overheating on conventional and hybrid insulations for power transformers

For more than a century, conventional mineral oil/cellulose insulation is being used as main insulation for power transformers. The life span of these major assets is determined amongst others by the insulation system’s mechanical resistance to withstand short circuit current forces. Since cellulose paper aging is accelerated by temperature, moisture and oxygen, poly-aramid-based synthetic insulation paper, with better thermal stability emerged for application in transformers. However the relatively limited number of studies on hybrid insulation and the cost of aramid paper still limit their use in distribution transformers. This contribution attempts to demonstrate that despite the high cost of aramid paper, hybrid insulation could be used in large power transformers which represent a significant financial investment. To do this, the quality of mineral oil aged with hybrid solid insulation and the thermal aging of the cellulose paper of this insulation are investigated. Comparison was made with traditional mineral oil impregnated cellulose paper to provide a benchmark for comparison. The samples underwent thermal accelerated aging procedure according to ASTM-D1934 standard and, furthermore, were submitted to local overheating. The condition of oil samples collected from each aging vessel was assessed using several diagnostic techniques, namely the relative amount of the Dissolved Decay Products (DDP), Turbidity, and Interfacial Tension (IFT). The Degree of Polymerisation (DPv) and Dissolved gas Analysis (DGA) were used to monitor the thermal aging of the cellulose paper samples. The results indicate that oil aged with hybrid solid insulation yield fewer decay products at some aging stages, especially after the application of a local overheating. The results also indicate that the cellulose paper within solid hybrid insulation is slightly less degraded compared to that of conventional insulation.

Oxidation stability assessment of a vegetable transformer oil under thermal aging

Vegetable oil is one of the alternatives to replace mineral oils in transformers. This paper focuses on oxidation stability assessment of a vegetable transformer oil under accelerated thermal aging at 110°C with various durations up to 2500 hours. Oil samples were aged under open condition to stimulate the oxidation process, as well as under sealed condition to provide a comparison. A number of parameters including acidity, DC conductivity, dielectric dissipation factor (DDF) and viscosity were measured to see their responses to oxidation process of the vegetable oil. The results showed that viscosity responds towards oxidation more than hydrolysis whereas acidity responds strongly to both oxidation and hydrolysis, since besides oxidation, hydrolysis of the vegetable oil also generates acid byproducts. Of more importance, oxidation stability, in the term of oxidation induction time (OIT), of thermally aged vegetable oils was investigated by using Pressure Differential Scanning Calorimetry (PDSC) method. As there was no standard on PDSC method available for vegetable oils, various measuring temperatures were tried and 130°C was found to be suitable for the investigated vegetable oil. The results showed that OIT of open samples decreases obviously with aging time until reaching the bottom plateau at 1000 hours, while OIT of sealed samples stays at a high level until the end of aging at 2500 hours. Therefore, OIT measured by using PDSC is appropriate to reflect oxidation stability of the vegetable oil. This was further confirmed by the corresponding consumption of antioxidant and generation of peroxides, dissolved decay products.

Comparative study of the electrostatic charging tendency between synthetic ester and mineral oil

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.

Characterization of aging transformer oil-pressboard insulation using some modern diagnostic techniques

This paper presents a study on the characterization of aging of transformer oil–pressboard insulation. Traditional and modern diagnostic tools and techniques were used to investigate aging of oil–paper insulation under controlled laboratory conditions. Insulation condition testing includes common chemical diagnostics such as acidity tests and new chemical techniques: determination of relative content of dissolved decay products by spectrophotometry, solid suspension by turbidimetry, Fourier Transform Infrared (FTIR) spectroscopy and electrical diagnostic techniques that have gained exceptional importance for utility professionals: polarisation/depolarisation current, recovery voltage measurement and frequency domain dielectric spectroscopy measurements. From the obtained results it is found that chemical tests depicted more sensitivity to insulation degradation. Also, different oil–pressboard insulation ratios were investigated to assess the influence of the relative amount of cellulosic materials on aged mineral oil decay products and vice versa. Emphasize is laid on the potentialities/avenues offered by FTIR to assess oil insulation condition. Notable experimental results have been reported, which could be valuable to researchers and practicing engineers in this field. Copyright © 2010 John Wiley & Sons, Ltd.