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Low molecular-weight acids (LMA) are by-products of and catalysts to transformer paper ageing. Their concentration can be well below 0.1 mg KOH/g. There has been no standard procedure to measure and control these acids at low acidity levels. In the present work, a technique using water to separate an LMA at low acidity from two types of transformer liquids, a mineral oil and a synthetic ester, was proposed and its effectiveness was studied. Two vessel configurations, Erlenmeyer flask (EF) and gas-tight syringe (GTS), were tested for the important step of separating the LMA from the liquids. Results showed that both vessels, which can be sealed tight, enable high percentage extraction of LMA (>80%) at acidities of 0.017 mg KOH/g and 0.06 mg KOH/g. The EF proved a preferred choice due to its ease of handling. Further validation of its suitability is confirmed by the high percentage extraction of LMA from a mixture of LMA and high molecular-weight acids (HMA) both added to the transformer liquids. The effective range of the total volume of transformer liquid and water has also been identified.

For nearly four decades 2-furfural (2-FAL) in transformer oil has been used as a chemical marker to indicate the ageing state of transformer paper insulation. Nevertheless, it has always been a challenge to define an acceptable limit for 2-FAL due to issues related to non-uniform temperature distribution in transformers, stability of 2-FAL in oil and its partitioning between liquid and solid insulation. Therefore, transformer users try to combine the laboratory findings, the operational experience and post-mortem analyses together for predicting ageing state of paper insulation through 2-FAL content in oil. Due to increase in the use of ester liquids in transformers, there is a need to investigate the ability to use 2-FAL as a paper ageing indicator for ester based insulation systems. Laboratory ageing studies have shown that 2-FAL is indeed generated in ester based insulation systems. However, there is a lack of understanding related to the partitioning of 2-FAL in ester based insulation systems. This paper studies the partitioning of 2-FAL in synthetic ester based insulation systems and compares to that in mineral oil based insulation systems with the focus on the effect of paper type and water content in paper. Under the investigated conditions the synthetic ester had higher partitioning coefficients (ratio between the 2-FAL content in oil and paper) than the mineral oil. Partitioning coefficients in both oil types decreased with the increase of the water content in paper. However, change in paper type did not significantly affect the partitioning coefficients in this study.

Laboratory scale thermal ageing experiments have been used not only to study the degradation of the insulation materials but also to understand the variation of chemical markers in liquid during the ageing process. However, trends obtained from different experiments often disagree with each other due to experimental variations such as material type, material ratio and ageing temperature. In addition to these well-known factors, there are lesser- known factors such as the duration for which samples are left at room temperature after an ageing experiment, which could affect the concentration of chemical markers in liquid. It is known that temperature could affect the partitioning of chemical markers among the liquid, solid and gas phase. Hence the concentration of certain chemical-markers in liquid could vary depending on the duration for which samples are left at room temperature prior to the sampling of oil for the measurements. This paper investigates the effect of this sampling period on the variation of chemical markers including water, methanol and ethanol in liquid after a laboratory scale ageing experiment conducted at 150 °C. Results showed that the concentration of the investigated chemical markers in liquid decrease continuously when the sample is left at room temperature, with water having the highest reduction followed by methanol and ethanol.

The applicability of using 2-furfural (2-FAL) and methanol as aging markers for synthetic ester impregnated paper insulation was investigated through laboratory aging experiments at temperatures of 80, 100 and 120 °C. Methanol in the synthetic ester increased linearly with paper aging until degree of polymerisation (DP) reached 400. During this period concentration of methanol in oil was higher than that of 2-FAL, which proved the concept of using methanol to indicate early paper aging in the synthetic ester. However, the rate of methanol increase against the reduction of DP in the synthetic ester was about three times higher than that seen in mineral oil. This is due to a difference in the partitioning of the marker between oil and paper. Since the synthetic ester is more polar than the mineral oil, it attracted more methanol molecules from the paper, resulting in a higher concentration of methanol in oil for the same reduction of DP. In addition, a reduction in methanol was observed at the later stage of paper aging at 120 °C. This could be due to either a change in the partitioning between oil and paper, or an esterification reaction which consumes methanol in the system.

Transformers being one of the most expensive equipment in the networks require proper utilisation, which includes regular maintenance and application of life-extension techniques in feasible situations. Ageing process of the mineral oil and cellulose paper insulation system creates by-products such as moisture, acids and sludge, which further accelerate the degradation of the transformer insulation. Therefore, the removal of these by-products through oil regeneration could potentially prolong the lifetime of transformers. Typically, oil regeneration process includes percolation of oil through an adsorbent system followed by a filtration and a degasification process. Various sorbent types such as fuller’s earth, alumina, molecular sieves, silica-kaolin-sand mixture are used for the regeneration process. This paper investigates the acid removal performance of a modified alumina based sorbent, against two conventional sorbents i.e. fuller’s earth and alumina. It was found that the overall performance of modified alumina was higher than the conventional sorbent materials. Furthermore, all the three sorbent materials showed higher absorption capacity for low molecular weight acids than the high molecular weight acids.

Managing a large fleet of ageing assets has become a technical challenge faced by many electricity utilities in developed countries. Asset managers are increasingly interested in techniques that can help extend the useful lifetime of a transformer. Oil regeneration is one of such techniques. In this paper, oil regeneration experiments were performed on a 6.4/0.4 kV retired distribution transformer to investigate the effect of oil regeneration on improving paper conditions. Oil regeneration was conducted in two stages, with the first stage aimed at ‘cleaning the oil’ and the second stage targeted at ‘cleaning the paper’. Oil samples were collected at regular intervals throughout the process and paper samples were obtained from the transformer before and after each oil regeneration stage. It was found that oil regeneration restores oil parameters, including moisture and acidity, similar to those of new oils at the end of stage 1. Analysis of paper samples indicated a reduction in paper moisture at the end of stage 2 by nearly 40%, while low molecular weight acids (LMA) in paper exhibited a reduction by around 30% on average. It is found that the extended oil regeneration period, i.e., stage 2, is necessary to improve the paper condition and hence to reduce the paper ageing rate.

Aging of transformer paper insulation is commonly investigated through indirect measurements conducted in the oil, e.g. furanic compounds. In addition to the conventional indicators, recent investigations showed that alcohols like methanol and ethanol in oil could be used as paper aging indicators. This paper focuses on the applicability of these new paper aging indicators in a novel, gas-to-liquid (GTL) technology based transformer oil through an accelerated laboratory aging experiment. The experiment was conducted at 120 °C for up to 280 days. A conventional mineral oil was also tested as a reference. Kraft paper aged in both oil types showed similar reductions in degree of polymerization (DP) and tensile strength (TS). Amount of methanol in oil is higher than that of 2-FAL in oil when DP is over 375, confirming the promise of using methanol as an early paper aging indicator. Both the new paper aging indicator, methanol and the conventional paper aging indicator, 2-FAL are applicable for the GTL oil without any modifications. In addition, oil only aging experiments showed that oil does produce methanol but it is negligible compared with oil paper aging experiment; however the amount of ethanol measured in the present oil paper aging experiment is believed to be originated from oil oxidation rather than from paper aging.

In HVDC converter transformers, the potential DC component in electric stresses imposed on valve windings would unfavorably cause space charge accumulation inside oil-paper insulation. Insulation degradation and aging may have great effects on space charge behaviors in oil-paper insulation. This paper focuses on the effect of the low molecular weight (LMA) and the moisture on space charge phenomenon in oil impregnated-paper insulation. Pulsed electro-acoustic (PEA) technique was used in this study to measure space charge behaviors in 400 $\mu$ m thick, dry and moist single-layer paper sample impregnated in mineral oil prepared with different amounts of formic acid. The measurements were carried out at $30 ^ { \circ } \mathrm { C }$ under a field strength of 20 kV/mm. It was found that LMA greatly strengthens the homo charge accumulation. The combination of LMA and moisture results in even more active space charge accumulations. In dry samples, the existence of LMA causes slower charge decaying rate. However, the introduction of the moisture accelerates the charge dissipation.

Ageing assessment of transformer paper insulation is commonly conducted through measurement of different chemical indicators in oil such as carbon oxide gases and furanic compounds [1]. Apart from these conventional paper ageing indicators, recent research success has shown that methanol and ethanol in transformers can also be used to indicate the degradation of transformer paper insulation [2].

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.

Ageing assessment of cellulose insulation in a transformer is normally conducted through indirect measurements of chemical markers in oil. In addition to the conventional paper ageing indicator 2-FAL, methanol in oil was proposed as an additional indicator to provide early ageing information. Relationships between the paper ageing and the concentration of the indicators in oil could be affected by the type of oil due to stability and partitioning issues. Therefore it is important to investigate the suitability of the paper ageing indicators for alternative insulating oils. This paper shows the results of a laboratory ageing experiment conducted to investigate the relationships between the paper ageing and the ageing indicators of 2-FAL and methanol in a Gas to Liquid (GTL) hydrocarbon oil. The ageing experiment was conducted at 100 °C for up to 126 days. A conventional mineral oil was also tested as benchmark. Variations of both methanol and 2-FAL against DP of paper aged in the GTL oil were found to be similar to those in the mineral oil.

Ageing assessment of transformer paper insulation is important due to its irreversible ageing nature. Due to the poor accessibility of the paper samples in field transformers, large efforts were made to seek for chemical parameters in oil to indicate paper ageing, e.g. furanic compounds and dissolved carbon oxides. In recent years, alternative parameters including methanol and ethanol in oil were proposed to be related with paper ageing. In this study, a laboratory ageing experiment was conducted at 120 °C with an inhibited mineral oil and an ordinary kraft paper. Methanol and ethanol in oil were measured through an in-house developed method based on a headspace gas chromatography mass spectrometry (HS-GC-MS) setup. Result showed that methanol in oil increases linearly with the chain scission in paper which suggests its suitability to indicate early paper ageing.

Paper ageing is an irreversible process, which has made paper insulation a lifetime determining factor for transformers. Chemical indicators in oil such as carbon oxide gases and 2-FAL, are used to indicate the ageing state of paper as it is difficult to obtain paper samples to measure DP or tensile strength. Methanol amount in oil was recently found to be an early-ageing indicator for paper. In this study, a headspace gas chromatography mass spectrometry setup was developed to measure methanol in transformer oil. This setup consists of an auto sampler with a gas tight syringe, a gas chromatography unit with 60 m VF-624ms column and a quadrupole ion trap type mass spectrometer unit. Measurement of several laboratory-aged and service-aged oil samples were conducted with both external standard and internal standard calibration methods. Higher methanol values obtained from internal standard method confirmed that it is more suitable than external standard method.

This paper presents performance comparison of coconut oil as an alternative liquid insulation for transformer mineral oil during ageing and under simulated electrical and thermal fault conditions. In the ageing study, 6 wet coconut oil, 6 wet and 6 dry transformer oil samples were aged (each half as sealed and unsealed with inclusions of Cu, Al, Fe and Zn metal substances) inside an oven at 120°C. Six sample sets were taken out at three different ageing intervals i.e. 2, 5 and 7 weeks and were tested by visual inspection (color), chemical tests (interfacial tension, acidity, DGA) and electrical tests (breakdown voltage, FDS). In the electrical fault study, 2 coconut oil and 2 transformer oil samples were exposed to corona for four hours and for 20 electrical breakdowns with a point-plane gap of 15mm. In the thermal fault study, 2 coconut oil and 2 transformer oil samples were aged at 160°C for 12 hours and at 200°C for 1 hour. The DGA and FDS measurements were taken on the aged samples. FDS Results were analyzed by using Davidson-Cole Model. It was found that the coconut oil samples gave comparable results with those for transformer oil.

Coconut oil has been considered as an alternative choice for power transformer liquid insulation. This paper presents ageing aspects of coconut oil in comparison to that of transformer oil. Wet coconut oil and wet and dry transformer oil was aged in sealed and unsealed conditions at 1200C. Variation of color, interfacial tension, acidity, breakdown voltage and frequency dielectric spectroscopy was observed during different aging period of 2, 4, and 7 weeks. In the aging process, the acidity and conductivity levels of coconut oil increased. However, its breakdown strength was considerably high compared to transformer oil showing a possibility of using as a transformer liquid insulation.