Interests on ester liquids as replacement of mineral oil in large power transformers are growing in recent years. This paper presents experimental studies on streamer and breakdown in synthetic and natural esters. Lightning impulse voltage was chosen when considering the practicality of standard tests of transformer industry. First, streamer length, velocity, shape and mode were analyzed based on optical images obtained by a multi-channel ultra high speed camera. Under the same voltage level, streamer in both natural and synthetic esters propagates faster and further, with more branches, than in mineral oil. Breakdown tests at various gaps from 15 mm to 100 mm were conducted, and it was found that fast streamer led breakdown becomes statistically dominant in a large gap under lightning impulse and ester liquids have lower breakdown voltage than mineral oil due to their low tolerance to fast streamer. Therefore a relationship between the results under lightning impulse and previously published results under step voltage was built up and an empirical formula was obtained to predict the breakdown voltage of esters at very large gaps.

Streamer characteristic and breakdown in synthetic and natural ester transformer liquids under standard lightning impulse voltage

The mineral or synthetic oil is mostly being used in conjunction with paper as the dielectric medium in most of the high voltage equipment. However, impact on environment, lack of petroleum oil reserves and disposal problems with used oils, have prompted researchers to direct their focus onto biodegradable and renewable insulating materials. The new insulating liquid materials development is guided by multiple factors such as environmental requirements and other safety and economic considerations. Therefore transformers manufacturer have to face new specifications related to these new requirements. The Vegetable-oil based transformer fluids increasingly replacing mineral oil-based products in the market place. They are successful because they perform better than mineral oil products and they provide definite environmental and safety gains. This paper reviews the current status of vegetable oils use as transformer oil, including their production, processing, and characterization. The vegetable oils most used as transformer oils are presented and their main advantages described in comparison with mineral oil. The various experimental work carried out in different countries is described, giving an overview of the current research carried out on the vegetable oils. In addition scope and challenges being faced in this area of research are clearly described.

Use of vegetable oils as transformer oils – a review

Dielectric response is a non-invasive diagnostic method with a potential to give information about the status of the solid insulation in a power transformer. To investigate what conditions the method can and cannot reveal, frequency domain spectroscopy has been applied to oil-impregnated paper and pressboard with and without moisture and/or acids added, and to paper at various stages of aging. Water and carboxylic acids are important aging indicators. It is found that moisture is clearly detectable. Also low molecular weight carboxylic acids will affect the dielectric response in a way similar to moisture, but high concentrations are needed for a contribution comparable to that of moisture. It is difficult to tell moisture and acids apart from the responses obtained. High molecular weight carboxylic acids make little or no contribution to the dielectric response of the solid insulation in a paper/pressboard - oil system. It was also observed that keeping paper or pressboard at elevated temperatures had a conditioning effect on the response

Dielectric response of mineral oil impregnated cellulose and the impact of aging

Acid catalyzed ageing of oil impregnated kraft paper has been studied experimentally. Five different carboxylic acids (formic, acetic, laevulinic, stearic and naphtenic acid) have been added to mineral oil to arrive at a neutralization value of 0.4 mg KOH/g. Thereafter, kraft paper of two different humidities has been allowed to equilibrate with the oil. The results show that the lower the molecular weight of the carboxylic acids is the more is absorbed by the paper. Furthermore, a clear synergy for the paper ageing rates between moisture and the lower molecular weight acids is found, while for the higher molecular weight acids almost no effects were found. It is concluded that present techniques for measuring acidity of oils do not reflect these detailed conditions. It is suggested to introduce a new measuring technique for neutralization value where water rinsing is used to identify the content of low molecular weight, water soluble acids.

Ageing of Mineral Oil Impregnated Cellulose by Acid Catalysis

Natural ester is considered to be a substitute of mineral oil in the future To apply natural ester in large transformers safely, natural ester impregnated solid insulation should be proved to have comparable dielectric strength and thermal stability to mineral oil impregnated solid insulation This paper mainly focuses on a comparative study of physicochemical, ac breakdown strength and thermal stability behavior of BIOTEMP natural ester/pressboard insulation and Karamay 25# naphthenic mineral oil/pressboard insulation after long term thermal ageing The physicochemical and dielectric parameters including moisture, acids and the ac breakdown strength of these two oil/pressboard insulation systems at different ageing status were compared The permittivity and ac breakdown strength of these two oil/pressboard insulation systems at different temperatures were also investigated And a comparative result of the thermal stability behavior of these two oil/pressboard insulation systems with different ageing status was provided at last Results show that though natural ester has higher absolute humidity and acidity during the long ageing period, the lower relative humidity of natural ester helps to keep its ac breakdown strength higher than mineral oil The pressboard aged in natural ester also has higher ac breakdown strength than that aged in mineral oil The lower relative permittivity ratio of natural ester impregnated paper to natural ester is beneficial to its dielectric strength Using natural ester in transformer, the resistance to thermal decomposition of the oil/pressboard insulation system could be also effectively improved

/pdf/a-comparative-study-of-physicochemical-dielectric-and-5e30k90olf.pdf

A comparative study of physicochemical, dielectric and thermal properties of pressboard insulation impregnated with natural ester and mineral oil

Hydrolysis of transformer insulating Kraft paper is experimentally investigated. Paper moisture content was varied and five different acids representative of what may be found in an aging transformer - were added to the oil. The low molecular weight acids were to a large degree absorbed by the paper, contrary to the high molecular acids that remained in the oil. The three low molecular acids accelerated the aging of the paper, and a clear synergy between these acids and water content was found. The high molecular acids did not influence the paper aging significantly. The results support a theory explaining the hydrolysis as governed by acid catalysis. Examples from measurements on real transformers are shown. It is concluded that low molecular acids constitute an important aging factor for transformer windings and should be included in diagnostic schemes, and when estimating the pay-back from maintenance actions.

Aging of Kraft paper by acid catalyzed hydrolysis

The aim is to improve the understanding of high-field phenomena (such as preinception currents/conduction, streamer initiation and propagation) in insulating materials in terms of the molecular properties of the substances involved. In high electric fields, ionization is a likely process, and in all such processes, the ionization potential is an important parameter. A fundamental question is how these processes depend on the electric field, and therefore, based on the interaction between a negative point charge and a molecular cation as modeled by density functional theory, a field-dependent model for the ionization potential is developed. In addition, the first excitation energies as a function of the electric field are calculated using time-dependent density functional theory. It is demonstrated that empirical high-field conduction models for cyclohexane and n-tridecane can be explained in terms of the difference between the ionization potential and the first excitation energy. It is also suggested that the reduction of the ionization potential with electric fields, can help explain how fast-mode streamers propagate.

Field dependence on the molecular ionization potential and excitation energies compared to conductivity models for insulation materials at high electric fields

In this letter, we experimentally demonstrate that the lightning impulse breakdown characteristics of natural ester liquids can be significantly enhanced by dispersing molecular additives possessing lower values of both the ionization potential and 1st excitation energies as compared to the base liquid One such additive contributed to an increase in the breakdown and acceleration voltage of the base liquid by 32% and 90%, respectively Apart from the expected influence of the low ionization potential of the additives, results also indicate a positive effect of lower value of 1st excitation energy

Enhancements in the lightning impulse breakdown characteristics of natural ester dielectric liquids

The solubility equilibriums of formic, acetic, levulinic, stearic and naphthenic acids in oil-paper (Kraft) insulation systems have been studied using titration techniques. Temperatures and moisture levels were varied. Low molecular weight carboxylic acids strongly favour the cellulose while the heavier stearic and naphthenic acids favour the oil. Paper moisture did not influence solubility in a systematic way. The presence of low molecular acids increases the water content of the oil. Oil and paper from service aged transformers, show that typically 10-15% of the oil acidity stems from low molecular weight, hydrophilic acids. The concentration of the same type of acids is a hundred-fold higher in the cellulose. Thus, most of the total acidity in the solid insulation consists of hydrophilic acids, while they constitute only a minor part of the acidity in the oil. The methods used here for extraction of low molecular weight acids are proposed as an addendum to international standards for determination of oil acidity.

S. Ingebrigtsen

Papers

Ageing of Mineral Oil Impregnated Cellulose by Acid Catalysis

Aging of Kraft paper by acid catalyzed hydrolysis

Field dependence on the molecular ionization potential and excitation energies compared to conductivity models for insulation materials at high electric fields

Enhancements in the lightning impulse breakdown characteristics of natural ester dielectric liquids

Solubility of carboxylic acids in paper (Kraft)-oil insulation systems