The role of electrical insulation is critical for the proper operation of electrical equipment. Power equipment cannot operate without energy losses, which lead to rises in temperature. It is therefore essential to dissipate the heat generated by the energy losses, especially under high load conditions. Failing to do so results in premature aging, and ultimately to failure of the equipment. Heat dissipation can be achieved by circulating certain liquids, which also ensure electrical insulation of energized conductors. The insulating-fluids market is therefore likely to be dominated by liquids, leaving to gases (such as compressed air and SF6) limited applications in power equipment such as circuit breakers and switchgear [1]-[3]. Several billion liters of insulating liquids are used worldwide in power equipment such as transformers (power, rectifier, distribution, traction, furnace, potential, current) [4], resistors [5], reactors [6], capacitors [7], cables [8], bushings [9], circuit breakers [10], tap changers [11], thyristor cooling in power electronics, etc. [12]. In addition to their main functions of protecting solid insulation, quenching arc discharges, and dissipating heat, insulating liquids can also act as acoustic dampening media in power equipment such as transformers. More importantly, they provide a convenient means of routine evaluation of the condition of electrical equipment over its service life. Indeed, liquids play a vital role in maintaining the equipment in good condition (like blood in the human body). In particular they are responsible for the functional serviceability of the dielectric (insulation) system, the condition of which can be a decisive factor in determining the life span of the equipment [13]. Testing the physicochemical and electrical properties of the liquids can provide information on incipient electrical and mechanical failures. In some equipment, liquid samples can be obtained without service interruption.

50 years in the development of insulating liquids

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

In this paper the dielectric strength, electrostatic charging tendency (ECT), ageing stability, gassing tendency, heat transfer efficiency and stray gassing issue of mineral and silicone oils, synthetic and natural esters are analyzed and discussed.

Experimental investigations on insulating liquids for power transformers: Mineral, ester, and silicone oils

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

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

With the fast-paced changing technologies in the power industry, new references addressing new technologies are coming to the market. Based on this fact, there is an urgent need to keep track of international experiences and activities taking place in the field of modern transformer design. The complexity of transformer design demands reliable and rigorous solution methods. A survey of current research reveals the continued interest in application of advanced techniques for transformer design optimization. This paper conducts a literature survey and reveals general backgrounds of research and developments in the field of transformer design and optimization for the past 35 years, based on more than 420 published articles, 50 transformer books, and 65 standards.

Transformer Design and Optimization: A Literature Survey

This paper is aimed at the optimization of power transformer insulation via the improvement of the characteristics of the most commonly used insulating liquid, i.e., mineral oil. For that purpose, mixtures consisting of mineral oil and other insulating liquids (namely silicon and synthetic ester oils) are investigated. The main properties such as the heat transfer, breakdown voltage (BDV), aging stability and electrostatic charging tendency (ECT) of different mixtures as well as the liquids alone are compared. The heat transfer is studied by means of kinematic viscosity. The measurements of breakdown voltage are analyzed by means of Gaussian probabilities. The results of aging tests at controlled temperature and in contact with atmosphere are presented, as well as the ECT effect versus the oil humidity and resistivity. It is shown that the best mixture enabling optimization of the power transformer insulation, (i.e. improving the dielectric properties and the aging stability of mineral oil without degrading its viscosity and ECT), is that of mineral oil with 20% by volume of synthetic ester oil

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Improvement of power transformers by using mixtures of mineral oil with synthetic esters

This paper reports on a comparative study between mineral and various ester oils based on Dissolved Gas Analysis (DGA). Electrical (breakdown of low energy) and thermal (stray gassing) faults were realized at laboratory scale, then gas formation as well as diagnosis methods were compared. Two extraction methods, described in IEC 60567 Ed.3 and independent from partition/solubility coefficients were used: multi-cycle vacuum degassing and stripping. Three different diagnosis methods were applied: IEC gas ratios, Duval's triangle and key gases. For electrical faults and especially for low energy discharges, the same main gases (Hydrogen and Acetylene) are created in relatively same ratios, thus confirming the efficiency of key gases method for ester oils. It is also shown that classic Duval's triangle (triangle 1) is well applicable for this type of fault confirming the results reported in literature. For thermal faults and especially for stray gassing issue (low thermal faults), Ethane (associated with Hydrogen) constitutes the key gas of natural esters. The new Duval?s triangles can be applied: Triangle 4 for mineral oil and Triangle 6 for ester oils. For extraction methods and in both electrical and thermal faults cases, it can be noted that results are comparable for mineral oil whereas they are slightly underestimated with stripping method for ester oils. The higher viscosity of ester could interfere with the extraction.

DGA comparison between ester and mineral oils

This paper presents the experimental results of a comparative study of mineral oils, synthetic ester oil and natural ester oils (vegetable oils) issued from different origins through the analyse of the initiation threshold voltage of streamers, their pattern and their stopping length (final length) Lf as well as the associated current and electrical charge, in a point - plane electrode arrangement submitted to a lightning impulse voltage (1.2/50 µs). It's shown that the streamers are filamentary in the different tested oils whatever the polarity of voltage. Lf is longer when the point is positive than when it's negative whatever the oil. It's is generally longer in ester oils (natural and synthetic) than in some mineral oils. When the point is negative, Lf can be ten times higher in ester oils than in mineral ones. Basing on the stopping length of streamers, the positive polarity appears as the most unfavorable one for industrial applications and the dielectric strength of mineral oils we tested are generally better (in the considered experimental conditions) than that of ester oils.

Comparative study of streamer phenomena in mineral, synthetic and natural ester oils under lightning impulse voltage

This paper is aimed at the improvement of power transformers through the improvement of the characteristics of mineral oil by mixing this later with other insulating liquids for transformers namely silicon and synthetic ester oils. A comparison of the main properties such as the heat transfer, breakdown voltage (BDV), ageing stability and electrostatic charging tendency (ECT) of different mixtures as well as liquids alone is presented. It is shown that the optimal mixture enabling to improve the dielectric properties and the ageing stability of mineral oil without degrading its viscosity and ECT is that of mineral oil with 20% of synthetic ester oils.

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Christophe Perrier

Papers

Experimental investigations on insulating liquids for power transformers: Mineral, ester, and silicone oils

Improvement of power transformers by using mixtures of mineral oil with synthetic esters

DGA comparison between ester and mineral oils

Comparative study of streamer phenomena in mineral, synthetic and natural ester oils under lightning impulse voltage

Improvement of power transformers by using mixtures of mineral oil with synthetic esters