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Engineering Dielectric Liquid Applications

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A special issue of Energies (ISSN 1996-1073).

Deadline for manuscript submissions: closed (30 April 2018) | Viewed by 59847

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Prof. Dr. Issouf Fofana

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Aging of Oil-Filled Equipment on High Voltage Lines (ViAHT), Department of Applied Sciences, Université du Québec à Chicoutimi (UQAC), Chicoutimi, QC G7H 2B1, Canada
Interests: electromagnetic compatibility; electrical engineering; electrical insulation; dielectric materials; problems associated with the transport and distribution of electrical energy
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Dear Colleagues,

Insulating liquids are used for impregnating solid insulations or filling products of very large number of electric materials. Several billion liters of insulating liquids are used worldwide in equipment such as transformers (power, rectifier, distribution, traction, furnace, potential, current), resistors, reactors, capacitors, cables, bushings, circuit breakers, tap changers, thyristor cooling in power electronics, etc. Their role is paramount in the sense that it is one of the fundamental conditions for the proper functioning of these equipment. Similar to blood in human body, the insulating liquid keeps responsibility to the entire equipment. The greatest challenges are related to the need for methods to assess the condition along with the improvement of the fluids efficiency by chemical admixtures and or application of biodegradable fluids.

The Special Issue will cover theoretical and practical developments with special emphasis on engineering problems in using these liquids in electrical equipment. The applications of new fluids and perspectives are also concerned.

Topics of interest for publication include, but are not limited to:

Application and performance of dielectric liquids
Electro-hydrodynamic phenomena and related applications
Breakdown and pre-breakdown phenomena,
Electrostatic charging tendency
Insulating liquids efficiency improvement by chemical admixtures
Nanofluids and synthetic/vegetable dielectric liquids
Measurement, monitoring and diagnostic techniques
Fundamental investigations and basic properties

Prof. Dr. Ing. Issouf Fofana
Guest Editor

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Keywords

electrical discharge
dielectric phenomena
electro-hydrodynamics
biodegradable fluids
nanofluids
aging and degradation
diagnostic and monitoring
high voltage direct current (HVDC) applications

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Published Papers (11 papers)

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Editorial

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4 pages, 171 KiB

Open AccessEditorial

Engineering Dielectric Liquid Applications

by Issouf Fofana and U. Mohan Rao

Energies 2018, 11(10), 2756; https://doi.org/10.3390/en11102756 - 15 Oct 2018

Cited by 5 | Viewed by 2851

Abstract

n/a Full article

(This article belongs to the Special Issue Engineering Dielectric Liquid Applications)

Research

Jump to: Editorial, Review

13 pages, 3550 KiB

Open AccessArticle

Lightning Impulse Withstand of Natural Ester Liquid

by Stephanie Haegele, Farzaneh Vahidi, Stefan Tenbohlen, Kevin J. Rapp and Alan Sbravati

Energies 2018, 11(8), 1964; https://doi.org/10.3390/en11081964 - 28 Jul 2018

Cited by 25 | Viewed by 3785

Abstract

Due to the low biodegradability of mineral oil, intense research is conducted to define alternative liquids with comparable dielectric properties. Natural ester liquids are an alternative in focus; they are used increasingly as insulating liquid in distribution and power transformers. The main advantages of natural ester liquids compared to mineral oil are their good biodegradability and mainly high flash and fire points providing better fire safety. The dielectric strength of natural ester liquids is comparable to conventional mineral oil for homogeneous field arrangements. However, many studies showed a reduced dielectric strength for highly inhomogeneous field arrangements. This study investigates at which degree of inhomogeneity differences in breakdown voltage between the two insulating liquids occur. Investigations use lightning impulses with different electrode arrangements representing different field inhomogeneity factors and different gap distances. To ensure comparisons with existing transformer geometries, investigations are application-oriented using a transformer conductor model, which is compared to other studies. Results show significant differences in breakdown voltage from an inhomogeneity factor of 0.1 (highly inhomogeneous field) depending on the gap distance. Larger electrode gaps provide a larger inhomogeneity at which differences in breakdown voltages occur. Full article

(This article belongs to the Special Issue Engineering Dielectric Liquid Applications)

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15 pages, 4824 KiB

Open AccessArticle

Dual-Temperature Evaluation of a High-Temperature Insulation System for Liquid-Immersed Transformer

by Xiaojing Zhang, Lu Ren, Haichuan Yu, Yang Xu, Qingquan Lei, Xin Li and Baojia Han

Energies 2018, 11(8), 1957; https://doi.org/10.3390/en11081957 - 27 Jul 2018

Cited by 18 | Viewed by 3999

Abstract

A high-temperature oil–paper insulation system offers an opportunity to improve the overloading capability of distribution transformers facing seasonal load variation. A high-temperature electrical insulation system (EIS) was chosen due to thermal calculation based on a typical loading curve on the China Southern Power Grid. In order to evaluate candidate high-temperature insulation systems, Nomex^® T910 (aramid-enhanced cellulose) immersed in FR3 (natural ester) was investigated by a dual-temperature thermal aging test compared with a conventional insulation system, Kraft paper impregnated with mineral oil. Throughout the thermal aging test, mechanical, chemical, and dielectric parameters of both paper and insulating oil were investigated in each aging cycle. The thermal aging results determined that the thermal class of the FR3-T910 insulation system meets the request of overloading transformer needs. Full article

(This article belongs to the Special Issue Engineering Dielectric Liquid Applications)

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13 pages, 4923 KiB

Open AccessArticle

Impact of Low Molecular Weight Acids on Oil Impregnated Paper Insulation Degradation

by Kakou D. Kouassi, Issouf Fofana, Ladji Cissé, Yazid Hadjadj, Kouba M. Lucia Yapi and K. Ambroise Diby

Energies 2018, 11(6), 1465; https://doi.org/10.3390/en11061465 - 6 Jun 2018

Cited by 34 | Viewed by 4668

Abstract

Aging of a power transformer’s insulation system produces carboxylic acids. These acids—acetic, formic and levulinic—are absorbed by the paper insulating material, thus accelerating the degradation of the whole insulation system. In this contribution, the effect of these acids on the aging of oil-impregnated paper insulation used in power transformer is reported. A laboratory aging experiment considering different concentrations of these three acids was performed to assess their effect on the insulation system’s degradation. Each acid was individually mixed with virgin oil, and a mixture of acids was also blended with oil. The paper’s degradation was assessed by the degree of polymerization (DPv). It was found that the DPv of paper aged with formic acid decreased much faster in comparison to the other acids. Full article

(This article belongs to the Special Issue Engineering Dielectric Liquid Applications)

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13 pages, 2562 KiB

Open AccessFeature PaperArticle

Characteristics of Negative Streamer Development in Ester Liquids and Mineral Oil in a Point-To-Sphere Electrode System with a Pressboard Barrier

by Pawel Rozga, Marcin Stanek and Bartlomiej Pasternak

Energies 2018, 11(5), 1088; https://doi.org/10.3390/en11051088 - 28 Apr 2018

Cited by 29 | Viewed by 4065

Abstract

This article presents the results of the studies on negative streamer propagation in a point-to-sphere electrode system with a pressboard barrier placed between them. The proposed electrode system gave the opportunity to assess the influence of the insulating barrier on streamer development in the conditions close to the actual transformer insulating system where the liquid gap is typically divided in parts by using pressboard barriers. The studies were performed for five commercial dielectric liquids. Among them two were biodegradable synthetic esters and two were biodegradable natural esters. Mineral oil, as the fifth liquid, was used for comparison. The measurements were based on electrical and optical experimental techniques. From the results obtained it may be concluded that, independently of the liquids tested, the electrical strength of the insulating system considered was increased by about 50%. In the case of streamer development assessed using photomultiplier-based light registration it is not possible to indicate clearly which of the liquids tested is better under the conditions of the experiment. In all cases streamers always developed slowly (2nd mode) at all voltage levels applied during the studies. In turn, the intensity of the discharge processes, comparing the same voltage levels, was mostly higher when streamers developed in ester liquids, however, the differences noticed were minimal. Full article

(This article belongs to the Special Issue Engineering Dielectric Liquid Applications)

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11 pages, 1413 KiB

Open AccessArticle

Experimental Study on Breakdown Characteristics of Transformer Oil Influenced by Bubbles

by Chunxu Qin, Yan He, Bing Shi, Tao Zhao, Fangcheng Lv and Xiangrui Cheng

Energies 2018, 11(3), 634; https://doi.org/10.3390/en11030634 - 13 Mar 2018

Cited by 27 | Viewed by 4869

Abstract

Bubbles will reduce the electric strength of transformer oil, and even result in the breakdown of the insulation. This paper has studied the breakdown voltages of transformer oil and oil-impregnated pressboard under alternating current (AC) and direct current (DC) voltages. In this paper, three types of electrodes were applied: cylinder-plan electrodes, sphere-plan electrodes, and cone-plan electrodes, and the breakdown voltages were measured in both no bubbles and bubbles. The sphere-sphere electrodes were used to study the breakdown voltage of the oil-impregnated pressboard. The results showed that under the influence of bubble, the breakdown voltage of the cylinder-plan electrode dropped the most, and the breakdown voltage of the cone-plan electrode dropped the least. The bubbles motion was the key factor of the breakdown. The discharge types of the oil-impregnated pressboard were different with bubbles, and under DC, the main discharge type was flashover along the oil-impregnated pressboard, while under AC, the main discharge type was breakdown through the oil-impregnated pressboard. Full article

(This article belongs to the Special Issue Engineering Dielectric Liquid Applications)

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13 pages, 2492 KiB

Open AccessArticle

Electronic Properties of Typical Molecules and the Discharge Mechanism of Vegetable and Mineral Insulating Oils

by Yachao Wang, Feipeng Wang, Jian Li, Suning Liang and Jinghan Zhou

Energies 2018, 11(3), 523; https://doi.org/10.3390/en11030523 - 28 Feb 2018

Cited by 36 | Viewed by 4590

Abstract

Vegetable insulating oil may replace the mineral insulating oil used in large power transformers due to its extraordinary biodegradability and fire resistance. According to component analysis, 1-methylnaphthalene and eicosane are considered the typical molecules in mineral oil. Triolein and tristearin are considered the typical molecules in vegetable oil. The ionization potential (IP) and the variation of highest occupied molecular orbital (HOMO) of typical molecules under an external electric field are calculated using quantum chemistry methods. The calculation results show that the IP of the triolein molecule is comparable to that of the 1-methylnaphthalene molecule. The mechanisms of losing electrons are discussed, based on the analysis of HOMO composition. The insulation characteristics of the triolein and tristearin are more likely to be degraded under an external electric field than those of 1-methylnaphthalene and eicosane. Due to the fact that the number density of low IP molecules groups in vegetable oil is much greater than that in mineral oil, the polarity effect in vegetable oil is more obvious than that in mineral oil. This eventually leads to different streamer characteristics in vegetable oil and mineral oil under positive polarity and negative polarity. Full article

(This article belongs to the Special Issue Engineering Dielectric Liquid Applications)

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15 pages, 4117 KiB

Open AccessArticle

Modeling the Insulation Paper Drying Process from Thermogravimetric Analyses

by Amidou Betie, Fethi Meghnefi, Issouf Fofana and Zie Yeo

Energies 2018, 11(3), 517; https://doi.org/10.3390/en11030517 - 28 Feb 2018

Cited by 11 | Viewed by 3837

Abstract

It is now well-established that moisture in the oil paper insulation used in power and instrument transformers significantly reduces the transformers’ lifetimes, and can eventually lead to premature failure. This moisture should, therefore, always be removed, not only during production but also after repairs. At the final stage of manufacturing, the drying process should be carried out to remove water and air vacuoles contained in the cellulose-based paper before impregnation. Successful drying helps increase the residual life of transformers, because the presence of moisture and air vacuoles accelerates the aging/degradation process of the oil paper insulation. Proper estimation of residual moisture before impregnation and the determination of the time required for drying play key roles in the time-consuming process of drying. In this paper, the disadvantages of inadequate drying are addressed, followed by a mathematical approach to model the paper drying process. A mathematical model describing the kinetics of drying according to temperature, initial moisture, paper weight, final moisture, and extraction rate is proposed. This model also estimated the amount of moisture removed at the end of the drying process. Full article

(This article belongs to the Special Issue Engineering Dielectric Liquid Applications)

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16 pages, 9203 KiB

Open AccessArticle

Development of a Biodegradable Electro-Insulating Liquid and Its Subsequent Modification by Nanoparticles

by Vaclav Mentlik, Pavel Trnka, Jaroslav Hornak and Pavel Totzauer

Energies 2018, 11(3), 508; https://doi.org/10.3390/en11030508 - 27 Feb 2018

Cited by 42 | Viewed by 5616

Abstract

The paper is focused on the possibility of replacing petroleum-based oils used as electro-insulating fluids in high voltage machinery. Based on ten years of study the candidate base oil for the central European region is rapeseed (Brassica napus) oil. Numerous studies on the elementary properties of pure natural esters have been published. An advantage of natural ester use is its easy biodegradability, tested according to OECD–301D (Organisation for Economic Co-operation and Development) standard, and compliance with sustainable development visions. A rapeseed oil base has been chosen for its better resistance to degradation in electric fields and its higher oxidation stability. The overall ester properties are not fully competitive with petroleum-based oils and therefore have to be improved. Percolation treatment and oxidation inhibition by a phenolic-type inhibitor is proposed and the resulting final properties are discussed. These resulting fluid properties are further improved using titanium dioxide (TiO₂) nanoparticles with a silica surface treatment. This fluid has properties suitable for use in sealed distribution transformers with the advantage of a lower price in comparison with other currently used biodegradable fluids. Full article

(This article belongs to the Special Issue Engineering Dielectric Liquid Applications)

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16 pages, 9824 KiB

Open AccessArticle

Comparison of Positive Streamers in Liquid Dielectrics with and without Nanoparticles Simulated with Finite-Element Software

by Juan Velasco, Ricardo Frascella, Ricardo Albarracín, Juan Carlos Burgos, Ming Dong, Ming Ren and Li Yang

Energies 2018, 11(2), 361; https://doi.org/10.3390/en11020361 - 3 Feb 2018

Cited by 28 | Viewed by 8630

Abstract

In this paper, a comparison of positive streamer diffusion propagation is carried out in three configurations of oil transformers: mineral transformer oil, mineral oil with solid dielectric barriers, and a nanofluid. The results have been solved using a finite-element method with a two-dimensional (2D) axi-symmetric space dimension selected. Additionally, previous results from other research has been reviewed to compare the results obtained. As expected, it is confirmed that the nanoparticles improve the dielectric properties of the mineral oil. In addition, it is observed that the dielectric solid blocks the propagation of the streamer when it is submerged with a horizontal orientation, thus perpendicular to the applied electric field. The computer used, with four cores (each 3.4 GHz) and 16 GB of RAM, was not sufficient for performing the simulations of the models with great precision. However, with these first models, the tendency of the dielectric behavior of the oil was obtained for the three cases in which the streamer was acting through the transformer oil. The simulation of these models, in the future, in a supercomputer with a high performance in terms of RAM memory may allow us to predict, as an example, the best concentration of nanoparticles to retard the streamer inception. Finally, other dielectric issues will be predicted using these models, such as to analyze the advantages and drawbacks of the presence of dielectrics inside the oil transformer. Full article

(This article belongs to the Special Issue Engineering Dielectric Liquid Applications)

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Review

Jump to: Editorial, Research

31 pages, 9171 KiB

Open AccessReview

Review of Research Progress on the Electrical Properties and Modification of Mineral Insulating Oils Used in Power Transformers

by Xiaobo Wang, Chao Tang, Bo Huang, Jian Hao and George Chen

Energies 2018, 11(3), 487; https://doi.org/10.3390/en11030487 - 26 Feb 2018

Cited by 94 | Viewed by 11342

Abstract

In November 2017, the first ±1100 kV high-voltage direct-current power transformer in the world, which was made by Siemens in Nurnberg, passed its type test. Meanwhile, in early 2017, a ±1000 kV ultra-high voltage (UHV) substation was officially put into operation in Tianjin, China. These examples illustrate that the era of UHV power transmission is coming. With the rapid increase in power transmission voltage, the performance requirements for the insulation of power transformers are getting higher and higher. The traditional mineral oils used inside power transformers as insulating and cooling agents are thus facing a serious challenge to meet these requirements. In this review, the basic properties of traditional mineral insulating oil are first introduced. Then, the variation of electrical properties such as breakdown strength, permittivity, and conductivity during transformer operation and aging is summarized. Next, the modification of mineral insulating oil is investigated with a focus on the influence of nanoparticles on the electrical properties of nano-modified insulating oil. Recent studies on the performance of mineral oil at molecular and atomic levels by molecular dynamics simulations are then described. Finally, future research hotspots and notable research topics are discussed. Full article

(This article belongs to the Special Issue Engineering Dielectric Liquid Applications)

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