Natural Ester Liquids Research Articles

Numerical modeling of positive streamer propagation in alternative natural ester and traditional naphthenic oil under sub-microsecond pulsed voltages

In this paper, a two-dimensional axisymmetric numerical model of streamer discharge in alternative natural ester liquid and conventional naphthenic oil under a positive sub-microsecond impulse voltage is proposed. The model is based on the charge drift-diffusion approximation in a divergent electric field. Simulation shows that streamer propagation in naphthenic oil is divided into two stages: stage I with a small streamer velocity of 0–0.91 km s−1 and stage II with a rapidly rising streamer velocity up to 213 km s−1. In natural ester, the streamer velocity gradually slows down from 4.84 km s−1 to 2.11 km s−1. The electric field at the head of the streamer in natural ester ranges from 2.4 × 108 V m−1 to 2.5 × 108 V m−1 during propagation, which is visibly lower but more consistent than that in naphthenic oil (3.5 × 108 V m−1 to 8.4 × 108 V m−1). It is revealed that the different streamer features within the two types of oil are caused by the different competition between the Laplacian electric field and space charge effect at the streamer head. For streamer propagation in naphthenic oil, stage I is dominated by the Laplacian electric field whereas stage II is dominated by the space charge effect. In natural ester, the whole streamer propagation is dominated by the Laplacian electric field. Further, it is indicated that the streamer head consists of two specific discharging regions, i.e. the front region of the streamer head which guides new space charge accumulation and the back region of streamer head which guides density reduction of accumulated space charge. The expansion of the streamer tunnel is caused by the alternating space charges in these two regions. The work in this paper provides a theoretical reference for the practical application of alternative transformer liquids.

Investigation of electrical properties of developed indigenous natural ester liquid used as alternate to transformer insulation

<p>The <span>performance of every electrical system depends on the different electrical devices especially transformers. Petroleum-based mineral oil is widely used for insulation and cooling purpose. The disadvantage of mineral oil is its low biodegradability and is a major threat to the ecosystem due to its poor oxidative stability. To remedy the drawbacks, focus on alternative fluids that can replace traditional mineral oil. Alternative liquids such as natural esters are used which do not panic the ecosystem. With the support of additives in natural esters liquids, the productivity of the oil can be increased, paving the path for the green conversion of liquids in high voltage applications. The purpose of this article is to analyze the electrical properties of the newly developed indigenous oil. The inhibited oil was insulating oil to which antioxidants were added such as 2,6-ditertiary-butylparacresol, butylated hydroxyl anisole and tertiary butyl hydro qunine to slow down the oxidation rate and to check the electrical properties. This article discusses the electrical properties of mineral oil, developed indigenous oil with and without antioxidants as per IEC62770 standards. A 1.1 kVA transformer was then designed in a laboratory for load tests and Indigenous oil performance under load was evaluated.</span></p>

Influence of Gelling in Natural Esters Under Open Beaker Accelerated Thermal Aging

The present work is focused on the oxidation aspects (focused on gelling) of natural esters and its influence on the performance of transformer insulation systems. Even though natural esters are not suitable for breathing units, the scenario considered simulates a defect in the sealing system and thus bringing the liquid in direct contact with atmospheric air. Commercially available natural ester liquids are thermally degraded under open beaker conditions in the presence of cellulose with a controlled aging history. The accelerated aging conditions are maintained until the formation of sol and gel is evident in the bulk of the liquid. Later, the insulating liquids and insulating papers are analyzed to understand the behavioral impact of sol and gel on the insulation. The discussions in this article are supported by the physicochemical and electrical characterizations of the degraded liquids and papers. The characterizations include Fourier transform infrared spectroscopy (FTIR), ac breakdown voltage (BDV) failure probability rate, X-ray diffraction (XRD), degree of polymerization (DP), and viscosity. It is found that before the formation of gelling, sol is witnessed in the bulk of the liquid. The presence of the sol highly influences the viscosity of the liquid and aids in rapid acceleration of the oxidation process. It is understood that that the gelling phenomena in natural esters do not have a noticeable impact on the liquid BDV failure rate and paper degradation.

Pre-Breakdown and Breakdown Behavior of Synthetic and Natural Ester Liquids under AC Stress

In the last decades, a large focus is being placed on the sustainability and safety of the power transformer spectrum. Ester liquids, which have interesting properties such as high fire point and biodegradability, are gaining needed attraction. Since in-service condition, thermal aging deteriorates the physicochemical and electrical properties of liquid dielectrics, it is important to study their long-term behavior. In this contribution, the pre-breakdown and breakdown behavior of ester fluids (synthetic and natural) under AC stress are investigated. Important characteristics, such as partial discharge pre-inception voltage, partial discharge inception voltage, breakdown voltage, average streamer velocity, and inception electric field, were assessed. The influence of the radius of curvature (of high voltage needle electrode) as well as the thermal degradation of typical ester liquids are also discussed. Mineral oil was also included in the tests loop as a benchmark for comparative purposes. It is found that the pre-inception voltage of ester liquids was, in most cases, higher than that of mineral oil. For a given radius of curvature, the streamer inception and breakdown voltages decreased with thermal aging. During the streamer initiation, the electric field at the electrode tip decreased with the increase in the radius of curvature. The velocity of the streamers seems to increase with the decrease in the radius of curvature. The period of vulnerability, the so-called “delay time”, seems to be independent of the aging or the radius of curvature for a given condition of the liquid.

Evaluation and Interpretation of Dissolved Gas Analysis of Soybean-Based Natural Ester Insulating Liquid

The electrical industry needs a better understanding in the topic related to the fault diagnosis, since these fluids are chemically different in comparison with conventional mineral oils, therefore, its performance in field changes, for example the generated gases. This paper presents an investigation related to the gas generation of a new soybean-based natural ester liquid, which includes a data compilation of distribution and power transformers diagnosis during factory tests by dissolved gas analysis (DGA) method. This work covers an evaluation of the generated gases for three common types of transformer faults (overheating, partial discharge and high energy discharges) at lab scale testing and their interpretation by using different diagnostic methods to predict the fault type. It is found that overheating problems in natural esters can be associated with ethylene, while acetylene and hydrogen are linked for high energy fault and partial discharge faults, respectively. On the other hand, it is confirmed that Duval triangle is the most reliable method to diagnose natural esters liquids. The authors consider that the DGA data presented in this work represent valuable information for the transformer industry.

Thermal Aging Performance of Cellulose Insulation in Natural Ester Liquid

Natural esters have the capability to enhance the life expectancy of the insulating cellulose paper used in oil-filled transformers. This work presents the aging behavior of an insulating paper when it is combined with a new natural ester (based on soybean oil) and compared with an industry-proven system according to the latest version of the standard IEEE Std. C57.100 (2011). The temperatures for the candidate system (natural ester + cellulose paper) are 160, 175, and 190 °C. Experimental results show that the average mechanical tensile retention from the industry-proven system aging system is 44.2%, which is used as reference to evaluate the candidate system. According to this investigation, the insulation system with the new natural ester liquid shows an improvement in the behavior of the aging performance compared to the mineral oil system. The Arrhenius life plots demonstrate that the system of natural ester-thermally upgraded Kraft paper shows a life expectancy higher than for the industry-proven system. Therefore, this alternative could be considered either to increase the lifetime of a transformer or to improve its overload capacity during operation.

Dissolved Gas Analysis in Mineral Oil and Natural Ester Liquids from Thermal Faults

This work addresses the gas formation mechanisms for paraffinic-type mineral oil and soybean-based natural ester with a predominantly linoleic fatty acid under thermal faults. Based on a literature review, possible decomposition paths due to pyrolysis are proposed. A case study of hot spots of different temperatures is presented and its results are compared and analyzed with the gas profiles of the IEEE C57.155-2014 guide. The results obtained for both cases are consistent with the proposed mechanisms. For the same thermal fault, the gas profiles are different. The natural ester presents significant proportions of ethane and carbon dioxide concerning mineral oil. For temperature faults above 700 °C, the behavior of the chemical structures of the natural ester is similar to the carbon structures of the mineral oil and is subject to similar reactions. Therefore, the major differences in the gases generated for thermal faults are at low temperatures.

Impulsive Breakdown of Mineral Oil and Natural and Synthetic Ester Liquids When Containing Varying Levels of Moisture

This article reports and discusses the results of impulse breakdown study of different insulating liquids under highly divergent electric field conditions. Samples of a natural ester (Envirotemp FR3), a synthetic ester (MIDEL 7131), and a naphthenic mineral oil (Shell Diala S4 ZX) at different levels of relative humidity were exposed to HV impulses with a nominal 7- $\mu \text{s}$ rise time and 150-kV peak voltage of both positive and negative polarity. A strong dependence of the breakdown voltage and time to breakdown of the investigated dielectric liquids was observed with respect to the polarity of the applied HV impulses. It was shown that the FR3 natural ester liquid has a higher dielectric strength when exposed to positive impulse than when under negative impulse stress. The opposite breakdown behavior was observed for the synthetic ester MIDEL 7131 and the naphthenic oil Shell Diala S4 ZX which exhibited lower breakdown voltage under positive energization as compared with the negative energization. The breakdown voltage and prebreakdown time obtained in the present tests of the naphthenic oil, Shell Diala S4 ZX, lie between that of the ester liquids irrespective of impulse polarity. It has been established that no statistically significant variations exist in the breakdown parameters (breakdown voltage and time to breakdown) of the studied dielectric liquids as their relative humidity is increased under either impulse polarity. The obtained results will help in coordination of practical applications of low environmental impact dielectric fluids in power and pulsed power systems and components.

Comparison of breakdown voltage of vegetable olive with mineral oil, natural and synthetic ester liquids under DC voltage

This paper deals with a comparative study of the negative and positive DC breakdown voltages of different insulating liquids namely two natural esters (an extra virgin olive oil (OO) without additive and rapeseed oil), two synthetic ester fluids (a methyleolate obtained by transesterification of olive oil and tetraester), and a naphthenic mineral oil (MO); an oils mixture namely 80% OO + 20% MO is also investigated. Statistical analysis of the experimental data is also performed. The obtained results are also compared with those obtained in our previous work under AC and lightning impulse voltages. It is observed that the negative and positive DC breakdown voltages of natural esters are higher than those of mineral oil. On the other hand, the DC breakdown voltage of (80% OO + 20 % MO) mixture shows comparable behavior to that of mineral oil. The DC negative breakdown voltages are higher than the positive ones in the tested liquids. It is also shown that the DC breakdown voltage values of the investigated liquids generally obey the normal distribution.

Statistical Investigation of Lightning Impulse Breakdown Voltage of Natural and Synthetic Ester Oils-Based Fe3O4, Al2O3and SiO2Nanofluids

The characteristics of lighting impulse breakdown voltage (LI-BDV) constitute key performance indicators of insulating materials, especially in insulations based on liquids and liquid-paper for high voltage applications. Encouraged by environmental policies worldwide, new insulating liquids have been developed in recent years. Among these are natural and synthetic esters that are currently considered as potential substitutes for traditional mineral oils due to their environmentally friendly properties. Recent technological advances have shown that the addition of specific nanoparticles (NPs) to insulating liquids can improve both the dielectric withstand voltage and the thermal behavior of liquid insulations. This paper examines the effects of some NPs on the negative LI-BDV of natural and synthetic ester liquids, namely MIDEL 1204 and MIDEL 7131, respectively. The used NPs are Fe3O4, Al2O3 and SiO2. The LI-BDV measurements are executed in accordance with IEC 60060 standard, and the breakdown voltage with 1% probability risk (i.e., the lowest possible breakdown voltage) was also estimated. The experimental results showed that these NPs improved the LI-BDV of both synthetic and natural ester liquids and an optimal concentration of NPs can be determined. Statistical analysis was also performed to check the compliance of the experimental results with the most common probability distributions, normal and Weibull distributions.

On Some Imperative IEEE Standards for Usage of Natural Ester Liquids in Transformers

From the recent past, biodegradable dielectric fluids are becoming popular across the global utilities for use in oil-filled transformers. To date, several utilities have started using biodegradable insulating fluids for new and retro-filled power and distribution transformers. The intent of this article is to compare and analyze various aspects that are involved in the successful operation and maintenance of mineral oil and ester filled transformers. The comparative analysis in this article is aimed at properties, condition monitoring, diagnostic, and reclamation aspects of ester filled transformers. The imperative and pertinent standards, including IEEE Std C57.106, IEEE Std C57.104, IEEE Std 637, IEEE Std C57.147, and IEEE Std C57.155 remain the target objectives for potential analysis. It is hoped that this critical analysis will be useful for utility and condition monitoring engineers in understanding several key aspects of ester fluids vis-a-vis traditional insulating fluids. The present analysis might also be useful for researchers and industry interested in alternative dielectric fluids for transformer insulation technology.

Comparison of creeping discharges propagating over pressboard immersed in olive oil, mineral oil and other natural and synthetic ester liquids under DC voltage

This paper presents an experimental study on the characteristics of creeping discharges propagating over pressboard immersed in olive oil, rapeseed oil, tetra-ester, methyl oleate, and mineral oil, under both negative and positive DC voltages. The investigated characteristics are mainly the shape of discharges and their stopping lengths. Two experimental test cells are used: in the first one, the pressboard is inserted between pointe and plane electrodes so that the tip is perpendicular and at the center of the pressboard sample; and in the second one, the pressboard is placed between a point and bar electrodes in a way that allow a propagation in one direction (tangentially to pressboard). It is observed that the creeping discharges under DC voltage are not radial. Their shapes are different from those observed in previous work under AC and lightning impulse voltages. On the other hand, for given voltage and pressboard thickness, the stopping length L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</sub> is shorter when the point is negative than when it is positive indicating that the flashover voltage will be greater with a negative point This also evidences a difference in the processes involved in each polarity especially the space charge resulting of the injected charges and the charge accumulated at the interface (including the charges resulting of double layer) as well as the evolution of the electric field in the vicinity of the point electrode and along the pressboard surface. For a given pressboard thickness, L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</sub> increases with the voltage; it decreases when the thickness of pressboard is reduced.

Understanding the influence of ambience on thermal ageing of natural ester liquid

Ambience during thermal ageing of ester liquid has a high impact on corona inception voltage (CIV) under AC and DC voltages. The CIV is high with ester liquid thermally aged in nitrogen ambience and is the least with air aged. CIV is high under negative DC voltage followed by positive DC and AC voltages. CIV reduces with increase in harmonic voltage and the reduction is predominant with harmonic voltages with higher total harmonic distortions. Corona activity radiates ultra-high-frequency (UHF) signal with its dominant frequency at 1 GHz. Phase-resolved partial discharge analysis carried out using UHF signal indicates that corona discharges occur at the peak of the voltage waveform. The magnitude of discharges is less in thermally aged liquid but the number of discharges increases. Fourier transform infrared spectroscopy analysis indicates the formation of carbonaceous particles with thermal ageing. Thermal ageing of ester liquid causes to form new chromophoric moieties causing red shift of absorption band in ultraviolet-visible spectra. Fluorescence spectroscopy traces the characteristic variations due to thermal ageing, which can be adopted for condition monitoring.

Significantly Improved Electrical Breakdown Strength of Natural Ester Liquid Dielectrics by Doping Ultraviolet Absorbing Molecules

In this paper, we experimentally demonstrate that molecules with maximum absorption wavelength corresponding to the first excitation energy of base liquid molecules are able to significantly improve the electrical performances of natural ester oils. The positive lightning impulse breakdown and acceleration characteristics, ac breakdown characteristics, and streamer structure of the natural ester oils, as well as the electronic properties of the molecules, are presented. The molecules with different absorptions in ultraviolet lights were doped into the natural ester oil. The positive lightning impulse breakdown characteristics of natural ester oil with different doped molecules indicated that the doped molecule with great absorption in the medium wavelength ultraviolet irradiation was able to significantly enhance the positive lightning impulse breakdown and acceleration voltages of natural ester oil by 15% and 60%, respectively. In addition, it also enhanced the ac breakdown voltage by 17%. The base liquid molecules exhibited first excitation energy of 4 eV, which corresponds to the maximum absorption wavelength of such molecules, indicating that the improved electric breakdown strength was a result of the reduced the number of excited molecules and weakened ionization of molecules during the streamer propagation.

Comparison of dielectric properties of olive oil, mineral oil, and other natural and synthetic ester liquids under AC and lightning impulse stresses

This paper presents experimental results of a comparative study of the characteristics of streamers propagating in olive oil and breakdown voltage with mineral oil, rapeseed oil (natural ester) and tetra-ester and methyl oleate (synthetic ester liquids). The comparison focuses more particularly on the pattern, stopping length, associated current and electrical charge of streamers propagating in a point-plane electrodes geometry under (1.2/50 μs) lightning impulse (LI) voltage as well as breakdown voltages under AC and LI voltages. A statistical analysis of breakdown voltages is also achieved. It is shown that the streamer characteristics (streamers shape, stopping length, associated current and electrical charge) of olive oil in small gaps are comparable to those of mineral oil. Breakdown voltages under AC and LI voltages are found to be higher in olive oil than in mineral oil. Consequently, olive oil can constitute a potential substitute for mineral oil in in high voltage power transformers.

Increased loadability of transformers using natural ester and cellulosic materials as high temperature insulation systems

The use of natural ester–filled transformers started in 1996, with two industrial transformers installed at the Cooper Power System factory to provide power for two core annealing ovens. Investigations conducted according to the IEEE C57.100 accelerated transformer life test method (Lockie Test) [1] and the sealed tube accelerated insulating system material life test method [2]–[4] indicated a possible extension of transformer life, associated with reduction of the paper degradation rate when impregnated with natural ester liquids.

Lightning Impulse Withstand of Natural Ester Liquid

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.

Teardown of a compact distribution transformer after 12 years of severe loading conditions

In 2003, CPFL Energia, a Brazilian utility, initiated investigations for high temperature compact distribution transformer, using Natural Ester liquids. Based on a 45 kVA transformer, a final rated capacity of 88 kVA has been reached after modifications in the cooling ducts and radiators. Two prototypes were installed at a heavily loaded network point, replacing mineral oil filled transformers of 112.5 kVA. After 12 years of continuous operation, one of the prototypes was removed for the study, while the other remains in service. The transformer was still in perfect operational conditions, submitted to an average loading along the 12 years of 82 kVA and peak loading reaching 123 kVA (273.3% of the original 45 kVA rating). A complete teardown was performed and samples of main insulating materials were assessed by two different laboratories, one in Brazil and one in USA. The visible wearing of the external painting is the main indication of the years, while the core and coil, when untanked, gave the impression of handling a brand new transformer. Full details will be presented in this article. Based on this long term/real life test, the long-term performance of the materials at high temperature operating conditions has been confirmed.

Comparative analysis of lightning breakdown voltage of natural ester liquids of different viscosities supported by light emission measurement

This article presents the results of the studies on lightning impulse breakdown voltage measurements performed for two commercial natural ester liquids of different viscosities. These viscosities measured at 100°C were 8.0 mm2/s and 4.6 mm2/s respectively. The dielectric test results presented in this paper concern the measurements performed using the recommendations of the IEC 897 standard where point-to-sphere electrode system is dictated. In addition to measuring the breakdown voltage other experimental techniques were applied such as time to breakdown estimation, light emission registration using the photomultiplier and static photograph collection. The results showed that the differences between the liquids tested concern only positive polarity of lightning impulse voltage. In the case of negative polarity from the standard based calculations and statistical analysis using Weibull distribution the breakdown voltages may be treated as a very similar. In turn the other discharge characteristics such as light waveforms, static photographs and propagation velocities did not showed any differences. In the case of positive lightning breakdown voltage the results were assessed only on the basis of the average values calculated because a deep statistical analysis was not possible due to the small scattering of data. These results indicated on slightly better lightning properties of the natural ester of higher viscosity. Confirmation of this conclusion shall constitute the light waveforms registered where the discharges developing in the low viscosity natural ester are characterized by higher intensity than corresponding discharges developing in the “classical” natural ester. However the static photos did not indicate any differences between the esters tested.

Revised sealed tube test procedure for thermal verification of liquid-immersed transformer insulation systems

Over the last few years, there have been numerous claims that the thermal performance of cellulose can be improved by the presence of ester liquids. However, solid verification of this phenomenon by a recognized, standardized test procedure is still missing. The article will present a summary of different historical studies that suggest some improvement of cellulose when immersed in ester liquids. Inaccuracies of these studies and a shortage of some important data will also be discussed. The recent industry discussion on the actual thermal performance of cellulose in natural or synthetic ester liquids resulted in a need for performing thorough studies on this subject. It was agreed within IEC Technical Committee 14 “Power Transformers”, that comprehensive studies shall be made. It is likely that the sealed tube test methodology discussed in this article could be used for developing more systematic conclusions on cellulose-ester thermal performance.A number of standardized test procedures were proposed in the past for new insulation system thermal evaluation. They included the Lockie Test, sealed tube testing and other laboratory testing methods. The Lockie Test was one method used to verify the higher thermal capability claimed for this new insulation system. However, the method uses complete transformers in the procedure and as such is specific to the individual manufacturer’s design. More recently, the dual temperature test has been proposed and has been shown to be a useful procedure, although the equipment necessary is very specialized and complicated. However, this procedure only tests the wire insulation, although it is still the best procedure for testing a system with widely differing thermal capability between the liquid and the solid insulations.The latest revision of IEEE Std C57.100 defines the Lockie Test procedure along with the dual temperature test and the sealed tube test. The sealed tube test was originally developed only as a means of determining equivalent material substitution and has been maligned in the past as an inadequate laboratory test. As with all simulated accelerated testing, this procedure certainly has its weaknesses. However, the 2011 revision of IEEE Std C57.100 now uses a relative thermal index technique rather than the original absolute method, mitigating one of the most problematic issues with the procedure. This new version also addresses many of the other weaknesses of the original method. The end of life criteria is based on either degree of polymerization (DP) or retained tensile as the determining factor. However, since DP is specific to cellulose products, tensile retention is suggested as the end of life criteria which works universally for almost all solid materials for both power and distribution transformers.An example of a new insulation system based on aramid enhanced cellulose paper immersed in mineral oil will be presented in detail, in order to clarify the procedure of full thermal evaluation following the IEEE Std C57.100-2011 sealed tube test procedure.