Synthetic Ester Liquids Research Articles

Temperature Dependency of Ion Mobility in Synthetic Ester by Dielectric Response Analysis in Time and Frequency Domain

Synthetic ester is a promising alternative to mineral oil for emerging applications relying on high-power static conversion, such as solid-state transformers (SST). The highly compact components used in SSTs must fulfill MV-DC insulation requirements to guarantee the safe operation of the system, therefore, their design relies on the characterization of the dielectric stress caused by the electric field. The prediction of the electric field induced by DC voltages requires analyzing the conduction phenomena in the liquid, including the effect of charge transport due to ion drift. In this scenario, one of the critical parameters that determine the transport of charge and the resulting electric field distribution is the mobility of ions μ; the value of μ in synthetic ester is available only at room temperature, however, investigations on other dielectric liquids indicate a significant temperature dependency, which therefore needs to be characterized. In this paper, the temperature dependency of μ in synthetic ester liquid is determined by comparing alternative methodologies based on the analysis of the dielectric response in the time and frequency domain, and the viscosity of the fluid. The characteristics obtained by these methodologies are consistent; the temperature dependency of μ obeys the Arrhenius law and an increase of one order of magnitude starting from 1.1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">–10</sup> m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs is observed in the 303K–353K temperature range.

Streamer and Breakdown Characteristics of Dielectric Liquids Under Impulse Waveforms With Different Tail-Times

Impulse waveforms have been widely used in the studies of streamer and breakdown phenomena in dielectric liquids. In this article, both experiments and analyses were carried out to investigate the effects of impulse tail-time on streamer and breakdown characteristics of a mineral oil and a synthetic ester liquid. A range of positive impulse waveforms with the same front-time of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.8~ \boldsymbol {\mu } \textrm {s}$ </tex-math></inline-formula> but different tail-times from 8 to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3200~ \boldsymbol {\mu } \textrm {s}$ </tex-math></inline-formula> were considered. Needle-plane electrodes with a needle tip radius of 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\boldsymbol {\mu } \textrm {m} $ </tex-math></inline-formula> and a gap of 10 mm were used. It was found that reduction of tail-time leads to an increase of 50% impulse breakdown voltage, while the instant breakdown voltage and the time-to-breakdown remain constant. On this basis, an analytical method is developed to estimate the 50% impulse breakdown voltage under waveforms with various tail-times. The method is valid when streamers remain of second mode and in divergent fields. It was also observed that some “self-regulation” by branching explains the constancy of streamer propagation velocity when the applied voltage is increased.

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.

Pre-breakdown Phenomena and Influence of Aging Byproducts in Thermally Aged Low Pour Point Ester Fluids Under AC Stress

Application of low pour point esters in transformers insulation serving in cold climatic regions is still a challenge. To address this concern, the pre-breakdown and breakdown analyses of thermally aged low pour point ester fluids are investigated. The breakdown measurements are performed at different aging conditions under AC stress. Needle-plane electrode geometry (with varying radii of the tip) is employed since this configuration is necessary for any practical diagnostic based on partial discharge (PD). This allows studying the effect of the chemical impurities (aging by-products) as assessed by some physicochemical properties (acidity, particles count, turbidity, and dissolved decay products) on the partial discharge and breakdown characteristics. The complete analysis is carried out for liquids having the pour points less than -50 °C, including a mineral oil (MO) and two synthetic ester liquids (SE1 and SE2). Changes in the streamer properties, such as partial discharge inception voltage (PDIV), breakdown-streamer inception voltage, and breakdown voltage are reported as a function of the needle tip radius and aging condition. It is observed that the breakdown properties of SE1 are better than that of SE2 and are comparable to MO. To understand the influence of aging on the PD, correlations between decay products, acidity, BSIV, and PDIV are reported. It is observed that the acidity and concentration of decay products directly impact the PD characteristics.

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.

A Review on Synthetic Ester Liquids for Transformer Applications

Synthetic esters have become more and more popular in last few decades, explaining the increasing number of units filled with this liquid year by year. They have been investigated under different aspects, both from the fundamental point of view and breakdown mechanisms, well as from the application point of view. However, their use in high voltage equipment is always a challenge and deeper knowledge of the various aspects that can be encountered in their exploitation is needed. The intent of this review paper is to present the recent research progress on synthetic ester liquid in relation to the selected issues, most important for ester development in the authors’ opinion. The described issues are the breakdown performance of synthetic esters, lightning impulse strength and pre-breakdown phenomena of synthetic esters, synthetic esters-based nanofluids, combined paper-synthetic ester based insulating systems, application of synthetic ester for retro-filling and drying of mineral oil-immersed transformers, DGA(dissolved gas analysis)-based diagnosis of synthetic esters filled transformers as well as static electrification of synthetic esters. The different sections are based both on the data available in the literature, but above all on the authors’ own experience from their research work on synthetic ester liquids for electrical application purposes.

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.

Lightning impulse breakdown voltage of synthetic and natural ester liquids-based Fe3O4, Al2O3 and SiO2 nanofluids

This article deals with the study the negative lightning impulse breakdown voltage (LI-BDV) of MIDEL 1204 natural and MIDEL 7131 synthetic ester liquids when adding small amounts of specific nanoparticles (NPs); the considered NPs being magnetic (Fe3O4) and insulating (Al2O3 and SiO2) NPs. It is shown that these NPs enhance the LI-BDV of investigated esters. In addition, for a given type of NPs, there is an optimal concentration for which the U50% LI-BDV is the highest. This latter is enhanced by about 16.8% for natural ester with Al2O3 at a concentration of 0.05 g/L; and by about 25.6% for synthetic ester with Fe3O4 at a concentration of 0.05 g/L.

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.

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.

Effects of temperature on partial discharges and streamers in an ester liquid under AC stress

Insulating liquids in transformers experience the transformer operational temperature rather than the ambient temperature that is often used in discharge studies in laboratories. This paper presents the effects of temperature on partial discharge (PD) and streamer characteristics of a synthetic ester liquid under AC stress. A needle-to-plane electrode configuration was employed to perform the PD and streamer measurements. The PD pattern, streamer images, current signals were chosen to explore the PD and streamer characteristics at temperatures of 20, 40, 60 and 80 °C. It was found that both PD and streamer characteristics of the synthetic ester are temperature dependent. As the temperature increased, the magnitude and repetition rate of positive PDs increased slightly, but both the magnitude and repetition rate of negative PDs dropped dramatically. In addition, it was observed that when the temperature increased, the stopping length of positive streamers did not change appreciably while a significant reduction of negative streamer length and area was witnessed. A mechanism based on the electrohydrodynamics (EHD) phenomenon and the space charge theory has been used to explain the different effects of temperature on positive and negative PDs.

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.

Streamer characteristic and breakdown in a mineral oil and a synthetic ester liquid under DC voltage

This paper presents experimental studies of streamer propagation and breakdown in a mineral oil and a synthetic ester liquid under both positive and negative DC voltages. A point-plane electrode configuration with a point tip radius of 10 μm was used. Characteristics of streamer including length, propagation velocity and shape were analysed based on shadowgraph images obtained at a gap distance of 10 mm using a multi-channel ultra-high speed camera. Under positive polarity, the streamers in both liquids propagate in the 2nd mode; the stopping length of the synthetic ester is longer than that of the mineral oil at the same applied voltage level. However there is no obvious streamer propagation (less than 10% of the gap distance) under negative polarity even when the applied voltage approaches to the breakdown voltage; the streamers in both liquids stay in the 1st mode. Breakdown tests at various gaps of 2 mm, 5 mm, 10 mm, 20 mm and 30 mm were also investigated. 50% breakdown voltages of the synthetic ester liquid are lower than those of the mineral oil at all the gap distances investigated under both polarities.

Lightning properties of selected insulating synthetic esters and mineral oil in point-to-sphere electrode system

This article presents the results of the studies on lightning properties of two synthetic ester liquids and mineral oil. The measurements concern the standard lightning impulse voltage at both polarities and a point-to-sphere electrode system representing highly divergent electric field distribution. The studies were based on the IEC 60897 Standard wherein the standard recommendations were extended to include light emission registration using a photomultiplier technique. Comparisons of the lightning impulse breakdown voltages (LIBV), times to breakdown (TtB) and light waveforms were made. The results concerning LIBV and TtB showed that at negative polarity both the esters have almost the same lightning characteristics which are slightly below the characteristics of mineral oil. At positive polarity the results indicated an improvement. Regarding the light emission, the mineral oil tested seemed to be more resistant against the appearance of fast light pulses indicating the intense stepwise propagating discharges.

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.

Dielectric properties of natural ester, synthetic ester midel 7131 and mineral oil diala D

The insulating liquids used in industrial applications are typically mineral oils. In recent years however, significant attention has been paid to alternative insulating fluids, including synthetic and natural ester liquids. In order to expand their practical applications, it is important to have detailed information on their dielectric properties. In this present paper, the dielectric properties of synthetic ester, Midel 7131; mineral oil, Shell Diala D; and vegetable (rapeseed) oil have been investigated. It has been shown that Midel 7131 has a higher ac breakdown voltage (27.6 kV) as compared with Diala D oil (26.4 kV) and rapeseed oil (24.6 kV). However, the breakdown voltage of the Diala D oil has the smallest standard deviation (7%) amongst the tested liquids (13% for Midel 7131 and 11% for rapeseed oil). Statistical analysis of the breakdown voltages has been conducted and it has been shown that the ac breakdown voltages can be described by a normal distribution. dc I-V characteristics have been measured and the space charge saturation regime has been observed for all three liquids starting from ~9 kV for positive energisation and ~10 kV for negative energisation in the point-plane topology. Apparent mobilities of the charge carriers in the tested liquids have been obtained using I <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> -V curves; these mobilities can be used for calculation of the space charge influenced distribution of the electric field in liquid insulators stressed with dc voltage. Such analysis can be important for design and exploitation of HVDC power systems.

Streamer characteristic and breakdown in synthetic and natural ester transformer liquids with pressboard interface under lightning impulse voltage

This paper presents experimental studies on streamer and breakdown of synthetic and natural ester liquids with the presence of pressboard interface in a divergent point-plane field under lightning impulse voltage. Based on the current, light signals and streamer images, characteristics of both positive and negative streamers on pressboard surface were described, which resemble the features of streamers in open liquid gaps. Introducing pressboard in parallel does not influence the streamer stopping length and thus not weaken the oil gap breakdown voltage, under both positive and negative polarities at gap distances up to 75 mm. However, presence of parallel pressboard significantly reduces the acceleration voltage in mineral oil but seems have no influence on ester liquids under positive polarity, while it tends to accelerate the streamer velocity at larger gap distance and under higher voltage in both ester liquids and mineral oil under negative polarity. Generally, streamer on ester/pressboard interface propagates further than on mineral oil/pressboard interface at the same voltage level; both breakdown voltage and acceleration voltage on ester/pressboard interface are lower than those on mineral oil/pressboard interface at the investigated gap distances up to 75 mm.

Secondary reverse streamer observed in an ester insulating liquid under negative impulse voltage

The so-called ‘secondary reverse streamer (SRS)’ in this study depicts a newly observed streamer-in-liquid phenomenon which subsequently occurs well after the extinction of the primary streamer (PS) propagation within a single shot of negative impulse voltage and has reverse polarity to the PS. Tests were carried out in a synthetic ester liquid at a 50 mm needle–plane gap under negative impulse voltages up to 140 kV. Based on the current and light waveforms as well as the photographic images, two types of SRS were found: one has a bright trunk with a high velocity of 9.79 km s−1 belonging to the 3rd mode streamer; and the other has a weak tree-like channel with a low velocity of 1.28 km s−1 resembling the 2nd mode streamer. It should be emphasized that SRS under negative impulse voltages has positive polarity being opposite to the negative PS, which is believed to be due to the reverse electric field induced by the residual space charges left by the PS. This has been confirmed by further verification tests under tail-chopped impulse voltages, for SRSs are significantly enhanced in both axial and lateral directions after the early removal of the external applied field.

Preliminary investigations for the retrofilling of perchlorethylene based fluid filled transformer

The main results from investigations aiming into the retrofilling conditions of a transclene (perchlorethylene based liquid) filled transformer are presented. The investigations were carried out on insulating liquids with less environmental risks than perchorethylene (Perc) but of almost similar electrical properties which were a mineral based transformer oil and a synthetic ester liquid. Also the influence of a small detachment of hydrochloric acid (HCl), which could be formed by Perc and water during operation is discussed. It has been generally acknowledged, that the life of an insulating liquid in service depends primarily on its initial quality, but service conditions need to be considered too. Additionally, the influence of aging was investigated. A comparison of the measured data with the pre-determinations of the relevant standards and the consequences for the use of the mixtures in transformers are presented.