Oct 30, 2012 - with mineral oil, as this is a fluid we are familiar with. ... Index Terms â Power transformers, mineral oil, synthetic ester fluid, Electrostatic.
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M. Talhi et al.: Comparative Study of the Electrostatic Charging Tendency between Synthetic Ester and Mineral Oil
Comparative Study of the Electrostatic Charging Tendency between Synthetic Ester and Mineral Oil 1
M. Talhi1, I. Fofana2 and S. Flazi1
University of Sciences and Technology of Oran – Mohamed Boudiaf (USTO MB). BP 1505 EL M’nouar Oran 31000, Algeria 2 Canada Research Chair on Insulating Liquids and Mixed Dielectrics for Electrotechnology (ISOLIME), Université du Québec à Chicoutimi, Québec, Canada
ABSTRACT The growing demands for improved fire safety, source material sustainability, environment friendliness, and asset life extension have driven the research and development efforts of synthetic esters, less-flammable fluids. However the relatively limited number of work on these esters and their cost still limit their uses to specific applications. This contribution reports some investigations onto the Electrostatic Charging Tendency (ECT) of a commercially available ester fluid. Comparison is made with mineral oil, as this is a fluid we are familiar with. Fresh (unaged) and aged fluid samples were investigated in a spinning disk system under laboratory controlled conditions. Changes in static electrification were compared to some aging indexes (interfacial tension, turbidity, relative dissolved decay products). This contribution is not only intended to provide a fresh review in this domain of research, but also contains a substantial amount of new material with a view of closing some gaps in the present state of knowledge of oil streaming electrification. The obtained results show that static electrification currents are affected by fluid motion velocity, type of paper, moisture content and the aging byproducts. It was also found that the removal of dissolved oxygen by nitrogen blanketing contributed to the reduction in static electrification. Index Terms — Power transformers, mineral oil, synthetic ester fluid, Electrostatic Charging Tendency, Dissolved Decay Products, Turbidity, Spinning disk system.
1 INTRODUCTION IN today’s global warming conditions, environemental concerns are being strong factors to take into account in all engineering fields. Environmental impacts (toxicity of the products) in terms of accidental rejection and treatment at the end of the lifetime of the insulation systems (recycling, re-use, elimination, incineration, destruction) are therefore essential components to be taken into account. In power transformers, insulating fluids are used alone as electrical insulation only in areas of a transformer where, by design, voltage stresses are relatively low. Whereas solid insulating materials are used in transformer areas where voltage stresses are high, or where a particular physical configuration is needed. These solid insulating materials, commonly used as wrapping and spacers, are cellulose papers and boards made with special care from wood pulps. Specially chosen wood and dense synthetic polymers are generally used as insulating support structures. These porous papers, pressboards, and wooden parts have to be processed under heat and vacuum to remove moisture and air, before they are impregnated with oil to increase their resistance to electrical breakdown. When the insulating paper is adequately Manuscript received on 30 October 2012, in final form 8 July 2013.
impregnated with oil, it offers the user a material with insulating and mechanical properties of remarkable suppleness. The ready supply of cellulose and mineral oil has, therefore, made these the materials of choice for nearly a century [1]. Power transformers which employ forced cooling to pump the oil from the transformer and through a cooling system, are sometimes beset by problems arising from the electrostatic charging of the oil [1-3]. This phenomenon only occurs when there is relative motion between the fluid and any surface in contact with it. Such surface can be stationary, as are the pressboard, paper, metal and pipe-work of a transformer or in motion, as are the moving parts of a pump. Static charges are produced at the interface of solid / liquid flow of oil causing a portion of the electrical double layer formed by preferential absorption of negative ions by the cellulose surface [1-5]. Thus, paper has generally a negative charge and oil positive charge [6-9]. Increase in the velocity of the circulating oil for greater cooling and also compact designing owing to space constraints; have further augmented the phenomena in power transformers. The static electrification processes are complex and depend on the properties of the oil/solid material interface constituting the electric charge double layer seat and its evolution in time. The characteristics of this latter depend in turn on the physicochemical properties of oil (ageing,
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IEEE Transactions on Dielectrics and Electrical Insulation
Vol. 20, No. 5; October 2013
moisture, charges carriers) and the intrinsic properties and surface condition (geometry, roughness, porosity...) of the solid insulation as well as external parameters such as the streaming velocity of oil and temperature. The measurement of the electrostatic charging tendency (ECT) in power transformers is still as important as about fifteen years, when research on the static problems of electrification started. Static electrification continues to be a significant worldwide problem jeopardizing the reliability of large forced-oil-cooled shellform and core-form transformers [10, 11]. Even after extensive research all over the world, flow induced electrification phenomena still remains poorly understood. The changes in the bulk characteristics of the oil/cellulose system with varying load conditions in service makes it very difficult to comprehend the phenomena. Previously published work demonstrates the suitability of substituting synthetic ester fluids (introduced in the 70s to replace mineral oil in transformers as a safer alternative where fire safety and protection of the environment are primary considerations) for mineral oil in liquid-paper insulation systems [12, 13]. Esters are a broad class of organic compounds synthesized from organic acids and alcohols. The two main categories are synthetic and natural esters. Synthetic esters, most commonly polyol (pentaerythritol), are generally limited to traction and mobile transformers and other special applications due to their high costs compared to other lessflammable fluids. This comparative study was undertaken to better understand and quantify the streaming electrification of a commercially available synthetic ester fluid.
2 EXPERIMENTAL PROCEDURE The investigations were performed using a spinning disk system designed in our laboratory, in which the disk is covered on both sides with different cellulose paper. This system has been adopted by CIGRE for international comparative measurements of both insulating liquid and solid transformer materials [1, 14, 15] and is relatively often used in streaming electrification studies [16-18]. A disc with a diameter of 40 mm and a thickness of 5 mm was used in these investigations. The spinning disk system and the electrometer are placed in a Faraday cage (Figure 1). The rotating disk was driven by a proportional-integral (PI) based speed controlled DC motor. The rotating velocity of the disk was varied between 100 to 600 rpm. The container as well as the rotating disk was made of aluminum. The temperature of oil was set and controlled within the range of 20 0.1 °C using a heating system. The electrification currents and the rotation speed measured using an encoder, were simultaneously recorded via a data acquisition Excelinx system developed by Keitley. The data are stored as excel files to ensure further analyses using other software applications. The static electrification current (leakage current) created by the charge concentration gradient was measured with a programmable electrometer (Keitley 6514) inserted between the tank and the earth. The spinning disk system is placed in a Faraday cage. Due to the centrifugal force, the charges created by the rotating motion of the disk in oil are drained toward the tank
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wall where they are collected. The streaming current was measured as leakage current (in pA), to ground from the container using an electrometer inserted between the tank and the ground. 9 1 2 5
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5 10 Electrometer
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Figure 1. Diagram of the system with a spinning disk for studying the electrification of insulating liquids: (1) DC motor; (2) rotating mandrel; (3) Faraday cage; (4) cover; (5) insulator; (6) liquid; (7) disk; (8) measurement container (9) encoder, and (10) coaxial cable.
The electrometer, based on the National Semiconductor LMC6001 BiFET op-amp, had an input resistance 1015 Ω and an input bias current no greater than 25 fA. The 1000 MΩ feedback resistor made possible measurements as low as ±5nA. The 500 kΩ input resistor limited input current to safe levels in cases of significant electrostatic potential on the electrometer input. Two different papers were used to cover the rotating disk: (i) pressboard having a thickness of 1 mm, and (ii) and a 1 mm thick kraft paper. These paper samples were vacuum dried in an oven at 105 °C for 48 hours, and then impregnated with dehydrated, degassed naphtenic type based inhibited oil or ester fluids. Insulation life is normally determined by measuring the time to breakdown. Doing this in "real time" would have been rather exhausting, given that transformer insulation systems are expected to last several decades before failure occurs. It is therefore appropriate to perform a sealed vessel ageing procedure, whereby the ageing process is accelerated in laboratory tests in order to greatly reduce the lifespan of liquid and/or solid insulation systems. A sealed vessel ageing procedure is more rapid, less expensive, and provides samples with different properties. It is also more amenable to the exploration of the materials and their condition during ageing. To assess the impact of aging on the ECT, fluids and impregnated paper samples were aged in laboratory conditions by placing the specimens in a convection oven at 115°C and heating them for different periods. The electrostatic charging tendency (ECT) was studied for two different fluids: a pentaethryol-based synthetic ester fluid and a naphtenic based mineral oil. Table 1 summarizes some of the main properties of both fluids.
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M. Talhi et al.: Comparative Study of the Electrostatic Charging Tendency between Synthetic Ester and Mineral Oil
For each sample of fluid, the electrification current was registered at different rotational velocities. At each velocity, the current were registered with a sampling rate of 210 samples / min. From each registered samples set, the mean value of electrification current was calculated, that was used in the further analysis. In addition to the static electrification the dielectric properties were assessed, i.e. the moisture content by Karl Fisher titration [19], turbidity in oil by a ratioturbidimeter [20], the interfacial tension (IFT) [21] and the dissolved decay products (DDP) by UV/Vis spectrophotometer [22]. Table 1. some technical data of the tested fluids. ASTM TESTS Dissipation factor @ 60Hz, 100°C, D924 @ 50Hz, 90°C, IEC 60247 Breakdown voltage (kV) ASTM D 877 CEI 60156 Volume resistivity (10-12 .cm), IEC60247 Gassing tendency (µL/min), D 2300B Water content (ppm), D 1533 Interfacial Tension (dynes/cm @ 25°C), D 971 Total Acid Number Viscosity (cSt @ 40°C ), D 445 color, D 1500 Flash point (°C), D 92 Pour point (°C), D 97
Mineral oil
Ester -
0.07
