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Long-term and fault-free operation of power transformers depends on the electrical strength of the insulation system and effective heat dissipation. Forced circulation of the insulating liquid is used to increase the cooling capacity. A negative effect of such a solution is the creation of the phenomenon of streaming electrification, which in unfavorable conditions may lead to damage to the insulating system of the transformer. This paper presents results of research confirming the possibility of using fullerene C60 to reduce the phenomenon of streaming electrification generated by the flow of liquid dielectrics. The volume charge density qw was used as a material indicator to determine the electrostatic charging tendency (ECT) of nanofluids. This parameter was determined from the Abedian-Sonin electrification model on the basis of electrification current measurements and selected physicochemical and electrical properties of the liquid. The electrification current was measured in a flow system with an aluminum pipe of 4 mm diameter and 400 mm length. All measurements were carried out at a temperature of 20 °C. The influence of flow velocity (from 0.34 m/s to 1.75 m/s) and C60 concentration (25 mg/L, 50 mg/L, 100 mg/L, 200 mg/L and 350 mg/L) was analyzed on the electrification of fresh and aged Trafo En mineral oil, as well as Midel 1204 natural ester and Midel 7131 synthetic ester. The density, kinematic viscosity, dielectric constant, and conductivity were also determined. A negative effect of the C60 doping on the electrostatic properties of fresh mineral oil was demonstrated. For other liquids, fullerene C60 can be used as an inhibitor of the streaming electrification process. Based on the analysis of the qw parameter, the optimum concentration of C60 (from 100 mg/L to 200 mg/L) resulting in the highest reduction of the electrification phenomenon in nanofluids was identified. [ABSTRACT FROM AUTHOR]

Extensive efforts have been made for many years by the power generating industry to replace conventional solid and liquid insulation with synthetic materials. Those measures are aimed at increasing the load capacity, improved fire safety and extending transformer life during exploitation. Modern insulating materials include aramid fibre-based paper and insulating fluids made of synthetic and natural esters. The paper presents research results of the electrostatic charging tendency (ECT) of mixtures of fresh and aged mineral oil Trafo En with synthetic ester Midel 7131 and natural ester Midel 1204. The measurements were taken in a flow-through system using the pipes made of metal, cellulose and aramid paper. The influence of the liquid flow velocity, the type of material of the measuring pipe and the mixture content on the level of the streaming electrification current generation was determined. [ABSTRACT FROM AUTHOR]

In this paper, a device with swinging plate electrodes has been proposed to measure contact electrification of a liquid sample. The proposed structure is composed of two parallel metallic plates immersed in a dielectric liquid. One of the plates is swinging with a constant frequency in a range from 0.4 to 4 Hz. The paper investigates the dependence in time and frequency of electrode velocity and streaming electrification. The measured current occurs for a very low intermittent velocity of less than 10 mm/s. In this range, the electrification current is around 50 pA. For higher velocities of up to 150 mm/s, the current is at the level of 1200 pA. The time–frequency characteristic using short-time Fourier transform shows no temporal changes in the frequency spectrum. The dependence of electrification on shuttle speed was calculated and it can be approximated with a second order polynomial model with the determination coefficient higher than 0.9. The advantage of the sensor is the ability to measure electrification phenomena without the necessity of having rotating electrodes or having a large volume of flowing liquids.

The subject matter of this study was the problem of the ECT (electrostatic charging tendency) of mineral insulation oils during their flow. The electrostatic charges generated may lead to partial discharges, and as a consequence, to the breakdown of a power transformer insulation system. In this study, the results of the ECT of mineral oils used in transformers were compared. The method of streaming electrification of insulation liquids using a flow-through system was used. The influence of flow speed, temperature, and the pipe material on the values of the electrification current and volume charge density qw were analyzed. The results obtained in this study should be taken into account regarding the operation of power transformers.

Copyright of Przegląd Elektrotechniczny is the property of Przeglad Elektrotechniczny and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Przegląd Elektrotechniczny is the property of Przeglad Elektrotechniczny and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The paper presents results of measurements and analysis of the flow electrification process in the Swinging Cylinder System. This system is similar to the Couette system. The proposed system has the ability to perform different movement function of the internal electrode. The identification of the electrification process was made based on small step movements of the electrode in both directions. The battery powered measurement device was mounted inside the movable inner cylinder. In addition, it allows for the wireless transmission of the measured data. A series of the electrification current measurements were made in response to the impulse changes of the speed of the measuring electrode. The identification of simple process models of first and second order was made on the basis of the results. [ABSTRACT FROM PUBLISHER]

The article presents the results of measurements and analysis of the flow electrification process. The research is presented with a new type of measurement instrument, in which one of the electrodes moves in an oscillating manner. The arrangement comprises two parallel plates and the liquid is relocated in one dimension. One of the electrodes is in the form of a copper wire. As a result, the liquid distribution is assumed as very simple and similar to the classic Stokes boundary layer. The electrification current measurements for different oscillation frequencies were performed in such an arrangement. The basic characteristics of the measured current and the determined process models were tested. The nonlinear approach and the linear approach were used. In the case of the nonlinear mode, the displacement of electrode was employed as an input. Hammerstein's mode with a piecewise linear input nonlinearity estimator was used for identification. In the case of the linear mode, the calculated velocity magnitude was used as an input into the system. The discrete time Output Error model and the Transfer Function model were used as a linear model. In each case, very good compliance was obtained between the simulation responses and the actual current measurements. [ABSTRACT FROM PUBLISHER]

This paper is related to the phenomenon of streaming electrification. The selected results of nonlinear identification of the electrification process are presented. We described the signals used for identification and validation and then two structures of the models used: the nonlinear autoregressive model and the Hammerstein-Wiener model. We then conducted parametric identification and compared the obtained results. We used the Gauss function as the input signal. In the case of the autoregressive model, we used the sigmoid network to estimate the nonlinearity. In the case of the Hammerstein-Wiener model, we tested various types of input nonlinearities. The piecewise linear nonlinearity estimators turned out to be the most effective. Results for the most effective models of both structures: the results of the nonlinear ARX model and the Hammerstein-Wiener model were similar and had varying fit values in the range of 1 to 3 percent. They also had similar generalisation features. In all cases, we also obtained similar shapes of the processes of residues and correlation functions. They indicate a relatively faultless representation of the responses of models as well as certain differences, especially in the end part of the processes representing the given process. [ABSTRACT FROM PUBLISHER]

Determining the Electrostatic Charging Tendency (ECT) level of insulating liquids requires no large and therefore costly transformers or modelling systems. Small laboratory systems are characterised by simple design and methodology, low volume of liquid sample required for testing, low cost of construction and the following tests. This also applies to pipe flow systems. This paper presents comparative studies of the phenomenon of streaming electrification of mineral TRAFO EN insulating oil and MIDEL 7131 synthetic ester flowing through a pipe made of conventional cellulose paper and modern aramid paper. Electrification current vs. liquid flow speed was measured. The influence of pipe material was determined. Also the effect of temperature and sample aging time on the extent of the streaming electrification was studied. With the Abedian-Sonin model, volume density of the qw charge, which is a parameter describing ECT of insulating liquids, was determined. [ABSTRACT FROM AUTHOR]

The paper presents an analytically derived model of electrification phenomena in turbulent flow. Mechanisms of diffusion, convection, and conduction were taken into account. Distributions of current density where determined for the whole range of liquid conductivity and viscosity. The equations are derived assuming an isothermal incompressible Newtonian flow in a pipe in the steady state in time and space. [ABSTRACT FROM AUTHOR]

We have investigated the performance of silicone oil as an insulation medium with the aim of developing environmentally-friendly, flame-retardant electric power equipment. In this paper, the insulation characteristics of silicone oil and ester oil, which is the alternative insulating oil candidate, have been investigated and compared. The insulation characteristics examined in this paper are the streaming electrification characteristics including electrification phenomena generated when an insulating oil streams, dielectric breakdown characteristics with increasing temperature and the surface breakdown characteristic when the insulator is placed between electrodes. The range of temperature investigated was 30 to 130 °C. Both kraft and aramid paper were used for the insulator. In the comparison between silicone and ester oil, it was found that the insulation performance of the ester oil was better than that of the silicone oil. We have considered possible mechanisms for these results. [ABSTRACT FROM AUTHOR]

The paper presents streaming current distributions measured in an overflow system in both laminar and turbulent conditions. For lower values of Reynolds number the profile of current is parabolic. For higher values of Reynolds number the profile is flattened. In this paper also charge density distributions were determined in both cases. In the turbulent regime the volume charge density is distributed in the pipe more evenly compared with the laminar regime. [ABSTRACT FROM AUTHOR]

The mobility of charge carriers is the parameter used for describing conductivity caused by the electrification activity. It is studied for substances of various states of aggregation, in the range including gases, liquids and solid bodies, for conductors, semi-conductors and dielectrics. The knowledge of this value is of great significance for the research work on insulation liquids. Transformer insulation oils, where due to the range of interest this issue is especially significant, serve as a good example. The paper presents a new method of the research on the mobility of charge carriers in insulation liquids, based on the application of the streaming electrification measurements of such liquids. [ABSTRACT FROM AUTHOR]

In this paper the distribution of electrification current flowing across a pipe, for alternating charge near a surface, is presented. A distribution of charge density was arbitrarily assumed compatible to the Stern theory. Assuming that the velocity distribution is parabolic (in laminar regime) or logarithmic (in turbulent regime), the convective current density can be obtained. The distribution of current density shows changes of sign at different distances from the boundary. This may be the reason for the change of total electrification current sign that is often observed during experimental measurements. [ABSTRACT FROM AUTHOR]

We have investigated the performance of silicone oil as an insulating medium with the aim of developing environmentally-friendly, flame-retardant electric power equipment. We have studied the streaming electrification characteristics of silicone oil during flow, with regard to its use in the cooling of electric power equipment, at temperatures from 30 to 130 °C. The effects of the insulator material on the charge density in the oil and on the accumulated charge density in the insulator were investigated., We use four kinds of insulators, including aramid paper, in these experiments. The silicone oil was negatively charged by streaming electrification, in contrast to conventional mineral oil. The magnitude of the accumulated charge density in the insulator was influenced by the kind of insulator. It was also found that the charge density in the silicone oil and the accumulated charge density in the insulator changed with increasing, temperature. Moreover, we investigated the effect of the kinematic viscosity on the streaming electrification by performing experiments using oils with kinematic viscosities of 20 cSt and 50 cSt. We have considered the optimum combinations of silicone oil and insulator material. [ABSTRACT FROM AUTHOR]

We present a study of the performance of silicone oil as an environmentally friendly and flame retardant insulating medium for electric power equipment. We have investigated the streaming electrification characteristics for flowing silicone oil for cooling of electric power equipment. As the initial step, the electric charge density accumulated in silicone oil and on the surface of the insulator is investigated simultaneously. We have used 5 types of insulating material for the experiment, including aramid paper. Silicone oil becomes negatively charged by streaming electrification, in contrast to conventional mineral oil, and the magnitude of the electric charge density on the insulating material is influenced by the kind of insulating material. As the second step, the streaming electrification characteristics of silicone oil were further examined using an insulator for which the work function is known. Based on these results, streaming electrification for various kinds of insulator and silicone oil is evaluated with respect to the work function. We consider the optimum combination of silicone oil and insulating material. [ABSTRACT FROM AUTHOR]

Conventional electret polypropylene nonwoven fabrics were made by corona discharging method. The both outer surfaces of the nonwoven fabrics had a lot of charge, but the inner parts of the nonwoven fabrics had poorer charge than both outer surfaces. Therefore, this paper reported the streaming electrification which was to make insulation liquid stream through the polypropylene nonwoven fabric. It was found that the both of outer surfaces and inner parts of the nonwoven fabric were uniformly charged, and also the polarization with positive and negative was confirmed by observation of dust figures. The performance of the filtration was superior to the conventional electret nonwoven fabric. [ABSTRACT FROM AUTHOR]

Abstract: In order to build highly reliable transformers, it is important to understand the streaming electrification phenomenon. We therefore measured the electrostatic charging tendency of one type of insulating oil flowing on the surface of plates made of several kinds of metal and insulator. From these experiments, it was found that the streaming electrification of insulating oil depended on work functions of solid materials. We then examined our results from the viewpoint of electronic states of insulating oil and solid materials. We then applied a model based on contact charging of polymers. [Copyright &y& Elsevier]

The incidence of streaming electrification in large transformers (and other industrial equipment) has generated the need for both an understanding of the underlying physics and a mathematical description of the phenomenon. Modeling is needed since the mechanisms do not readily scale and small-scale laboratory experiments are limited in predicting performance. The development of both steady-state and transient descriptions of the streaming electrification process is described for a rectangular duct structure. The nature of the phenomenon is such that appropriate assumptions and approximations need to be made. As a consequence, some experimental verification of the outcomes, at least in qualitative form, is also included. The entrainment of charge from the interface charge layers also intimately involves the flow of the liquid. A description of the flow regimes and a transient solution of the charge development through a duct are thus an integral part of the modeling effort. The utility of the steady-state model developed is demonstrated by application to an oil-cellulose system typical of a large core-form transformer. The method advocated takes advantage of the repeat duct arrangement in such units. The results presented allow the internal behavior of such a unit to be studied in considerable depth, and provide insight into the complex interplay between generation and relaxation in circumstances when flow rates, temperature, external fields and influent charge are changed. [ABSTRACT FROM AUTHOR]

In this paper, a device with swinging plate electrodes has been proposed to measure contact electrification of a liquid sample. The proposed structure is composed of two parallel metallic plates immersed in a dielectric liquid. One of the plates is swinging with a constant frequency in a range from 0.4 to 4 Hz. The paper investigates the dependence in time and frequency of electrode velocity and streaming electrification. The measured current occurs for a very low intermittent velocity of less than 10 mm/s. In this range, the electrification current is around 50 pA. For higher velocities of up to 150 mm/s, the current is at the level of 1200 pA. The time–frequency characteristic using short-time Fourier transform shows no temporal changes in the frequency spectrum. The dependence of electrification on shuttle speed was calculated and it can be approximated with a second order polynomial model with the determination coefficient higher than 0.9. The advantage of the sensor is the ability to measure electrification phenomena without the necessity of having rotating electrodes or having a large volume of flowing liquids. [ABSTRACT FROM AUTHOR]

Power transformers are the main element of an electric power system. The service life of these devices depends to a large extent on the technical condition of their insulation system. Replacing mineral oils with natural or synthetic ester (retrofilling process) may increase the efficiency and operational safety of transformers, and also limit their adverse environmental impact. It is technically unfeasible to completely remove mineral oil from a transformer. Its small residues form a mixture with fluid ester, with different physicochemical and electric properties. Streaming electrification is one of the phenomena which, under unfavorable conditions, may damage the insulation system of a forced oil cooled transformer. It is necessary to run prophylactic tests for the ECT (electrostatic charging tendency) of insulating liquid mixtures from the point of view of transformer retrofilling, which is being used more often than before. The article presents the results of studies on selected physicochemical, and electrical properties, and the ECT of mixtures of fresh and aged Trafo EN mineral oil with Nycodiel 1255 synthetic ester. In this regard, the density, the kinematic viscosity, the conductivity, and the relative dielectric constant were measured. The molecular diffusion coefficient was determined using Adamczewski's empirical dependency. The streaming electrification was tested in a rotating disc system. The impact of the rotation time, the diameter, and the disc's rotation speed on the amount of the electrification current generated were analyzed. In addition, the co-relation between the electrification current and the composition of the mixture was determined using fresh and aged mineral oil. On the basis of the electrification model, the volume density of the qw charge was calculated, which is a parameter defining the ECT of insulating liquids. Based on the results, it was concluded that the synthetic ester is characterized by a higher susceptibility to electrification than the mineral oil. However, combining synthetic ester with a small amount (up to 20%) of fresh or aged mineral oil significantly reduces its ECT, which is beneficial from the point of view of retrofilling power transformers. [ABSTRACT FROM AUTHOR]

Many studies on solid-liquid triboelectricity have used electrons in water as a means to generate electricity. However, for such applications researchers have utilized the same mechanism as that formalized for solid-solid triboelectricity. In this paper, a direct charge control method for generating liquid-solid triboelectricity is proposed. This mechanism suggests a new method for solid-liquid state triboelectricity using the inherent properties of a liquid. The proposed direct charge control strategy employs a liquid-solid triboelectricity to separate positive and negative charges in water. In the case of indirect charge control, external electrons from the ground or an electrode are used and thus, suitable electrical parts are needed for rectification. This can reduce the usable power. In contrast, direct charge control does not require additional electrical components for rectification, and available power is conserved. Upon overcoming certain limitations, normal operation of a system without rectification was realized, and a capacitor (100 μF) was charged to over 14 mV. In the relationship between the streaming electrification equation and the obtained results, the electronegativity of the material, tube length, and number of tubes affect the charging voltage. Here, the charged voltage increased above 100 mV as the tube length was increased. Such finding indicates that the output of direct charge control can be enhanced by various factors, thereby pioneering new areas for solid-liquid triboelectricity. Image 1 • A novel DC power harvesting system without rectification parts has been proposed. • The mechanism based on the new sight of the streaming electrification is established. • OCE is advantageously reacted with this mechanism to amplify the output. • Only changing the tube length promotes the output 10 times increase. [ABSTRACT FROM AUTHOR]

Natural and synthetic esters are liquids characterized by insulating properties, high flash point, and biodegradability. For this reason, they are more and more often used as an alternative to conventional mineral oils. Esters are used to fill new or operating transformers previously filled with mineral oil (retrofilling). It is technically unfeasible to completely remove mineral oil from a transformer. Its small residues create with esters a mixture with features significantly different from those of the base liquids. This article presents electrostatic charging tendency (ECT) tests for mixtures of fresh and aged Trafo EN mineral oil with Envirotemp FR3 natural ester from the retrofilling point of view. Under unfavorable conditions, the flow electrification phenomenon can damage the solid insulation in transformers with forced oil circulation. The ECT of the insulating liquids has been specified using the volume density of the qw charge. This parameter has been determined using the Abedian–Sonin model on the basis of the electrification current measured in the flow system, as well as selected physicochemical properties of the liquids. It was shown that ECT is strongly dependent on the type of insulating liquid and pipe material, as well as the composition of the mixtures. The most important finding from the research is that a small amount (up to 10%) of fresh and aged mineral oil is effective in reducing the ECT of Envirotemp FR3 natural ester. [ABSTRACT FROM AUTHOR]

The subject matter of this study was the problem of the ECT (electrostatic charging tendency) of mineral insulation oils during their flow. The electrostatic charges generated may lead to partial discharges, and as a consequence, to the breakdown of a power transformer insulation system. In this study, the results of the ECT of mineral oils used in transformers were compared. The method of streaming electrification of insulation liquids using a flow-through system was used. The influence of flow speed, temperature, and the pipe material on the values of the electrification current and volume charge density qw were analyzed. The results obtained in this study should be taken into account regarding the operation of power transformers. [ABSTRACT FROM AUTHOR]

In order to investigate the electrostatic charging tendency (ECT) of transformer oil under thermal aging and understand its correlations to the other insulation parameters, a series of accelerated aging tests were carried out on two types of mineral insulation oils, A and B, both of which were mainly used in 500 kV power transformers in China. Oil samples were pre-processed by degassing and dehydrating, and then aged at 120°C for various periods up till to 675 h, meanwhile, different contents of oxygen and copper/pressboard/silicon steel sheet were also added into the oil samples for sensitive study. The results indicate that the ECT of oil A increases notably with aging time, the ECT of oil B however does not change so much during the whole aging period. Injection of oxygen in the samples enhances the ECT of oil A, such a promotion effect is minor for oil B. The correlations between the ECT and other parameters, such as tanδ, volume resistivity (ρv) and acid number were analyzed as well. There are linear correlations between ECT and tanδ, acid number and log (ρv) of oil A, of which ECT and tanδ has the highest correlation coefficient of 0.881. Regarding oil B, there is only a positive linear correlation between ECT and acid number. [ABSTRACT FROM AUTHOR]