dc field effects on streaming electrification in insulating oils

The results of an experimental investigation into the influence of impurities on electrification in insulating liquids in a tube model under an external dc field effect is described. Both conduction and streaming currents were measured as a function of temperature and flow rate using a paper-pressboard model with concentric cylindrical electrode geometry. Positive or negative dc voltage was applied to the inner electrode. The streaming current was measured from a shielded test chamber and the conduction current was measured from the ground electrode. Energization by positive dc voltage always enhanced the streaming electrification; however, when the applied voltage was negative, the polarity of streaming current was field strength dependent. At low fields (<0.52 kV/mm), the streaming current with negative dc field I_sn^c increased with increased field and with field strength >0.52 kV/mm, polarity reversal was observed. The field at which current crossed the zero value has been defined as the zero cross over point (ZCOP). Flow rate did not have any influence on the ZCOP; however, the additive concentration, moisture content and the temperature affected this ZCOP. In the presence of the corrosion inhibitor carbazole, the streaming current with negative dc field, I_sn attained zero at low fields. The conduction currents corresponding to fields where streaming currents were zero were also relatively low. It is therefore possible that by applying a moderate dc field of appropriate polarity the streaming current can be reduced to very low values by adjusting the carbazole concentration in the test liquid     

Static electrification by laminar flows through artificiallyroughed pipes

The mechanism of static electrification of hydrocarbon liquids flowing through artificially roughed metallic pipes is analyzed. Only the case of a laminar flow is considered. The equations of motion of the liquid are computed in the case of circular pipes with periodic roughness of rectangular cross section in the axial direction. The results reveal the formation of eddies, which depend on the size and the period of the roughness and on the Reynolds number. After determining the charge convected in a smooth pipe and the velocity profile in a rough one, it is possible to predict the convected space charge in terms of the roughness characteristics. The experimental results obtained with different rough pipes and several Reynolds numbers are in good agreement with the predictions, making it possible to explain the observations made during previous experiments on static electrification in rough pipes     

Influence of solid insulating phase on streaming electrification oftransformer oil

Due to increased demand of electrical power, the trend of the capacity and voltage rating of power transformers increases with reliable design; especially in large oil-cooled power transformers. Consequently, the use of polymer blended and polymer coated insulations in such transformers are the most recent advances in modern technology. The consequences of the type of solid insulating material, in contact with the streaming oil, on the ECT (electrostatic charging tendency) of oil are the principal aim of this paper. ECT of transformer oil is experimentally investigated using different types of oils and solid insulting materials. Two laboratory analog cycles are used; namely, a closed and an open cycle. In the closed cycle, the investigated factors are the type and polarity of the applied voltage (±10 kV and 10 kV_rms at 50 Hz), solid insulating material (polymeric and non-polymeric insulation), and oil type, temperature (20 to 70°C) and velocity (1.5 to 3.25 m/s). Good agreement is achieved between the measured data under ac and dc voltages of both the conduction and streaming currents, and the derived formulas. The currents increase exponentially with oil temperature, velocity and applied voltage. All solid dielectrics used show quantitative as well as qualitative changes in oil electrification especially at low temperatures. In the open cycle, both the rotating cylinder speed (430 rpm) and the oil volume are kept constant, and the oil temperature is raised to 120°C for different types of oils and solid dielectrics. ECT depends on the oil type and is activated with temperature. Cellulose-based dielectrics give the highest ECT and coating of these dielectrics with epoxy resin enhances the ECT. Different suppression characteristic is obtained using Theophylline in different fresh oils     

Magnetic properties of insulating RTiO3 thin films

The magnetic properties of epitaxial RTiO₃ (R?=?Y, Eu) thin films are reported. The films were grown on various substrates with different lattice mismatches using the pulsed laser deposition technique. Ferromagnetic YTiO₃ thin films could only be grown on LaAlO₃ (110) substrates, while EuTiO₃ films could be grown on various substrates. The magnetic properties of the grown films are discussed in light of the structural characterization.     

Degradation of insulating materials under electrical stress

An avalanche of physical and physicochemical phenomena, often self-sustained, leads to the end of life of an insulating material. The collapse of the insulating role of a material is mostly due to the electrical field stress. The path leading from the catastrophic situation, the breakdown, back to its origin is followed in the present paper. A defect, pre-existing or created by the in-service stresses, gives rise to an ionization process in a volume bounded by insulating walls. With time the process does not remain identical due to both changes in the material and gas evolution, which makes it difficult to interpret the detected electrical signal. The end of life will occur through microcracks in which new discharges are initiated, activating the propagation of these structural failures. This change of structure of the insulating material when due to the existence of water molecules will lead to the same kind of final degradation. The polymeric insulating materials could contain additives but they lead only to a partial healing. From polymers of low complexity such as the PE used in cables, assuming that there is no pre-existing defect, one endeavors to understand how a microstructure reacts with a local field or accumulated charges. The present paper summarizes the thoughts of the author pertaining to the problems of aging and dielectric breakdown in organic insulating materials, processes often given a concrete form through experiments.     

Dielectric behavior of insulating materials under liquid nitrogen

Measurements were made to assess the AC breakdown voltages in liquid nitrogen (LN/sub 2/) with different electrode configurations such as sphere-sphere, needle-needle, hemisphere-hemisphere, plane-plane, sphere-needle, etc. Experimental results reveal that the breakdown voltage is a function of electrode geometry and gap length. This study also addresses the effect on the breakdown strength of solid insulating materials under LN/sub 2/ environment with a sphere-sphere electrode configuration. In this paper, special emphasis has been attributed to the effect on loss index of a variety of dielectrics dipped in LN/sub 2/. The measured values of breakdown strength and loss index have been compared with those obtained under atmospheric condition. The study reveals that the breakdown strength of cellulosic materials like paper or pressboard increases manifold while the loss index decreases significantly when dipped in LN/sub 2/ with variations of the order of 50% to 90%. However, for impervious noncellulosic materials like Perspex (acrylic glass) or presspahn, the increase in breakdown strength is not that pronounced and the decrease in loss index is of the order of 2 to 30%.     

Towards a better knowledge of insulating polymers under stress

This contribution to a better understanding of insulating materials which are evolving in the domain of electrical engineering, concerns two polymers: a low density polyethylene free of additives, and an amorphous and biaxially oriented poly(ethylene-2,6 dicarboxylate). Through conductivity, dielectric relaxation and luminescence measurements the importance is shown of the role played by the structure of the material on its electrical properties. Also, the evolution of these properties under electrical stress or environmental conditions, such as electrical discharges, are studied     

Insulation resistance under DC stress and electrification characteristics of GIS epoxy insulator

Gas‐insulated switchgear (GIS) has widely been used for AC power distribution because of its high reliability and compactness. Recently, DC GIS has been developed with various investigations for dielectric breakdown characteristics of DC gas insulation. GIS insulation is composed of SF₆ gas and solid spacers, and it has been recognized that the dielectric performance of DC GIS is mainly influenced by solid spacers. Under DC stress, the electric field is directed one way, the effect of electrification for charges to be accumulated in the spacer must be taken into account and also the effect exists in AC GIS because the switching operations may leave the remnant DC charge on the AC GIS spacer. This paper first describes the effective resistivity (the bulk or the surface) of the solid spacer under the DC stress from the experimental investigation, and the critical factor on the solid spacer that causes reduced dielectric performance of the GIS insulation is studied. Second, the present paper deals with the electrification on the GIS with various levels of surface roughness of the epoxy insulator and metallic electrode. Finally, the DC insulation characteristics of GIS insulator are investigated based on the experimental results. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 168(4): 6–13, 2009; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20788     

A critical approach to measure streaming current: case of fuels flowing through conductive and insulating polymer pipes

Electrostatic charge generation phenomenon in the case of hydrocarbons flowing through pipes has been studied for a long time. The charge generation depends on the nature of both the liquid and the solid. Because of the introduction of new plastic materials and due to fuel composition evolution, it became necessary to perform a comprehensive study of the subject in the particular case of an automotive environment. The streaming current measurement permits one to characterize charge generation when a fuel flows through a plastic pipe. However, this measurement must be carefully managed. In this paper, we are working on the validity and the accuracy of such measurements. The case of conductive and insulating pipes, and the double-layer development influence, are analyzed and discussed. In particular, we give a detailed methodology in order to measure reliable streaming currents and to wisely derive the space-charge density at the wall (/spl rho//sub w/). Experimental results are presented for some solid/liquid couples.     

Study of samples of composite insulating system under electricaland thermal stresses

Factors affecting the performance of small composite samples are investigated, and the capabilities of the uniaxially oriented polyethylene (UOPE) tape/dodecylbenzene (DDB) oil insulation system are evaluated. The apparatus used to study the influence of the combination of electrical and thermal stresses on the performance of the samples is described. Experimental results are reported for both uniform and nonuniform electrode arrangements. The discharge energy/material damage tests showed that a satisfactory correlation between the total surface area damage of the tapes and the energy supplied by the discharge exists. This suggests that the measurement of total discharge energy is a very useful technique in evaluating electrical insulation and may prove useful as a diagnostic tool for predicting the remaining life of insulation     

中外电气化发展历程探讨

论述了电气化的涵义与分析指标,分析了美国、日本、韩国和中国的电气化水平变化历程,以及主要行业电气化水平差异,用因素分解法分析了各国电气化水平提升的原因,指出了未来中国电气化水平的发展趋势,提出了相关的措施建议.     

Time evolution of current through insulating paper under DC voltage stress

The use of paper in electrical insulation, especially in power transformers, remains a topical subject. The interest for the insulating paper, despite the emergence of many synthetic materials is ascribed to its low cost price and its ability to an oil impregnation. However its strong hydrophilic character remains the main difficulty for its implementation in electrical insulation. This affinity to water absorption and/or adsorption shows that a good understanding of the moisture effect becomes imperative. Several studies were carried out on the subject but a systematic study of the moisture effect on overall behavior of paper has not been achieved. This work is meant to be a contribution for this systematic investigation. The first step of this study, which is the object of the present article, concerns an examination of time variation of the electrical currents, under DC stress, for different moisture levels. A particular attention is given to the polarization and depolarization currents. These currents are examined during alternate charges and discharges sequences. An increase of the current, during the application of continuous voltage was noticed. This unexpected phenomenon would be related to an electro-dissociation and/or polarization processes.     

Analysis of surface conditions of electrical insulating structures under influence of precipitation

The method of estimating the humidity insulator’s surface under the influence of rain is considered. A way of increasing the water-repellent properties (water repellency) of a surface is proposed. Tests are conducted and positive results are obtained.     

Motion characteristics of a charged metal particle in insulating oil under flow state

Problems due to discharge by charged metal particles in insulating oil under flow state are examined in this study. The motion characteristics of a charged metal particle in a horizontal transformer oil tract are investigated, and a mechanical model of a charged metal particle under flowing oil and an AC field is proposed. The particle's equations of motion are numerically solved using the fourth‐order Runge–Kutta computational algorithm. The trajectory of a spherical iron particle in typical motion state under flowing oil and AC field is simulated. The influence on the motion characteristics of a charged metal particle in terms of the oil flow rate, electric field strength of the oil tract, particle scale, initial state, and particle rotation is analyzed. Results reveal that when a charged metal particle is moving fast along the direction of oil tract flow, reciprocating oscillation simultaneously occurs in the vertical direction. Its trajectory is significantly affected by the oil flow rate, the electric field strength of the oil tract, particle scale, initial state, and particle rotation. Moreover, the degree of oscillation of the charged metal particle may be the main cause of degradation of the insulating oil under flow state. © 2017 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Characterization of static electrification in power transformers

Static electrification due to oil now causes many field failures of large forced-oil cooled power transformers. Also, in practice under normal operating conditions, the oil volume in these power transformers is reduced due to the internal heat generated (from the energized windings) which accelerates the chemical processes occurring inside. To complete filling of the oil tanks of such transformers, either new oil of the same or different type is used. Laboratory analogs of these transformers; namely a closed and an open cycle, have been used for investigating this phenomenon. Such tests have been performed upon different types of fresh and aged oils and oil mixtures to measure the electrification current. Investigations of the effect of oil temperature, oil velocity, frequency and type of the applied voltage, type of the solid-phase material, and the length of the oil gap are introduced. Also, a 200 kVA, 3.3 kV/380 V distribution power transformer is used where the oil is forced and heated externally to examine and demonstrate this phenomenon for unenergized and energized cases and for both fresh and aged oils     

Physicochemical aspects of gassing of insulating liquids under electrical stress - [Feature Article]

This article describes the physico- chemical aspects of the gassing phenomenon and proposes a new method to determine the resistance to gas formation of insulating liquids.     

Rural electrification and expansion planning of off-grid microgrids

This paper presents the past and current practices for rural electrification and the current trend in using off-grid microgrids to provide energy to the customers with no access to the central electricity network. The challenges correspond to the capacity expansion of off-grid microgrids including the financial and business models for establishing these technologies, the economic and reliability considerations, the environmental issues, the expansion and feasibility studies, and the uncertainties in the operation horizon were presented.     

Electrostatic painting of insulating surfaces

The purpose of this work was to develop methods for painting insulating surfaces by the conventional electrostatic painting technique used to coat conducting samples. This technique is not applicable to insulators since the charges carried by the paint accumulate on insulating targets, thereby preventing a proper coating. A detailed investigation of the decay of charges deposited on insulators by a corona discharge simulating the charged paint has confirmed that a grounded counterelectrode (CE) on the back face of the sample speeds up the decay and that the better the contact, the faster the decay. Because a material CE with proper contacts is inconvenient for painting in production configuration, it was suggested that ionized air might behave as a satisfactory CE. Several ionizers have been tested, and good coatings have been obtained, provided that the flows of paint and ionized air are well separated. The possibility of self-creation of ionized air by the voltage of the early paint deposit has also been successfully tested