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     

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     

Streaming electrification of thin insulating pipes under electricfield

Streaming electrification under dc and ac electric fields was investigated by using test pipes and oil circulation apparatus. We used two different test pipes, one made of pressboard, and the other of insulating paper. At first, the amount of static charges which flowed into the test pipes was measured without electric field. For the experiment using an insulating paper pipe, when the oil temperature increased, the measured current curve shows a peak. When the velocity increased, the current increased proportionally from the 2nd to 4th power of velocity. For the experiment using a pressboard pipe, the current increased proportionally from the 3rd to 5th power of temperature, and by the 1st power of velocity. Then, we applied ac and dc electric fields on the test pipes and the current from a relaxation tank to ground was measured. The ions in oil carry the charges, so an electric field affects ions in the oil. It makes the mathematical approach to streaming electrification difficult. We know that as the electric field intensity increased, the current due to selective adsorption of the test pipe was increased. The current vs. velocity curve under ac electric field had a peak point in velocity     

Effect of streaming potential on heat transfer in transformer oil

The results of an experimental study of the effects of streaming electrification on the heat transfer of transformer oil (Voltesso 35) in Plexiglas and paper/pressboard pipe models are reported. The magnitude and the polarity of the charge in the oil entering the test section was varied by using two different filters (Millipore cellulose and Sartorius fiber glass) in the upstream of the oil flow. An enhancement in heat transfer was observed with both types of filters in the Plexiglas model and with cellulose filter in the paper/pressboard model; whereas a reduction in heat transfer was observed with fiber glass filter in paper/pressboard model under similar flow rates and temperatures. With uncharged oil, heat transfer was either inhibited or no enhancement was noticed in both models. The results have been discussed considering the temperature gradients, the flow conditions and the electrical forces due to static charge in the liquid     

Mineral insulating oil in transformers

Nearly all load bearing transformers in electric power delivery systems around the world are filled with liquid. The liquid functions both as an electrical insulation and as a heat transfer fluid. The liquid in almost all of these units is transformer oil, petroleum-based insulating oil refined specifically to meet the requirements of this application. Transformers in electric power distribution and transmission systems are expected to function reliably and efficiently and to do this for many years. The quality of the oil in a transformer plays an important role in performing this function, and the characteristics of transformer oil have been examined and reported on for decades. This article discusses some of these characteristics and the relationships between these characteristics and the production and in-service performance of mineral transformer oil. Transformer oil is used around the world, but the emphasis here is on practices in the United States     

Development and calibration of a streaming electrification modelfor a cellulose duct

Streaming electrification occurs in large power transformers where insulating oil circulated for cooling purposes acts to separate charge from the interfacial double layer present on a multitude of cellulose insulation surfaces. In this paper, the electrification phenomenon is addressed from a microscopic viewpoint. An empirically calibrated analytical model is provided for calculating the amount of charge separated in a core-form transformer duct. The model incorporates a boundary layer approach in approximating the wall charge distribution which provides a critical length scale for the analysis. This length scale is used to specify the appropriate model for charge development under turbulent flow conditions. The duct model considers both laminar and turbulent flows subject to ac electric fields and varying temperature. This concept forms the elemental building block of a network-based methodology for examining electrification in a complete transformer geometry     

Effect of filler concentration on electrical conductivity andultralow-frequency dielectric properties

The effect of filler concentration on the dielectric properties in the ultralow-frequency region and on the electrical conductivity was studied for ethylene propylene rubber. First, we investigated the relation between the electrical conductivity and filler concentration: as the volume fraction of fillers q_a increased, the conductivity decreased in the low filler concentration region but increased abruptly in the high filler concentration region. The decrease and increase in conductivity can be explained with the action of carrier traps at the interface between EPR and fillers and with the formation of highly conductive paths of filler across the sample, respectively. Secondly, we studied the dielectric properties in the ultralow-frequency region which was obtained from the discharge current. As q_a increased, the relaxation time decreased in the low filler concentration region and then rose in the high filler concentration region. The polarization in the high filler concentration region can be explained by two-layer interfacial polarization between filler and rubber     

浅析大型变压器油流带电现象

从油流带电产生的机理、影响油流带电的主要因素、油流带电的测试方法和广东省大型变压器油流带电的测试实例等方面探讨了油流带电现象,提出如何抑制油流带电现象的产生是当前急需解决的问题。     

Effect of Ca co-dopant on the electrical conductivity of Gd-doped ceria

Co-doped ceria of Ce_0.8Gd_0.2?x Ca _x O_2-δ (x?=?0?0.2), were prepared by oxalate co-precipitation method. Their structures and conductivities were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and AC impedance spectroscopy (IS). All the electrolytes were found to be single phase with cubic fluorite structure. SEM cross-section image showed relatively uniform grains with distinct and clean grain boundaries. The chemical states of the surface of the prepared samples were analyzed by XPS. Though Gd and Ca were present in their characteristic chemical state, Ce was found in single Ce⁴⁺ state or in mixed Ce⁴⁺ and Ce³⁺ states. IS measurements indicated that the conductivities for Ce_0.8Gd_0.2–x Ca _x O_2-δ pellets increased with increasing the sintering temperature. Moreover, co-doping with appropriate ratio of gadolinium and calcium was found to effectively enhance the conductivity in comparison to the singly doped ceria. The isothermal conductivity plots showed that sample Ce_0.8Gd_0.1Ca_0.1O_2-δ had the maximum conductivity with minimum activation energy (σ _700°C?=?0.0742?S/cm, Ea?=?0.58?eV), which is much higher compared to the conductivity exhibited by most of the reported codoped ceria compositions.     

Chemical resistance of electrical insulating epoxyisocyanates compound

The chemical resistance of hardened epoxyisocyanate compound to alkalis, acids, sodium carbonate, and chloride solutions, as well as to hot water, is examined. According to the test results, the compound can be classified as a chemically resistant substance and recommended for application in the insulating systems of electrical machines that operate under conditions with increased chemical contamination, as well as for electrical machines used in seagoing ships.     

Nonlinear dc and ac conductivity in electrically insulating composites

The non-linear electrical behavior of zinc oxide, used to practical advantage in surge arresters on distribution power systems, has been replicated in epoxy resin, which has been filled with an appropriate volume fraction of this intrinsically semi-conducting material. Comparison has been made with epoxy resin containing a similar volume fraction of aluminum oxide in order to establish that the non-linearity observed in zinc oxide filled resin is a function of the zinc oxide itself and not simply an artifact resulting from the presence of any filler and the interfacial phenomena thereby arising. Previous work in this field, in both thermo-setting and thermo-plastic insulation has concentrated on dc voltage application. This work, in addition to extending the range of host materials to include epoxy resin using dc, has also explored the ac behavior. The results demonstrate that the inclusion of zinc oxide in epoxy resin is capable of producing non-linear conductivity when the filler volume fraction exceeds the percolation threshold ( /spl sim/ 15%).     

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.     

A parametric study of streaming electrification in a full-scalecore-form transformer winding using a network-based model

Streaming electrification remains a problem for large power transformers which require forced-oil cooling. Oil flowing against cellulose insulation surfaces results in the separation of charges from the electrical double layer at the interface. This process can generate hazardous static potentials and volume charge densities in the transformer dielectric system. While streaming electrification has been studied in great detail using small-scale laboratory models, extension of these results to full-scale transformers is rather difficult. This issue has been addressed through the development of a network-based analytical approach which is calibrated against empirical data at the elemental duct level. The model has been utilized to perform a parametric study of streaming electrification in the full-scale high-voltage winding of a core-form transformer. Volume charge densities exiting from each duct in the structure and the local surface potentials are predicted as a function of influent charge, oil volume flow rate, temperature and applied voltage     

Simulation of electrical tree growth in solid insulating materials

Contents In this paper the electrical tree growth in solid insulating materials is modeled using von Neumann's Cellular Automata (CA). The model is based upon the assumption that the electric stress at the end of a conducting tip quite often approaches the dielectric strength of the material and that progressive breakdown can occur by electrical tree formation. Because of tree advancement, the potential distribution into the insulating material changes with time and is calculated at each time step. An algorithm for the simulation of electrical tree growth in solid dielectrics based on this model has been developed. The algorithm is also used to simulate breakdown in solid dielectrics containing square or spherical voids.

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Simulation des Wachstums elektrischer Bäume in Festisolierstoffen

Übersicht In diesem Artikel wird das Wachstum der elektrischen Bäume in Festisolierstoffen mittels von Neumannscher zellularer Automaten modelliert. Dieses Modell geht davon aus, daß das elektrische Feld an der Spitze der Elektrode oft in der Nähe der dielektrischen Festigkeit vom Material liegt und das ein fortschreitender Durch-schlag durch elektrische Bäume möglich ist. Durch das Wachsen der elektrischen Bäume ändert sich die Spannungsverteilung innerhalb des Festisolierstoffes mit der Zeit und wird in jedem Zeitschritt berechnet. Ein Algorithmus für die Simulation des Wachstums eines elektrischen Baums in Festisolierstoffen ist entwickelt worden. Der Algorithmus kann auch den Durchschlag in Festisolierstoffen mit zylindrischen oder kugelförmigen Hohlräumen simulieren.

     

Flow electrification of transformer oil effects of mixed fields

Static electrification in transformer oil is investigated experimentally using a laboratory synthetic closed cycle, where the oil is pumped in a coaxial electrode arrangement. The electrode system is electrically energized radially with dc and/or ac, and the electrostatic charging tendency (ECT) of the oil is quantified by measuring the streaming current. The results indicate that for the same voltage ratio (K=Vˆ/(Vˆ+V)), increasing the ac or the dc voltage component leads to higher ECT of oil, although the ECT under only ac field is much lower than that under dc field. The unenergized streaming current is inherently affected by the electrode material and configuration; where electrodes having a lower work function give higher positive ECT of oil at high temperature. The energized streaming current increases with oil temperature, oil velocity and electric field; where negative dc voltage application to the outer electrode gives both higher conduction and streaming currents. While the conduction current decreases with oil velocity, increasing the frequency of mixed ac voltage has no significant effect on streaming current. Moreover, the effect of combined radial electric (ac or dc) and axial magnetic (ac or dc) fields on ECT of oil is investigated, and the results reveal that the ECT is enhanced by the magnetic field while the radial conduction current decreases. Derived formulas for the streaming current for unenergized and ac energized cases are also presented together with expressions of the volume and surface charge densities for the coaxial electrode system     

Influence of various compounds on electrostatic charging tendency in insulating oil

The influence of various compounds on the increase of the electrostatic charging tendency (ECT) in insulating oil was investigated. Sulfide compounds, which are contained in new mineral oil, are identified for the first time as the original materials involved in the increase of the ECT. This is verified by heating tests of alkylbenzene with the addition of various compounds. Coexistence with copper and injection of oxygen enhance the increase of the ECT. Some sulfide and sulfoxide compounds increase the ECT markedly. On the other hand, the influence of sulfones, which are oxidation products of sulfoxides, is negligible. Although sulfonic acids, which are oxidation products of sulfones, markedly increase the ECT, the polarity is reverse. Thus, the ECT is influenced by the kinds and the molecular structures of additive sulfur compounds. The influence of nitrogen compounds and that of oxygen compounds are confirmed to be smaller than that of sulfur compounds. Since the amounts of nitrogen compounds and oxygen compounds are much smaller than those of sulfur compounds in mineral insulating oils, the influence of these compounds is considered negligible. Since the ECT of oils with the addition of sulfides or sulfoxides increases after heating, the other compounds should be created from these compounds that increase the ECT directly. Thus, the increase of the ECT is mostly caused by the oxidation of sulfides, which creates sulfoxides. Then certain compounds, which directly increase the ECT, are created from sulfoxides. If sulfones or sulfonic acids are created by the oxidation of sulfoxides, the ECT decreases because of the influence of these compounds on the ECT. It is most likely that some process other than oxidation occurs in the creation of the compounds that increase the ECT directly. © 2011 Wiley Periodicals, Inc. Electr Eng Jpn, 176(3): 29–36, 2011; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/eej.21112     

Flow electrification characteristics of transformer oil by rotatingelectrode systems

Flow electrification has been found to be the principal cause of a number of failures of forced oil cooled power transformers. Flow charging characteristics of oil/cellulose system with factors like electrode configuration, electrode material, presence of Benzotiriazole (BTA), metallic contaminants and copper coils were investigated for paraffinic oil by employing a rotating electrode system. A few hydrodynamic parameters (viz. Reynolds number, boundary layer thickness and friction factor) were correlated with flow charging characteristics of oil for varying temperatures concentrations of BTA. With concentrations of BTA in oil (viz. 10 ppm and 25 ppm) a marginal reduction in flow charging of oil was noticed, but about 40% reduction was observed with 150 ppm of BTA. A significant reduction in the flow charging characteristics of untreated and BTA treated oils was also observed in the presence of copper coils and metallic particle contaminants