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Copper sulfide deposition on cellulosic insulating materials in oil-immersed transformers is evaluated by heating tests stipulated by IEC 62535. Although this method is suitable for evaluation of the corrosivity of unused insulating oil, the subject remains in diagnosis of the insulating oil used. It is because dibenzyl disulfide (DBDS), which is the prime compounds that cause copper sulfide deposition, is consumed by the reaction with copper during operating of transformer. Therefore, it is necessary to detect the existence of DBDS in insulating oil diagnosed as non-corrosivity by IEC 62535. The proceeding paper [6] reports that Bibenzyl (BiBz) and Dibenzyl sulfide (DBS) will be formed in the insulating oil as by-products of copper sulfide formation between copper and DBDS in oxygen-poor atmosphere which imitated the closed-type transformers. These by-products are considered to be the indication of copper sulfide formation. However, the influence of oxygen and addition agent (e.g. 2,6-di-tert-butylp- cresol) on the mechanism of the by-product formation is unclear. The purpose of this study is to elucidate the influence of oxygen and DBPC in by-products formation for development of diagnosis. It became apparent that benzyl alcohol, benzaldehyde and benzoic acid generate as by-products of copper sulfide formation in the air atmosphere which imitated the open-breathing transformers. Furthermore, it became apparent that the reaction product of benzyl radical and the radical of DBPC generates as by-product of copper sulfide formation in the insulating oil containing DBPC. These by-products are detectable even if DBDS disappears. Therefore, it is thought that diagnosis of the open-breathing transformer is possible by detecting these by-products. [ABSTRACT FROM AUTHOR]

The suppressive effect of triazole-based passivators such as 1,2,3-benzotriazole (BTA) and Irgamet 39 and duration of the suppressive effect on copper sulfide deposition are investigated by changing the time when passivators are added to oil. All corrosive sulfur tests were performed by the method stipulated by IEC 62535. In order to simulate the timing for adding passivators into actual transformers, preheated and non-preheated oils were used for corrosive sulfur test. Adding a passivator into preheated oil simulates the condition where a passivator is added into the transformer after a period of operation, and adding a passivator into non-preheated oil simulates the condition where a passivator is added before operation. Although the deposition rate of copper sulfide in preheated oil with the addition of passivator was lower in preheated oil without passivator, copper sulfide deposition still continued even while the passivator was detected in the oil. The result suggests that the precursors of copper sulfide have been already generated in preheated oil before adding a passivator. After the passivator was depleted in the oil, the period until onset of copper sulfide deposition in the preheated oil was shorter than in non-preheated oil since a protective complex layer was created on the copper surface and reaction with the passivator continued to prevent the deposition of copper sulfide precursors. [ABSTRACT FROM AUTHOR]

The temperature distribution is vital to the lifetime and reliability of converter transformer bushings. However, the temperature distribution of epoxy impregnated paper converter bushings under a certain current has usually been calculated through the two dimensional model, without considering the heat convection and radiation effects. In this paper, using the computational fluid dynamics (CFD) and finite element method (FEM) analysis, a 3-D electromagnetic-fluid-thermal analysis method has been proposed. With the considering of heat conduction, heat convection and radiation, the temperature distribution of the converter transformer bushing has been simulated according to the actual operating environment. The temperature and velocity distribution of the converter transformer bushing has been obtained. Moreover, the simulation results have been compared with the experimental results to verify the accuracy of the simulation method. The simulation results, especially for the inner conductor, external conductor and the epoxy impregnated paper, are very close to the experimental results and the maximum temperature difference percentage is within 10 %, which indicates the accuracy and effectiveness of the electromagnetic-fluid-thermal analysis method proposed. [ABSTRACT FROM AUTHOR]

Copper sulfide deposition on cellulosic insulating materials in oil-immersed transformers was investigated by heating tests stipulated by IEC 62535 with controlled atmospheres to which mineral insulating oils were exposed. Both oxygen in the atmosphere and 2,6-di-tert-butyl-p-cresol (DBPC) in the oil were found to accelerate the copper sulfide deposition on the insulating paper when the oil contains dibenzyl disulfide (DBDS). The weight of copper sulfide deposition on the insulating paper was found to increase when the oxygen concentration was increased up to 20 volume %. X-ray absorption near edge structure (XANES) analysis was applied to study the chemical bonding state of copper atoms in copper deposits on the insulating paper for the first time. It can be clarified the copper deposits on the insulating paper are mainly Cu2S and small amounts of CuO are contained. [ABSTRACT FROM PUBLISHER]

The hot spot temperature inside the windings of the oil-immersed power transformers may exceed 200 °C due to some winding deficiencies, posing negative impact on the paper insulation. A series of heating experiments are deliberately conducted with oil-paper insulation samples to study the influence of the hot spot temperature ranging from 120 to 280 °C, and the insulation degradation is characterized by the breakdown voltage (BDV) and the degree of polymerization (DP). Furthermore, surface morphology, water content and cellulose degradation mechanism are analyzed so as to reveal the causes of the above phenomena. The experimental results indicate that with increase of the hot spot temperature, the breakdown voltage increases after the first drop, while the DP continues to decline in the tested temperature range. No obvious linear relationship is observed between the hot spot temperature and the DP or the BDV. Regarding the test samples the authors have studied, the inflection point of the hot spot temperature for the insulation deterioration is found to be between 240 and 260 °C. The proposed research presents a fundamental reference for assessment of the winding insulation degradation from the viewpoint of hot spot temperature. [ABSTRACT FROM AUTHOR]

Power transformer is one of the most expensive equipments in a high voltage AC power system. Adequate monitoring system for power transformers can help in reducing the failure rate and thereby enhancing system reliability and economic efficiency. The failure of power transformer has serious consequences on system reliability. Long term and comprehensive testing cannot be done in working transformer, so prorated model of transformer is preferred. In the proposed work, a new dual temperature test cell is developed based on IEC 62332 and IEEE C57.100. The cell provides for a current carrying conductor wrapped with insulating paper and further insulation is provided with high-density pressboard as per arrangements in power transformer. In this paper a new mathematical model is developed using activation energy and preexponential constant calculated from experimental results at different temperatures. The expected life based on degree of polymerization (DP) value, shows nonlinear aging of insulation based on investigation carried out on prorated model. [ABSTRACT FROM AUTHOR]

In a search for mineral oil substitutes, we focus on silicone and ester oil. These insulating oils are fire-resistant and environmentally friendly. In previous studies, we investigated streaming electrification and dielectric breakdown phenomena in these new oils and found that their performance was similar to or better than that of mineral oil. In the present study, we focus on the thermal degradation of insulating oils. Thermal degradation tests in the presence of a solid insulator and copper carried out for 1000 h at 130 and 180 °C. Color changes, chemical analysis and dielectric breakdown characteristics were also investigated during thermal degradation of the insulating oils. We also investigated the mechanical strength of the solid insulator. The characteristics of aged oils were compared with those of fresh oils. Tested silicone oil presents better performance than esters and mechanical characteristics of paper remain unchanged in all resistance liquids. [ABSTRACT FROM AUTHOR]

The power transformer is one of the most important equipment in the electrical power system due to its cost and potential failure????s consequences. To ensure a continuous energy supply, it is needed to assess abnormal condition such as thermal and electrical behavior. From this viewpoint, dissolved gas analysis (DGA) is very useful in detecting internal defects in oil-filled electrical equipment. A system has been developed that allows to simulate partial discharge and overheating in transformer????s insulation. By using this system, we have studied on DGA under low temperature overheating in mineral oil and vegetable oil. [ABSTRACT FROM AUTHOR]

For more than a century, conventional mineral oil/cellulose insulation is being used as main insulation for power transformers. The life span of these major assets is determined amongst others by the insulation system?s mechanical resistance to withstand short circuit current forces. Since cellulose paper aging is accelerated by temperature, moisture and oxygen, poly-aramid-based synthetic insulation paper, with better thermal stability emerged for application in transformers. However the relatively limited number of studies on hybrid insulation and the cost of aramid paper still limit their use in distribution transformers. This contribution attempts to demonstrate that despite the high cost of aramid paper, hybrid insulation could be used in large power transformers which represent a significant financial investment. To do this, the quality of mineral oil aged with hybrid solid insulation and the thermal aging of the cellulose paper of this insulation are investigated. Comparison was made with traditional mineral oil impregnated cellulose paper to provide a benchmark for comparison. The samples underwent thermal accelerated aging procedure according to ASTM-D1934 standard and, furthermore, were submitted to local overheating. The condition of oil samples collected from each aging vessel was assessed using several diagnostic techniques, namely the relative amount of the Dissolved Decay Products (DDP), Turbidity, and Interfacial Tension (IFT). The Degree of Polymerisation (DPv) and Dissolved gas Analysis (DGA) were used to monitor the thermal aging of the cellulose paper samples. The results indicate that oil aged with hybrid solid insulation yield fewer decay products at some aging stages, especially after the application of a local overheating. The results also indicate that the cellulose paper within solid hybrid insulation is slightly less degraded compared to that of conventional insulation. [ABSTRACT FROM AUTHOR]

Copper sulfide deposition on Kraft paper in oil-immersed transformers is found to be accelerated by di-tert-butyl p-cresol (DBPC), which is widely used as an antioxidant, and paraffinic hydrocarbons in their oils, which is verified in laboratory experiments. DBPC can prolong the lifetime of precursors of the copper sulfide deposition by reacting decomposition products of copper dibenzyl disulfide (Cu-DBDS) complex and by creating the Cu-DBPC complex. Because the lifetime of the Cu-DBPC complex is far longer than that of the Cu-DBDS complex, considerable amounts of copper sulfide are deposited on Kraft paper instead of being formed on the surface of copper. A good correlation is obtained between the concentration of paraffinic hydrocarbons and the amount of copper sulfide deposited on Kraft paper in the three commercial oils. [ABSTRACT FROM PUBLISHER]

An isothermal microcalorimeter has been used to study ageing of oil impregnated kraft paper. Under the assumption that the heat flow is proportional to the ageing rate of paper it is found that activation energy seems to be lower for oxidation than for hydrolysis. This observation corresponds well with results obtained by traditional ageing methods. The results have also been confirmed by measuring the changes in DP of the samples used in the calorimetric measurements. Comparing degradation of aged and unaged oil impregnated paper in air gave a higher heat flow for the unaged paper, but the activation energy for the processes remained the same. The process was also studied with another cellulosic material and in addition varying size of the sample holder of the calorimeter, giving similar results. Hence, since the microcalorimeter appears to give the same qualitative ageing characteristics for oxidation and hydrolysis as more time consuming methods, calorimeter could be a useful tool for quick ageing investigations. [ABSTRACT FROM PUBLISHER]

Copper sulfide is found to be more likely formed on Kraft paper in oil-immersed transformers with an increase in either dibenzyl disulfide (DBDS) concentration or heating duration, which is verified by heating tests stipulated by IEC 62535. Two effects of DBDS concentration on the formation rates of copper sulfide were obtained. The formation rates of investigated oils are proportional to the DBDS concentration when the concentration is low. However, the amounts of copper on the Kraft papers are constant when the DBDS concentrations are more than certain values, which is considered to reflect the saturation of dissolving DBDS-Cu complex in the oil. These concentrations depend on oil brand. Copper sulfide begins to form after some duration of heating and the duration decreases with an increase in the DBDS concentration. Copper sulfide formed on the Kraft paper is a portion of the total copper reacted with DBDS by comparing the amount of copper formed on the Kraft paper with the total amount of reacted copper calculated from the amount of consumed DBDS. [ABSTRACT FROM PUBLISHER]

The main objective of this study was to evaluate the effect of transformer construction materials, such as kraft insulation paper, varnish, core steel and gasket materials (rubber), on the physicochemical properties of natural ester-based insulating oils (NEIs) and mineral insulating oil (MIO) under heating. We evaluated two NEIs, one formulated with soya oil (Envirotemp FR3) and other with corn oil (Biovolt A). The test specimens were placed in a 1-L jar with 800 mL of insulating oil and then the jar was placed in an oven at 100 °C for 716 h. The insulating fluids were analyzed before and after the heating tests by physicochemical analyses, such as kinematic viscosity, acid number (or neutralization number), dissipation factor and interfacial tension for MIO. Properties of the aged reference and the test specimen oils for a standard testing method for compatibility of construction materials with NEIs are proposed. The only parameter that showed variation after the NEI compatibility testing with different materials was the dissipation factor at 90 °C. The results show that besides compatibility and aging tests, mechanical tests for the gasket or seal materials should also be performed to conclude whether a material is suitable for construction of transformers insulated with NEIs. [ABSTRACT FROM AUTHOR]

This paper describes work performed in an attempt to bring more clarity regarding the differences in potential corrosive effect between sulfides and disulfides at specified reaction conditions as defined in ASTM 1275 B. SEM/EDS was used to further evaluate testing results. The results show that Dibenzyl disulfide (DBDS) has a strong tendency to form copper sulfides while Dibenzyl sulfide (DBS) display no tendency at all to form copper sulfides in the virgin insulating oils, but the reactivity of DBS increase if in the thermally degraded oils. In addition, the insulating paper decrease the oxidation of the copper surface due to that oxygen is adsorbed and consumed when migrating through the cellulose phase. With the improvement of surface smoothness, the specific surface area accessible for reactions will decrease and consequently it is more difficult for the copper to be corroded by DBDS. [ABSTRACT FROM AUTHOR]

This paper proposes an explanation for the aging of solid insulating materials exposed to high frequency voltage impulses based on thermal stimulating processes triggered by losses generated in insulating materials due to damped oscillations of molecular dipoles in high frequency electrical fields. For this purpose, total dissipation of insulating materials is calculated based on frequency domain description of applied voltage and frequency dependent dielectric properties of insulating materials. To validate the proposed theory, different experimental investigations are carried out on thin oil impregnated paper exposed to high voltage square wave shape pulses with different pulse durations, rise times and repetition rates. The trends obtained in experimental studies can be satisfactorily explained using the proposed theory. [ABSTRACT FROM AUTHOR]

This paper presents the frequency and temperature-dependence complex permittivity (ε = ε' + jε") of three layers (the mesocarp, shell and kernel) in palm fruit var. Tenera. The complex permittivity was measured and reported at frequencies between 0.5-6 GHz from 10-70?C. Dominated by the frequency characteristic of free water in these three layers, the ε' and ε" decreased significantly with the increase in temperature at the measured frequencies. The temperature characteristic of ε at 2.45 GHz was investigated. It was found that ε', ε" and the loss tangent for the mesocarp layer were higher than those of the kernel and shell layer. This was especially so for the loss tangent case; this means that the mesocarp layer can absorb microwave energy and convert it to heat more readily than the other two layers. The penetration depth,Dp, for each layer at 2.45 GHz between 10-70°C was computed and confirms volumetric heating does occur in all three layers. Temperature-dependence models for εof all three layers at 2.45 GHz were formulated by applying regression analysis to the measured ε with a 95% confidence interval. The three models were applied to form a multilayer concentric model for palm fruit and can be applied for microwave heating. The temperature distribution profiles in the palm fruit predicted from the model and the experimental results showed good agreement. [ABSTRACT FROM AUTHOR]

Since surface flashover of insulators in vacuum restricts the operation of the power equipment, improvement of surface flashover performance is of considerable importance in various fields. Previous studies show that surface flashover performance is closely related to the trap parameters of materials. In this paper, the crystallization behavior of low density polyethylene (LDPE) is modified by phenolphthalein. It results in the change of the trap parameters, leading to the enhancements in surface flashover performance. Results of differential scanning calorimeter (DSC) and scanning electron microscope (SEM) show that the crystallization behavior of LDPE is greatly changed by phenolphthalein. The crystallinity increases initially and then decreases with the increase of phenolphthalein concentration, and its maximum value occurs at 0.4 wt%. The spherulite size of LDPE composites decreases obviously with the increasing phenolphthalein concentration. The results of thermally stimulated current (TSC) show that both the shallow trap (? peak) and the deep trap (? peak) become deeper with the increase of phenolphthalein concentration. Furthermore, the impulse and dc surface flashover voltages present the optimum values at 1 wt% phenolphthalein concentration, which can be enhanced by 30.8% and 48.4%, respectively. It is concluded that LDPE with the smaller spherulite size indicates the deeper trap and the better surface flashover performance. This paper reveals the relationship between surface flashover performance and crystallization behavior of LDPE, and proposes a new method to enhance the surface flashover performance of semi-crystalline polymers in vacuum. [ABSTRACT FROM AUTHOR]

It is well known that the presence of a highly conductive water film over ice-covered insulator is a prerequisite to form local violet arcs across air gaps. This paper develops a model for calculation the thickness and surface conductivity of the water film formed over ice-covered insulators due to Joule heating effect. Different conductivities for water film layers are considered in the model mainly due to the rejection of impurities to the ice surface during the solidification process. The continuity equation and the Navier-Stokes equations of motion for the water film flowing down an inclined insulator string due to gravity are used to obtain the velocity distribution. Water film thickness of and ice layer melting time are determined by taking into account the heat flux at the inclined plane due to the viscous dissipation and Joule heating and by assuming a quasi-steady-state model. As well, the influence of angle of inclination on thickness and surface conductivity of water film was studied. The model is integrated with a static arc model for flashover voltage prediction to improve its capability to be used for inclined insulator strings and its accuracy compared to present models. [ABSTRACT FROM PUBLISHER]

Ageing is strongly related to the external factors acting upon the polymer and to the manufacturing process. It mainly depends on the anti-ageing performance whether heat-shrinkable tubes for switchgear busbar can maintain excellent electrical and mechanical properties during the long term operation. In this paper, the collected samples were applied to the switchgear busbar of 10 and 35 kV in substations of Tianjin Power System. The 10 kV tubes have operated for one, two and nine years and 35 kV tubes for four and six years respectively. Besides, two more kinds of unused samples were chosen to compare with the aged ones. Dielectric permittivity, dissipation factor and AC impedance were tested to assess the extent of ageing. The breakdown characteristics under AC voltage and the trap distribution calculated by measuring the dynamic behavior of surface potential under DC voltage were obtained at room temperature. Moreover, the tests of tensile strength, including breaking strength and elongation at break, were performed to analyze the mechanical properties. Results show that dielectric permittivity and dissipation factor increase with the operating time while the tensile strength decreases. The decline of breakdown strength and AC impedance indicates that insulation capability is reduced by long time natural ageing. The results obtained from the investigation also show the correlation between the trap distribution and the dielectric strength with the operating time. [ABSTRACT FROM PUBLISHER]

Polyimide (PI) film, as a special type of engineering plastic film, is a kind of basic insulating material and is widely used in electrical and electronic industry. However, polyimide film has a relatively low thermal conductivity so it may be damaged by thermal breakdown. One approach to obtain a higher thermal conductivity composite is to introduce inorganic fillers with high thermal conductivity. The purpose of this paper is to study the effect of boron nitride (BN) particles on thermal accumulation and tracking failure process. In this paper, polyimide samples were prepared with different weight ratios. The thermal conductivity of 2.58 W/m-K was obtained for the composites that was loaded h-BN(hexagonal boron nitride) with the mass fraction of 80 wt%. Tracking failure process under different pulse frequency was measured with a digital camera and an infrared thermal imager. The CCD camera was used to observe the failure process and the infrared imager was used to measure the thermal accumulation and decay process. Obtained results showed that the addition of BN fillers had a significant inhibitory effect on tracking failure process. With increasing the mass fraction of BN, the time to tracking failure increased obviously. The ability to inhibit tracking was improved by reducing the temperature at the concentrated discharge region and increasing the heat dissipation region. High frequency discharges were more easily to cause the thermal accumulation, which was unfavorable to the reliability of materials. It is concluded that the tracking resistance can be improved by adding BN particles. [ABSTRACT FROM AUTHOR]

Overheating is one of the severe defects in gas-insulated switchgear (GIS). However, researches on decomposition characteristics of SF6 under overheating conditions are relatively limited. In this paper, SF6 decomposition products are studied under three overheating conditions which are overheating of solid insulation, overheating of metal, and overheating of metal adjacent to insulating material. Influence of overheating temperature, gas pressure, initial moisture content and adsorbent on the decomposition characteristics of SF6 is discussed. The results show that typical decomposition products of SF6 under overheating of solid insulation are H2, CO, CO2, CH4, SOF2, SO2, H2S, COS and CF4, and typical decomposition products of SF6 under overheating of metal are SOF2, SO2, CO and CO2. Besides, typical decomposition products of SF6 under overheating of metal adjacent to insulating material are CO, CO2, SOF2, SO2, CF4, CS2 and COS. Types of overheating can be recognized by characteristic ratio of (SOF2+SO2)/(CO+CO2): for overheating of solid insulation, the ratio is 0 to 0.25, and for overheating involving metal, it is greater than 1. Two types of overheating involving metal can be distinguished by their characteristic products such as CS2, COS and CF4. [ABSTRACT FROM AUTHOR]

The paper describes a study of an electrical discharge in a supersonic rarefied air flow. The discharge is created by applying negative dc potential difference between two electrodes flush mounted on the surface of a quartz flat plate placed in Mach 2 continuous airflow. The electrodes are arranged in spanwise direction. Two discharges are studied. In the first one, the negative potential is applied to the electrode placed close to the leading edge. In the second one the potential is applied to the electrode placed close to the trailing edge. Current-voltage characteristics, spectroscopic analyses, electrostatic probe and surface thermal measurement will be presented. Luminous emission pattern and potential measurements show that the plasma is similar to a low pressure tube discharge modified by a non uniform density field of the flow. A comparison between current voltage characteristics of the upstream and downstream discharge shows the effect of the density field on the discharge. Thermal and spectroscopic measurements also show that the plasma induces a surface and volume heatings. [ABSTRACT FROM AUTHOR]

There has been a growing concern over recent years regarding the phenomenon of local heating of composite insulators. In this paper, the influences of humidity, contamination and local electric field on the temperature and dielectric properties of field-aged insulators were studied experimentally. Aging of the silicone rubber (SIR) material on the weathershed surface, rather than resistive current flow from contamination, was found to be the major cause of local heating observed by thermography. The aged SIR material exhibited strong dielectric loss under high localized AC electric field and high relative humidity. A hypothesis was developed to mitigate the moisture induced local heating on composite insulators. [ABSTRACT FROM AUTHOR]

In the context of increasing transmission capacities during the last years, the investigation of thermal effects, especially for gas-insulated systems, is required. Higher operating currents decrease the gas density in the vicinity of the heated conductor significantly. Hence, the insulation strength of the system is reduced. Dimensioning for different load scenarios requires a comprehensive knowledge of the temperature influence on the insulation performance of AC and DC installations. This paper introduces an algorithm to predict the temperature-dependent breakdown voltage utilizing the streamer criterion and proves its functionality by experimental investigations. Necessary input parameters, like the electrical field strength and the gas density, can be gained from numerical calculations. The insulation strength of the investigated arrangement at a conductor temperature of 100 ?C is decreased by (10 ? 15) % in comparison to a conductor temperature of 20 ?C. The accordance between measurement and calculation proves the quality of the model. [ABSTRACT FROM AUTHOR]

The local heating phenomenon of composite insulator has drawn more and more attention gradually. From our analysis, the temperature rise of composite insulator is closely related to the moisture in atmospheric environment according to research results. Effect of moisture on the temperature rise of composite insulators is studied in this paper. The composite insulators, which have operated in power system for a long time and have been detected for local heating, and new insulator are taken as test samples. The temperature rise of operated composite insulators, which have been detected for local heating, increases with the rise of the relative humidity, while new insulator shows no temperature rise can be detected. The temperature rise of sample is not relevant to the core and the interface between core and housing material practically, but the high voltage end housing. The moisture content of the housing material is different at various relative humidity, and this leads to difference in dielectric loss, which is a significant source of local heating phenomenon. The difference of dielectric loss at different relative humidity leads to the temperature rise variation with the relative humidity. [ABSTRACT FROM PUBLISHER]

Transformer oil is widely used in electric transformers serving the dual functions as insulating medium and cooling material. However, safety and stability of power system is interrupted due to the operation failure of electric transformers in certain case. Therefore efforts should be focused on improving the property of transformer oil for the stable operation of insulating system. The purpose of this paper is to obtain a new type of nanomodified transformer oil with improved dielectric and thermal properties. Boron nitride (BN) nanoparticles with high thermal conductivity were dispersed into transformer oil to form the nanooil. Dielectric and thermal properties of the samples before and after modification were measured. Dielectric property experiments were performed in accordance with IEC standard. Discharge phenomenon in breakdown process was captured by a high speed CCD camera. In order to investigate the effect of BN nanoparticles on thermal property, temperature change and heat distribution of the samples in heat transfer process were measured by an infrared thermal imager and a temperature sensor. It is found that the nanooil has higher dielectric breakdown strength and lower dissipation factor which indicate better dielectric property compared with nonmodified oil. Obtained results show significant improvement in heat transfer process with increasing the concentration of BN nanoparticles. It is proposed that the nanoparticles create an interfacial region which contributes to the enhancement of dielectric strength. For thermal property the ballistic phonon transport of the nanoparticles acts as the key factor in the remarkable improvement. Furthermore, the improved thermal property of the nanooil has an inhibitory effect on the breakdown process according to the bubble theory. [ABSTRACT FROM AUTHOR]

This paper describes the suppression of dry-band arcing erosion of silicone rubber by alumina tri-hydrate and silica fillers in the DC inclined plane test, employing the wavelet based multiresolution analysis of leakage current. The third detail component of the leakage current as decomposed by the wavelet-based multiresolution analysis is shown to be an indicator of the effectiveness of the filler type in suppressing erosion by dry-band arcing. The addition of alumina tri-hydrate or silica filler to silicone rubber increases the thermal conductivity of the composites, retarding the development of the eroding temperature, and thus the evolution of the third detail. Additional effect is also obtained for the dehydration enthalpy, of alumina tri-hydrate in silicone rubber at a filler level of 30 wt%, in impeding the development of hot spots on the tested surface. A reduction in the magnitude of the third detail is evident with filler level, indicating that the increasing volume of silica or alumina tri-hydrate reduces the temperature of the dry-band arcing plasma. Comparable levels of the leakage current third detail is found between 30 wt% alumina tri-hydrate and silica filled composites which suggests the water of hydration plays a minor role in diluting the SiR but at 58 wt% an internal oxidation mechanism that produces gases diluting the arcing phase appears to suppress erosion. [ABSTRACT FROM AUTHOR]

Metallized polypropylene film capacitors (MPPFCs) are the key components in pulsed power systems. The self-healing property guarantees MPPFCs? application under high electric fields. This paper mainly concentrates on the effect of the pulsed discharge current on the lifetime performance. The end connection peak current density (Id) and reversal coefficient (?) are two critical parameters which affect the lifetime. Experiments are designed to investigate the lifetime under different Id and ?. The results indicate that the lifetime is sensitive to ?. Empirically derived lifetime-scaling equation is given to predict the lifetime under different ?. When Id is higher than 13 A/m, the electrode fracture might occur in the MPPFCs and the probability of the sudden capacitance drop will increase. Therefore, this phenomenon results in lifetime reduction. [ABSTRACT FROM AUTHOR]

In this paper, a proposed electro-thermal model of cable terminations is presented. The cable terminations under study are exposed to a square-pulse voltage applied at different switching frequencies. The purpose of the model is to predict the temperature distribution inside the cable termination as a means of determining the location and value of the maximum temperature. The model is based on fundamental heat transfer mechanisms: conduction, convection, and radiation. Two different distribution classes of cable terminations have been examined. A comparison of the simulated temperature rises with actual measurements reveals good agreement. [ABSTRACT FROM AUTHOR]

One approach to increase the low thermal conductivity of polymers is to add nanosized or microsized fillers with relatively high thermal conductivity. The concept of nanodielectrics is based on the fact that the dielectric behaviour of polymeric composites is tailored on or dictated by nanoscale manipulation. In this paper, the role of fillers in the thermal conductivity of polymeric composites is investigated and reference is made to a number of different studies. The main goal is to determine potential trends governing or dictating the process of thermal conduction in polymeric composites including nanocomposites and microcomposites. Shape, size and thermal conductivity are some of the particle properties considered. Also, the crucial role of interfaces and agglomerations of particles is discussed. Finally, three dimensional models are implemented in Comsol Multiphysics in order to demonstrate the experimental observations and fit the experimental results on the thermal conductivity of epoxy nanocomposites. [ABSTRACT FROM AUTHOR]

In this paper, a system of equations is presented, which describes time-dependant temperature variations for a dielectric surface exposed to an arc discharge. The formulas are derived for a circular (as well as any-shape) region interacting with the arc discharge. The final set of equations makes it possible to convert temperature values, measured at some depth beneath the sample surface which is exposed to the arc heating, to the corresponding surface temperature values. The theoretical results are verified by measurements according to the IEC 61621 test method, and numerical calculations are done for an epoxy composition that is used in composite insulator manufacturing. [ABSTRACT FROM AUTHOR]

Current techniques used for monitoring the levels of contamination on high voltage insulators, such as leakage current and infrared, are not effective in dry conditions since they require the surface of the insulator to be wetted by fog, rain or snow. If a buildup of contamination occurs during a prolonged dry period prior to a weather change there will be a significant risk that flashover may occur before there is time to implement preventative maintenance. Previous work has demonstrated the use of microwave radiometry to determine the levels of contamination on an insulator material under dry conditions, however practical applications are limited by low sensitivity. This paper reports the development of a novel technique based on microwave reflectometry to detect the power levels reflected from the surface of the insulator material. The level of contamination is then determined as a function of received power. A theoretical model establishes the relationship between equivalent salt deposit density levels on insulator surface and the dielectric properties of the contamination layer. A Finite Difference Time Domain (FDTD) model is used to simulate the total loss as a function of the contamination level. Experimental results verify the FDTD model and demonstrate the sensitivity of the reflectometer system to be approximately 100 times greater than the radiometer system. Therefore, the reflectometry system has considerably greater potential for practical applications to provide advance warning of the future failure of insulators under dry conditions for both HVDC and HVAC systems. [ABSTRACT FROM PUBLISHER]

When the high voltage direct current (HVDC) system operates in monopole ground-return mode, DC current flows through the neutral of AC power transformer with the neutral grounded which exacerbates the vibration of the transformer. During the field test of Yibin-Jinhua HVDC system in China, the vibration detection system is deployed to detect the vibration signal of the 500 kV transformer in Luzhou substation, which is close to the grounding substation. The spectrum of the transformer vibration signal is analyzed by means of the wavelet transform, through which the vibration acoustic fingerprint and three typical characteristic parameters, i.e., the distribution percentage of spectral energy, the energy ratio of odd and even harmonics, and the waveform distortion ratio, are extracted respectively. When the HVDC system current increases from 0 A to 5 kA, the impact of DC bias on the operation condition of the 500 kV transformer is investigated through the analysis of the variation of vibration acoustic fingerprint and three characteristic parameters. It is indicated that the energy radio of odd and even harmonics of the 500 kV transformer vibration signal decreases significantly with the increase of the current. In addition, the distribution percentage of spectral energy and the vibration acoustic fingerprint change significantly. This paper demonstrates that the effect of DC bias on the operating condition of the transformer can be accurately assessed by the proposed indices. [ABSTRACT FROM PUBLISHER]

In the present paper, Differential Scanning Calorimetry (DSC) has been applied to bring out the role of zinc naphthenate accelerator in the curing of epoxy-anhydride pressure impregnant used for impregnation of electrical machines. Effect of concentration of zinc naphthenate on curing behavior of bisphenol-A epoxy resin and methylhexahydrophthalic anhydride hardener has been studied by obtaining dynamic DSC thermograms for uncatalysed, and catalysed system with various concentration of zinc naphthenate. The investigations reveal that the un-catalyzed purified epoxyanhydride mix exhibits an endothermic peak at temperature around 260 °C in place of cure exothermic peak in the DSC curve. The analysis of DSC and TGA data of individual resin, hardener and their mix indicate that the endothermic peak is attributed to the evaporation of resin and hardener implying that the hardener does not react with resin in absence of accelerator. The addition of zinc naphthenate accelerator at concentration of 1 pph (parts of zinc naphthenate per hundred parts of resin- hardener mix) causes two exothermic peaks in the DSC thermogram. The position and area under these peaks vary with the accelerator content resulting in the variation in the processing parameters and the performance properties of the cured material. The first peak from esterification reaction appears between 90 and 200 °C and second peak from etherification reaction appears in the temperature range of 180 to 330 °C. The area under the first exothermic peak appearing at low temperature increases with increase of accelerator content and predominates at concentration of ⩾ 1.5 pph. The area under the second exothermic peak appearing at high temperature increases with the decrease of accelerator content and predominates at concentration of ⩽ 0.5 pph. The accelerator concentration of 2 pph provides maximum reactivity. [ABSTRACT FROM PUBLISHER]

Polyimide (PI) tape has been widely used as electrical insulation for wires and cables due to its excellent balance of electrical, mechanical and thermal properties. However, insulation materials are exposed to various environmental degradation mechanisms during service, which can cause deterioration of their insulation properties. In this paper, the effects of thermal exposure and immersion in water on dielectric strength of Kapton HN PI film are investigated statistically by two-parameter Weibull distribution analysis. The PI samples were heated at 475 °C for 1 to 4 h, and at 450 to 480 °C for 4 h, to study the influence of thermal exposure. Moreover, to explore the effect of moisture absorption, other PI samples were immersed in distilled water for up to 96 h at room temperature. After the degrading process, breakdown voltage and thickness were measured at 20 different points on each sample. The Weibull cumulative distribution function for each of the degraded samples was plotted, and compared with that of a sample dried by heating at 200 °C for 1 h. Significant decreases in both the scale parameter and the shape parameter of the Weibull distribution, obtained through least-squares regression, are observed, indicating a statistical decrease in dielectric strength and a more dense dispersion of electrical weak points in PI. This observed deterioration of insulation capability of PI is attributed to chemical degradation and formation of ionic side groups during heating and immersion in water, respectively. [ABSTRACT FROM PUBLISHER]

In the condition settings of a power-frequency withstand voltage test for substation equipment, an approach based on insulation reliability evaluation using Weibull distribution is available. However, the subjects which should be investigated remains in this method. One of the subjects is that evaluating the insulated reliability for temporary overvoltage caused in an aged equipment by the pre-aging Weibull parameter. In the first place, for an oil-immersed transformer, the breakdown voltage-time (V-t) characteristics of the oil gap were obtained as basic characteristics in the previous experiment. In this experiment the presence or absence of the oil flow was used as a parameter affecting the history of the applied voltage. Also, the validity of the present evaluation approach to temporary overvoltage during operation was examined. In this paper, an insulation characteristic was obtained in consideration of the age-related deterioration in the field. The results were then analyzed in comparison with the insulation characteristics of a new insulating oil. As a result, there was little change affecting the result of insulation reliability evaluation in the Weibull parameters. Consequently, it is reasonable to evaluate the insulation reliability by the V-t characteristic of the new insulating oil. [ABSTRACT FROM AUTHOR]

Polarization and depolarization currents obtained at a low dc field for two different brands of XLPE and EPR insulation subjected to water treeing were correlated with the insulation breakdown characteristics to assess the diagnostic value of various polarization/depolarization current parameters. It is shown that for all three materials the polarization currents change more consistently with aging than do the depolarization currents. It is also shown that although the RC time constant and the area under the polarization current curve change consistently with the aging time for all three materials tested, the consistency and the diagnostic value of the information they provide about impending insulation failure depends very much on the type of material. [ABSTRACT FROM AUTHOR]

Power cables with extruded crosslinked polyethylene (XLPE) insulation are used in HVAC and HVDC applications. These cables usually go through a degassing process to remove the methane formed in XLPE during the cross-linking reactions. In case of HVDC cables, the general belief is that the polar peroxide decomposition products (PDP) content significantly influences the conduction and space charge behavior of XLPE. This belief is mainly based on the results of experiments made on thin XLPE samples with different heat-treatments; but since heat-treatment also influences the morphology of the polymer, it is necessary to consider this effect as well. It is common to use polyethylene terephthalate (PET) film as a protective layer during sample press molding. Studies on the influence of the pressing film on the electrical properties of the sample are rare. In this work, the results of a series of experiments performed on 0.5 mm thick XLPE plaque samples in reference to additive free LDPE samples with different heat-treatment times are presented. Beside the PDP content, the morphology, DC conductivity and polarization properties are studied and analyzed. It was found that the pressing film used during sample preparation has a significant effect on the results and if not corrected, it may lead to wrong conclusions about the influence of the PDP content. Eliminating the effect of the pressing film, no clear correlation between the DC conductivity, dielectric loss and the PDP content was observed. The relation between the PDP content and DC conductivity is not found to be obvious, hence this correlation may need to be further evaluated. [ABSTRACT FROM AUTHOR]

In this work, a range of polypropylene (PP) based composites filled with micro or nano sized boron nitride (BN) particles at different loadings (0, 3, 6 and 9 wt%) were manufactured to investigate effects of thermal conductivity on surface dielectric breakdown (SDB). In addition to thermal conductivity measurement of various samples, thermal dissipation was also discussed to analyze the relationship between them. Meanwhile, in order to evaluate SDB strength of the manufactured samples, time to SDB, erosion depth and weight loss of the polymeric samples were measured through the test. Obtained results show that thermal conductivity of the doped PP composite is obviously improved. The increased thermal conductivity results in improvement of thermal dissipation ability, which contributes to inhibition of SDB occurrence. Thermal conductivity, thermal dissipation ability and SDB strength all represent positive correlation to filler loading level. More importantly, compared with nano-BN filled PP samples, PP/micro-BN samples can obtain higher thermal conductivity, better thermal dissipation ability and higher SDB strength at the same filler content. Experimental validation was also conducted with dc bulk dielectric breakdown (BDB) measurement of various samples. It is concluded that introduction of nano-BN filler into base resin favors improvement of BDB strength evidently, while the sample coated micro-BN exhibits a lower BDB strength than the base resin in any filler loadings investigated. [ABSTRACT FROM AUTHOR]

To clarify the breakdown mechanism and the conditions leading to the breakdown under non-uniform electric field in air, we studied the details of breakdown characteristics and mechanism in air by using an ultra-high speed measurement system. We clarified that there were two types of breakdown mechanism in air. The one is the channel heating breakdown, and the other is the leader-type channel development breakdown. These breakdown mechanisms are dominated by three conditions: arrival of secondary streamer from high-voltage anode to opposite grounded cathode, channel heating, and leader-type channel initiation. We studied these conditions quantitatively, and revealed the conditions leading to the breakdown, and the breakdown mechanisms in air were classified. Finally, we clarified the factors, e.g. critical instantaneous power for the secondary streamer arrival, dominating these conditions based on the breakdown mechanism under non-uniform electric field in air. [ABSTRACT FROM PUBLISHER]

The implementation of space charge measurements based on thermal perturbation on thin films requires an improvement of the temperature distribution estimation at the surface and in the depth of dielectric materials for getting reliable space charge profile measurements. Absolute temperature variations are needed, both in time and space. The present contribution addresses surface temperature measurements based on either thermoelectric or bolometric effects. Both responses have been measured on coppercoated silicon nitride layers and gold-coated polypropylene films heated with a Nd:YAG laser pulse. It is shown that high temporal resolution thermal response can be obtained through the bolometric response and that the information appears nearly independent on the nature of the coating electrode. The setup developed provides good signal to noise ratio for heated electrodes of a few ohm resistance. Strategies are still to develop to get the temperature profile in the insulating sample layer. [ABSTRACT FROM AUTHOR]

Scanning Pyroelectric Microscopy (SPEM) at micrometer resolutions has been employed to study permanent polarity and its spatial organization in various insect wings containing chitin, one of the most abundant biopolymers. Both measurements on wings of houseflies and wasps revealed a well defined pyroelectric response, showing a multidomain polarization pattern at several length scales, from millimeter sized domains in the wing veins, to 20 micrometer sized domains in the membranes. The size of the smaller domains correlates well with the average chitin fiber length of 18 micrometers. [ABSTRACT FROM AUTHOR]

The incubation criterion of the 62535 standard procedure of the International Electrochemical Commission (IEC) was modified to obtain the depletion profiles for different concentrations of each of 1,2,3-benzotriazol (BTA) and dibenzyl disulfide (DBDS) when both are present in a mineral oil matrix. Measurements on BTA concentrations ranging from 0 to of 70 mg L?1 show that its depletion profile after incubation for 72 h, at room temperature, and at 150°C in the presence and absence of a copper strip, is the same irrespective of the DBDS concentration. Similar measurements on DBDS at concentrations ranging from 10 to 300 mg L?1 show that identical depletion profiles are obtained as long as the BTA concentration is maintained in excess of 5 mg L?1. The results show that a minimum BTA concentration of 5 mg L?1 is needed to make the copper windings in contact with the mineral oil passive and in turn suppress their sulfur corrosion by DBDS. [ABSTRACT FROM AUTHOR]

An isothermal microcalorimeter has been used to study the effect of solid copper in hexadecane, vegetable oil (ester) and mineral oil. The experiments have shown a catalytic effect of copper in oxidation of hydrocarbons and also the anti-oxidative effect of dibenzyl disulphide when combined with copper addition. The catalytic effect of copper is found in hydrocarbon as base oil and hexadecane, but not in experiments using inhibited oil and ester as the dielectric liquid. [ABSTRACT FROM AUTHOR]

To operate power transformers long-term, as well as ensuring their insulating reliability, an adequate study must be conducted on the age-related decline of various insulating oil characteristics and the method to inhibit the same. Accordingly, in the present study, 1,2,3-benzotriazole (BTA) and 2,6-dibutyl-4-methylphenol (DBPC), which are expected to be effective in preventing oxidation degradation, were added to the test oil and an accelerated degradation test was conducted to examine their effect. Consequently, for new oil, an deactivating effect on copper by the addition of BTA was confirmed and the ability to prevent the decline in characteristics caused by the degradation of the dissipation factor and volume resistivity could be observed. For aged field oil, however, the effect was less than that for new oil. Since phenol compounds structurally similar to DBPC may have been present in aged field oil, a study was conducted on the increasing tendency of concentration and saponification value of the former. As a result, it emerged that the presence of phenol compounds inhibited the increase in the saponification value and boosted oxidation degradation inhibition, even if BTA or DBPC had not been added. Based on the above results, it was clarified that, where an oxidation degradation inhibitor was added or structurally similar compounds were contained, an inhibitory effect on the aging of insulating oil could be obtained. [ABSTRACT FROM AUTHOR]

Prevention of icing flashover on transmission lines has become an important problem. A new method for mitigating effects of glaze ice on energized insulators has been developed. Semiconductive silicone rubber was painted on the bottom of suspension disc insulators, while the upper ceramic surface was untreated. An electrically conductive path was formed when supercooled water or droplets bridged the uncoated areas. The power loss was limited when there was no precipitation. The heat and partial discharge energy generated by the leakage current melted the original ice accretion on the insulator surface. The electrical performance of the insulator string with this kind of coating under freezing rain conditions was comparable with that of a clean insulator string. The impact of the ambient temperature on the anti-icing performance was also evaluated. In addition, the performance of the insulator with bottom surface semiconductive coating under fog conditions was compared with an uncoated string. [ABSTRACT FROM PUBLISHER]

Several major parameters affecting the electrical behavior of natural snow, namely volume conductivity and density, liquid water content and conductivity of water melted from snow, are crucial for the characterization of the electrical performance of snow-covered HV insulators. However, little study has been devoted to this subject, despite of its importance. These parameters are found to vary significantly with snow composition and purity as well as with other parameters, such as temperature, airborne pollutants, electric field strength and polarity. From laboratory experiments carried out on a large number of snow samples, it was found that dc conductivity of snow shows a peak at about -2°C. This apparently curious behavior near the melting temperature is attributed to important changes to the microstructure of snow. A correlation between dc conductivity and snow temperature was established. [ABSTRACT FROM AUTHOR]

The authors investigated how changes in the molecular structure of sulfur compounds in mineral insulating oils affect their corrosivity. A method for the quantitative evaluation of corrosivity of insulating oils was developed and used in this study. Several types of sulfur compounds were found to cause sulfur corrosion. However, hydrotreated oil was noncorrosive because the sulfur compounds which cause sulfur corrosion were easily removed through hydrotreatment. The results of thermal aging tests using model sulfide compounds showed that the corrosivity of sulfide-added oils increased with longer heating times due to generation of sulfoxide, caused by the oxidation of sulfide. The corrosivity of aged oils depends on the concentration of sulfoxide in the oils. Meanwhile, the corrosivity of a commercial insulating oil did not change drastically in the thermal aging test, despite the fact that this oil contained a certain amount of sulfide. [ABSTRACT FROM AUTHOR]

A triggered vacuum switch (TVS) is one of the important switch apparatuses in the field of pulsed power system, due to its quick switching, rapid dielectric recovery, small volume, convenient trigger system, and simple structure. A sample of TVS was fabricated and its turning-on experiments were done. Initial plasmas play an important role in the turning-on process of TVS. The delays of TVS were measured, including wait time t1 and collapse time t2, which relate to the generation and development of initial plasmas. From thermodynamics equations, a model of cathode spot heat conduction was proposed. A formula deduced from the heat conduction model could be used to calculate the delay time of TVS. The calculated results of t1 and t2 were in accordance with the tested results. [ABSTRACT FROM PUBLISHER]

A comprehensive experimental investigation of XLPE and its nanocomposite with fumed silica (SiO2) has been performed by CIGRE Working Group D1.24, in cooperative tests conducted by a number of members; covering materials characterization, real and imaginary permittivity, dc conductivity, space charge formation, dielectric breakdown strength, and partial discharge resistance. The research is unique, since all test samples were prepared by one source, and then evaluated by several expert members and their research organizations. The XLPE used for preparation of the nanocomposites was a standard commercial material used for extruded power cables. The improved XLPE samples, based on nanocomposite formulations with fumed silica, were prepared specifically for this study. Results of the different investigations are summarized in each section; conclusions are given. Overall, several important improvements over unfilled XLPE are confirmed, which augur well for future potential application in the field of extruded HV and EHV cables. Some differences/discrepancies in the data of participants are thought to be the result of instrumental and individual experimental technique differences. [ABSTRACT FROM AUTHOR]