Insulation Samples Research Articles

Preventive Measurement and Online Insulation Evaluation of Power Transformer

Cellulose based non conducting materials and mineral oils have widely been used in liquid filled transformers. Environmental and fire-safety concerns have accelerated the use of Natural Esters (NE) as a sustainable alternative to mineral oil. To ensure a safe and long-term operation of the vegetable oil filled transformers, it is need to clarify the knowledge gaps with respect to the ageing of vegetable oil impregnated cellulose and insulation diagnostic methods. The condition monitoring of mineral oil filled transformers has been well characterised, due to the numerous laboratory and field studies conducted during the past several decades. The application of these available condition monitoring techniques for NE filled transformers is yet to be validated as the use of NE for power transformers is in the tested stage. In this paper the performance of NE as insulation in transformers by evaluating the ageing of NE impregnated cellulose pressboard is presented. For this purpose, an accelerated ageing experiment has been carried out in sealed tubes at 120 ⁰C. The ageing of oil impregnated pressboard has been characterised by decrease in the degree of polymerisation (DP) of pressboard, and the formation of both 2-furfuraldehyde and dissolved carbon dioxide (CO2) in the oil. The structural changes of cellulose have been supported by fuzzy logic algorithm. Moreover, the results have been compared with cellulose insulation samples aged under similar conditions in conventional mineral oil.

Preparation of Cobalt doped Bi2Se3 nano-topological insulators

Magnetically doped topological insulator samples, namely Co–Bi2Se3, were prepared at the nanoscale using a combined organic–inorganic hot-injection technique. In such a one-pot synthesis method, the nanoparticle sizes were tuned by varying the time at which the nanoparticles are drawn from the reaction flask. Highly crystalline, spherical, and dispersive ultra-small nanoparticles were imaged by High Resolution Transmission Electron Microscopy. It was shown that deviations from the spherical shapes and reduced crystallinity occur for samples taken at longer reaction times, however, with slight enlargement of the average particle sizes. The Infrared spectra taken for all samples showed two well-defined peaks located at 414 and 438 cm−1 which were assigned to Bi2Se3 stretching modes. A strong red-shift in the UV–visible absorption spectra for samples taken at different reaction times was also observed. The present system provides the zero-dimensional analogy to the rapidly progressing field of 3D topological insulators doped with magnetic materials.

Analysis of frequency domain dielectric response of pressboard insulation impregnated with different insulating liquids

This study investigates the influence of moisture, temperature and diverse oil properties on the frequency domain response of pressboard insulation samples, which are impregnated with four different insulating liquids including two commercially available natural esters, one synthetic ester and mineral oil. Measurements are performed over a wide frequency range from 0.1 mHz to 1 kHz at variable moisture contents and temperatures (35, 55 and 75°C). It has been found that dielectric response is extremely sensitive to moisture content regardless of the type of impregnated liquid. Furthermore, dielectric response behaviour of ester impregnated pressboard insulation is qualitatively resembles to that of mineral oil impregnated pressboard which contains two distinct relaxation processes including Quasi-DC (q-dc) conduction features at low frequencies, and loss peak behaviour at mid frequency range. This paper proposes that the loss peak arises due to cooperative microscopic motion in water cellulose mixed phase dipole clusters together with the microscopic level interfacial polarization occurring at the interface of oil filled cavities. It is also discussed the q-dc process in pressboard insulation due to the charge transport between water and oil cavity based charge clusters through a percolation network of water molecules and hydroxyl groups along the cellulose fiber wall. In order to provide better understanding on charge transport and polarization phenomena in oil impregnated pressboard insulation, this paper also proposes an equivalent circuit, which is mainly derived based on Dissado- Hill (DH) cluster framework theory. Furthermore, this study has used activation energy to characterize the temperature dependent dielectric response of pressboard impregnated in mineral oil, natural esters and synthetic esters.

The Impact of Thermal Degradation on Properties of Electrical Machine Winding Insulation Material

Interturn stator short circuits can develop quickly leading to serious damage of an electric machine. However, degradation mechanisms of winding insulation material are not yet fully understood. Therefore, the main contribution of this paper is the analysis of the impact of thermal ageing on the electrical properties of the thin-film winding insulation. The insulation samples have been aged thermally at 200 °C–275 °C and for 100–1600 h. After ageing, impedance spectroscopy measurements were undertaken on the samples and equivalent-circuit model (ECM) parameters fitted for each measurement. This allows the impact of thermal ageing on ECM parameters to be analyzed, giving insight into the changes in the electrical properties of the insulation. Finally, high voltage was applied to the samples aiming to identify the breakdown-voltage (BV) characteristics of the insulation material.

Quantitative Estimation of Mullite in Alumina Based Porcelain Insulators Samples

Although porcelain insulators in power lines have been in use for decades, several undergo catastrophic failures while early in service. A study addressing the materials aspects has been conducted on alumina based porcelain insulators. Apart from their electro mechanical stability characteristics, material aspects like composition, phase and microstructure play a very important role in their performance. The qualitative phase analyses on the insulator samples give an idea about the different major and minor phases present. Attempts have already been made to estimate α alumina and α quartz content in porcelain body. Mullite is another important phase present in the porcelain body, which gets formed during firing of raw materials during insulator manufacture and which has not been estimated quantitatively. An attempt has been made here to assess the mullite content in the alumina based insulator samples by X-Ray diffraction quantitative analysis. Internal standard method of quantification has been adopted successfully for the analysis. This article describes the quantification of mullite phase by the internal standard method based on X-Ray diffraction principle.

Impact of mist and acidic fog on polymer insulator samples exposed to corona discharges

Corona resulting from the metallic parts of the transmission line insulators and hardware has proven to be the potential factor in inducing the degradation of polymer insulators. Corona activity is found to increase substantially during the foul weather condition (rain, fog/mist, hoarfrost), which further promotes the ageing of polymer insulators. Present work investigates the possible corona activity under fog/moist condition and its effects on the long term performance of polymer insulators in service. The spread of corona generated plasma is observed to be different under fog condition than in normal air condition. The concentrated streamers that are originated from the corona electrode during fog condition are believed to damage the polymer shed material locally and more intensely. This was confirmed by the scanning electron microscopy and Fourier infrared spectroscopy analysis carried out on the treated polymer samples, which showed more damage on the corona treated under fog than on the samples treated under normal air condition. Further TGA and EDAX studies have shown that loss of ATH occurs on the sample due to intense corona activity.

Desempenho físico-químico de filmes hidrofóbicos de AlN e de Ti nanoestruturado crescidos por plasma frio sobre superfícies de isoladores cerâmicos de porcelana tipo pino

Resumo A resistência mecânica e a propriedade dielétrica de isoladores cerâmicos são características fundamentais para o suporte das redes de distribuição de energia elétrica. Porém, tanto a umidade quanto o teor de poluentes na atmosfera podem vir a ser prejudiciais e causar falhas na entrega de energia por elevação da sua corrente elétrica de fuga. O vidrado do isolador tem grande afinidade com a água resultante da interação da superfície do vidro com a atmosfera. No presente trabalho, foi realizada a deposição de filmes de nitreto de alumínio sobre o vidrado de amostras de isoladores elétricos cerâmicos, via plasma "magnetron sputtering" pulsado, de corrente contínua, e com uma mistura dos gases argônio e nitrogênio e de Ti sobre isoladores tipo pino para classes de tensão de 15 kV. A morfologia dos filmes e o desempenho dos isoladores foram medidos por MEV-FEG/EDS, AFM, máxima corrente de fuga RMS e grau de hidrofobicidade. O comportamento dos filmes depositados medido pelo grau de hidrofobicidade reduziu nas amostras de CM6 para CM4, com AlN, e de CM7 para CM3 quando foi depositado o Ti. A máxima corrente de fuga RMS medida nos isoladores sem e com filme nanométrico à base de Ti foi superior nos dispositivos sem a proteção, alcançando valores medidos de 5 mA e com desligamento do sistema computadorizado de medida (após 3 dias em Sauípe e 3 meses em Pituba) em consequência desta ter superado o intervalo de segurança de curto circuito estabelecido nos dispositivos eletrônicos de proteção (surtos superiores a 300 mA). Com a deposição dos filmes em 7 isoladores, foram verificadas alterações de comportamento em dois destes, com picos de máxima corrente de fuga atingindo 2,4 mA no campo. A maioria dos isoladores analisados teve como variações de correntes de fuga RMS, no período de medida, entre 0,2 e 2 mA, em condições adversas de clima (com chuva e diferentes velocidades de vento).

Ultrasonic phased array detection of internal defects in composite insulators

To reduce the risk imposed by use of defected composite insulators on the operation of power grids, this paper introduces a nondestructive ultrasonic phased array (UPA) technique that allows effectively test such insulators. The method offers a great potential by reducing inspection time as well as allowing for analyzing components characterized by a complex geometry. The UPA inspection system utilizes an openended rectangular waveguide sensor, operating at frequency of 2.5 MHz. The system is simple, safe and relatively inexpensive. In this work, samples of silicone rubber composite insulators with various types of detects are studied and the obtained results show that void defects in the bulk of the insulator housing are easiest to be detected. Holes under insulator sheds can also be detected by the edge of scanning range. For the defects near the core-shed interface, the detection becomes possible by comparisons with sample without defects.

Understanding the ageing aspects of natural ester based insulation liquid in power transformer

Cellulose based insulation materials and mineral oils have widely been used in liquid filled transformers. Environmental and fire-safety concerns have accelerated the use of Natural Esters (NE) as a sustainable alternative to mineral oil. There is a rapid growth of their application for distribution level transformers. To ensure a safe and long-term operation of the vegetable oil filled transformers, it is necessary to clarify the knowledge gaps with respect to the ageing of vegetable oil impregnated cellulose and insulation diagnostic methods. The condition monitoring of mineral oil filled transformers has been well characterised, due to the numerous laboratory and field studies conducted during the past several decades. The application of these available condition monitoring techniques for NE filled transformers is yet to be validated as the use of NE for power transformers is in the evaluation stage. In this paper the performance of NE as insulation in transformers by evaluating the ageing of NE impregnated cellulose pressboard is presented. For this purpose, an accelerated ageing experiment has been carried out in sealed tubes at 120 °C. The ageing of oil impregnated pressboard has been characterised by decrease in the degree of polymerisation (DP) of pressboard, and the formation of both 2-furfuraldehyde and dissolved carbon dioxide (CO2) in the oil. The structural changes of cellulose have been supported by Fourier Transform Infrared (FTIR) spectra. Moreover, the results have been compared with cellulose insulation samples aged under similar conditions in conventional mineral oil. Acidity value, dissipation factor and viscosity measurements have been used to compare the stability of NE in a transformer operating environment to mineral oil.

Corona degradation of the polymer insulator samples under different fog conditions

Corona discharges resulting from the metal parts of insulators and the line hardware affect the long term performance of the polymeric insulators used for outdoor application and can lead to its eventual failure. The authors previous work, involved in developing a new methodology to evaluate the performance of polymeric shed materials subjected to corona stresses in the presence of natural fog condition, results revealed more surface hydroxylation thereby resulting in more loss of hydropobhicity. With the increase in industrialization, there is an increase in acidic component of the rain as well as the fog (moisture). The present work, reports the effect of acid fog on the corona performance of the polymeric insulators for both AC and DC excitation, interesting results are obtained. A comparison of the experimental investigations revealed that the acidic fog has more effect than that of the normal fog. This fact has been confirmed by physico-chemical analysis like the scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), x-ray photoelectron spectroscopy (XPS) and contact angle measurement. The effect of DC corona is found to be lesser in comparison with the AC; however the hydroxylation induced by the DC corona under the presence of fog is similar with that of AC excitation.

Applicability Analysis of Selected Methods of Determining the Moisture Content of Highly Porous Thermal Insulations

Spray foam thermal insulations are one of new materials, which are now considerably applied in building practice and which are therefore also the aim of current research. In building practice is an effort to apply these materials to very rugged surfaces in both the external and the internal side of the structure. On the interior side, however, the application is connected with the risk of condensation, which is difficult to verify by the measurement. The contribution is focused just on the comparison of methods and measuring devices designed for immediate detection of moisture in the material of sprayed polyurethane foam. The samples of thermal insulation, which have undergone exposing of humidity in accordance with Czech Standards, were compared by the experimental assessment. The obtained results were compared and evaluated, while as expected, the moisture measuring of highly porous thermal insulation is significantly more difficult than the measuring of thermal insulations made of conventional homogeneous building materials.

Life evaluation of insulation paper used in transformer by determining the activation energy

Mechanical strength of insulation paper was the key factor for the life of large oil-immersed transformer. To more accurately assess the life of the insulation paper, the aging mechanisms of transformer insulation paper were analysed and a new method using the activation energy was proposed to access the paper's life. Then the life prediction model was defined. Accelerated aging tests of transformer oil-paper insulation samples were executed under different temperatures. According to the test results, the parameters of life prediction model were found out. So, the life of insulation paper under the working condition could be calculated. In the end, an example of insulation paper life evaluation was given to illustrate the effectiveness of this method.

Time to failure of medium-voltage form-wound machine turn insulation stressed by unipolar square waves

The lifespans of enamelled turn insulation samples are studied when stressed with square waves characterized by different rise times, duty cycles, switching frequencies, and applied voltages. Partial discharge activities under these test conditions have also been recorded using an RF antenna in an effort to correlate the time to breakdown with material degradation under fast repetitive pulse aging. The aging and PD features were independent of rise time for a 50 % duty cycle; however, slightly faster times to failure were observed at a 15 % duty cycle for the faster rise times, for the considered range. The higher duty cycle and the higher number of applied cycles accelerated the aging.

Study on glaze electrical erosion characteristics of porcelain post insulator by using inclined plane and graphite-layer-based method

In order to study the effect of glaze erosion on the performance of porcelain post insulator, an inclined plane (IP) test similar to IEC 60587 standard was conducted on a cylindrical insulator sample under laboratory conditions for ac voltage, and a new test method, called graphite-layer-based glaze erosion test was proposed in this paper and performed on the sheds of a 40.5 kV post insulator. The experimental details of the graphite-layerbased glaze erosion test was introduced, the technical points of which are coating a graphite layer on the surface of the shed using a brush mold which contains multiple hollowed-out rhombic grids, and arranging high voltage electrode at shed root and grounded electrode at shed edge respectively at both ends of the graphite layer, so as to form continuous and stable surface arcing under ac voltage and therefore achieve desired glaze electrical erosion effect. The graphite layer is a uniform viscous mixture made up with flake graphite and liquid glue according to a quality ratio of 1:1.2. The arcing process, arc current characteristics, surface temperature, and glaze erosion appearance were analyzed. Leakage current under power frequency voltage and flashover voltage of the eroded cylindrical insulator by IP test were measured, while porosity test and scanning electron microscopy (SEM) tests were conducted on samples taken from the sheds after graphite-layer-based glaze erosion test. The results indicate that glaze erosion will decrease both electrical and mechanical performance of the porcelain post insulator. The new test method proposed in this paper has been proved to be effective for glaze erosion test and consumes much less time compared with IP test, which provides a new idea for accelerated tracking and erosion test of glaze and porcelain materials so as to study the aging characteristics of porcelain insulators.

Aging degree evaluation for paper-oil insulation using the recovery voltage method

The paper presents new diagnostic method applied to assess aging degree for paper-oil insulation used in professional electrical power device, primarily including medium and high power transformers. The described method is based on modified diagnostics using the recovery voltage effect occurring in dielectrics (RVM – Recovery Voltage Methods). Modification of the RVM method applied so far, which is now used only to assess moistening degree for paper-oil insulation in power transformers, consists in a new parametric analysis of recovery voltage time characteristics. Simulation and experimental tests carried out in laboratory conditions for adequately prepared insulation samples have proven that the time-constant of recovery voltage decay (τR_MIN) is the most susceptible parameter as regards quantitative change in the function of aging of the examined insulation. This conclusion is also confirmed by statistical studies, in which a multi-parameter variance analysis method has been applied. An original equivalent circuit diagram of the insulation has been used to simulate aging degree of paper-oil insulation samples in numerical tests. The diagram is based on the Cole-Cole type dielectric response model.

Quantitative evaluation of ageing condition of oil-paper insulation using frequency domain characteristic extracted from modified cole-cole model

The service life of a transformer is determined by the condition of its oil–paper insulation. A modified Cole–Cole model was introduced to describe the frequency domain spectroscopy (FDS) curves based on the classic Debye relaxation model and thereby accurately determined the ageing state of the insulation of a power transformer using FDS. Oil–paper insulation samples with different ageing states are prepared under laboratory conditions, and a characteristic parameter is extracted to quantify their ageing states after a detailed investigation of the frequency domain dielectric properties of the samples. Results reveal that the modified Cole–Cole model, including the α relaxation components, dc and hopping conductivity components, can be employed to effectively model the frequency dielectric characteristics of oil–paper insulation. Furthermore, the extracted characteristic parameter from this model, τα, can be utilized in the quantitative evaluation of the ageing states of transformer insulation. A case example is discussed to demonstrate the applicability of the proposed technique in the condition assessment of field power transformers.

Electrical treeing behavior at high temperature in XLPE cable insulation samples

This paper reports on investigations aiming to understand the behavior of electrical trees formed in XLPE cable insulation at elevated temperatures. Samples cut from a real high voltage cable equipped with an embedded electrode system and composed of a needle separated by 2 mm from counter electrode were used. The temperature was varied between 50 and 90 °C and the range of voltage changed between 9 and 18 kV. The electrical treeing process was followed by means of microscopic observation from the moment of initiation until the leading branches connected the counter electrode and partial discharge (PD) activity was monitored simultaneously. The shape and fractal dimension of the resulting trees were analyzed. It was found that temperature and voltage level had pronounced effect on the process of electrical tree formation. At lower voltages different type of trees could be formed, whereas at higher voltages branch trees dominated. Increase of test temperature reduced the time for tree initiation, whereas the tree growth time was less affected, except for the lowest voltage level at which the slowest tree development was observed at the highest test temperature. It is postulated that the impeding effect of elevated temperature on tree growth is due to a change of tree channel sidewall conductivity, whereas the observed reduction of tree initiation time with the increase of temperature and voltage level is attributed to lowering of the threshold energy for damage at higher temperature as well as increasing of the injection current at higher voltage level.

Influence of pin corrosion on mechanical characteristic of UHVDC disc suspension insulators and solutions

Hardware corrosion has occurred upon lots of suspension-type porcelain and glass insulators of ±800 kV Chusui and Xiangshang UHVDC transmission lines in China since October 2011. The cause of pin corrosion and its influence on the mechanical property of insulators are researched in this paper. The mechanical load that insulator bears under the circumstance of conductor icing and galloping is calculated by means of ANSYS software. Also, the failure forms of insulators under large mechanical load condition are analyzed. On the basis of this, the method that installing organic material sleeve onto the zinc sleeve to vary its corrosion area is proposed. The problem of mechanical strength decreasing of the insulators caused by expansion of corroded by-products is solved. Insulator samples with organic material sleeves of three structures including zinc sleeve half package type, flush type, and whole package type are made, and pin electrolytic corrosion simulation tests are carried out upon the three kinds of samples respectively by means of the spray water method. It turns out that half package type organic material sleeve alters the corrosion part of zinc sleeve effectively and the pins of insulators with two other kinds of organic material sleeves were corroded seriously. The electric field intensity of insulators is calculated by means of ANSOFT software. Calculation results indicate that the semi-conductive rubber sleeve provides the insulator with better electric field intensity characteristic.

Initiation of vacuum insulator surface high-voltage flashover with electrons produced by laser illumination

In this paper, experiments are described in which cylindrical vacuum insulator samples and samples inclined at 45° relative to the cathode were stressed by microsecond timescale high-voltage pulses and illuminated by focused UV laser beam pulses. In these experiments, we were able to distinguish between flashover initiated by the laser producing only photo-electrons and when plasma is formed. It was shown that flashover is predominantly initiated near the cathode triple junction. Even dense plasma formed near the anode triple junction does not necessarily lead to vacuum surface flashover. The experimental results directly confirm our conjecture that insulator surface breakdown can be avoided by preventing its initiation [J. G. Leopold et al., Phys. Rev. ST Accel. Beams 10, 060401 (2007)] and complement our previous experimental results [J. Z. Gleizer et al., IEEE Trans. Dielectr. Electr. Insul. 21, 2394 (2014) and J. Z. Gleizer et al., J. Appl. Phys. 117, 073301 (2015)].

Non-destructive condition monitoring of aged ethylene-propylene copolymer cable insulation samples using dielectric spectroscopy and NMR spectroscopy

The causes of changes in dielectric response as a result of thermal and irradiative ageing of cable insulation of ethylene propylene copolymer rubber containing 38 wt.% filler were investigated. Samples were aged in three different combinations of irradiation dose rate and temperature, 0.42 kGy h−1 at 85 °C, and 1.58 kGy h−1 at 55 and 85 °C, and subsequently studied by dielectric spectroscopy, NMR spectroscopy using a portable spectrometer, and tensile testing. The extractable mass fraction and density were determined and related to the imaginary part of the dielectric permittivity at 100 kHz. The ageing led to an increase in the dielectric permittivity, stiffness, density and degree of oxidation, together with a decrease in both strain-at-break and relaxation time, as revealed by NMR spectroscopy. Except for the strain-at-break, the properties changed in a linear fashion with increasing imaginary part of the dielectric permittivity at 100 kHz, with particularly good agreement with respect to the density. As these properties are affected by the degree of oxidation, the results show that both NMR using a portable spectrometer and dielectric spectroscopy can be used as condition monitoring techniques to detect the degree of oxidation in complex systems such as filled copolymers.