Lanca, M. C., and J. Marat-Mendes,
"Dielectric breakdown statistics of polyethylene for progressively-censored data",
Advanced Materials Forum Ii, vol. 455-456, pp. 602-605, 2004.
AbstractThe dielectric breakdown of thin films of low-density polyethylene (LDPE) electrically aged in an aqueous solution of NaCl under an AC electric field was investigated. A two-parameter Weibull function was used for the dielectric breakdown time to failure. The probability of failure for a sample was obtained by the White method for progressively censored data. Samples aged at different temperatures were compared. The results show that initially the samples aged at lower temperature (approximate to25degreesC) are more prone to fail, while those aged at higher temperature (50degreesC) fail at longer times. This was attributed to a competition between oxidation and diffusion.
Lanca, M. C., E. R. Neagu, L. A. Dissado, and J. Marat-Mendes,
"Space charge studies in XLPE from power cables using combined isothermal ans thermostimulated current measurements",
Advanced Materials Forum Iii, Pts 1 and 2, vol. 514-516, pp. 935-939, 2006.
AbstractCross-linked polyethylene (XLPE) peelings from aged power cables from three different sources were studied using a combined procedure of isothermal and thermo-stimulated current measurements. Different parameters, such as electric field, temperature, charging/discharging times, can be selected in order to make an analysis of the space charge characteristics (such as, relaxation times and activation energies). Three different cables peelings were analyzed: A - electrically aged in the laboratory at high temperature, B - service aged for 18 years and C - thermally aged in the laboratory at high temperature. The results were compared for the different types of samples and also with previous results on laboratory aged and produced films of low-density polyethylene (LDPE) and XLPE.
Lanca, M. C., M. Fu, E. Neagu, L. A. Dissado, J. Marat-Mendes, A. Tzimas, and S. Zadeh,
"Space charge analysis of electrotherinally aged XLPE cable insulation",
Journal of Non-Crystalline Solids, vol. 353, no. 47-51, pp. 4462-4466, 2007.
AbstractCross-linked polyethylene (XLPE) is currently widely used as an insulating material for power cables due to its good physical properties, however when in use it undergoes an electrical ageing process. Its ability to trap electric charge can give rise to space charge accumulation in the bulk of the polymer and produce localised electric stresses that can lead to cable failure, since the electric field will be increased above the design stress in some regions favouring the initiation of degradation there. In this work the PEA (pulsed electro-acoustic) method was used to compare the charge dynamics in three samples (XLPE cable peelings) aged in different ways (electrothermally in the laboratory, field aged in service and thermally aged in the laboratory). Very different transient behavior was found depending upon the ageing history. This is related to differences in the migration of chemical species in the insulation layer, which are known to act as charge traps. All materials showed heterocharge peaks when the space charge reached stability, the magnitude of which seems to be related to the severity of the ageing. (c) 2007 Elsevier B.V. All rights reserved.
Lanca, C. M., S. Peuckert, E. R. Neagu, L. Gil, P. C. Silva, and J. Marat-Mendes,
"Electrical Properties Studies of a Cork/TetraPak (R)/Paraffin Wax Composite",
Advanced Materials Forum Iv, vol. 587-588, pp. 613-617, 2008.
AbstractLately the electrical and dielectric properties of cork and some cork-based materials (commercial and non-commercial) have been studied in order to understand their ability to store electrical charge. The main problem found so far is related to the water content in cork, only of a few % weight. but large enough to influence greatly the conductivity of cork and, consequently, the charge storage capability. To overcome this problem cork has been combined with hydrophobic materials. In this work a commercial wax (paraffin wax) was used to produce a cork/paraffin composite by hot pressing. After milled and mixed natural cork. TetraPak (R) containers waste and paraffin were pressed to make plaques of a new composite. Different concentrations of cork. TetraPak (R) and paraffin, different granules sire, different temperature and pressure were used to produce the samples. The electrical properties of the new composite were measured by the isothermal charging and discharging current method and the results compared to previously ones obtained for natural cork and other derivative products. The new composite has shown to have lower conductivity than the commercial agglomerate. which makes it a better material for charge storage.
Lanca, M. C., S. Peuckert, E. R. Neagu, L. Gil, P. C. Silva, and J. Marat-Mendes,
"Electrical Properties Studies of a Cork/TetraPak (R)/Paraffin Wax Composite",
Advanced Materials Forum Iv, vol. 587-588, pp. 613-617, 2008.
AbstractLately the electrical and dielectric properties of cork and some cork-based materials (commercial and non-commercial) have been studied in order to understand their ability to store electrical charge. The main problem found so far is related to the water content in cork, only of a few % weight. but large enough to influence greatly the conductivity of cork and, consequently, the charge storage capability. To overcome this problem cork has been combined with hydrophobic materials. In this work a commercial wax (paraffin wax) was used to produce a cork/paraffin composite by hot pressing. After milled and mixed natural cork. TetraPak (R) containers waste and paraffin were pressed to make plaques of a new composite. Different concentrations of cork. TetraPak (R) and paraffin, different granules sire, different temperature and pressure were used to produce the samples. The electrical properties of the new composite were measured by the isothermal charging and discharging current method and the results compared to previously ones obtained for natural cork and other derivative products. The new composite has shown to have lower conductivity than the commercial agglomerate. which makes it a better material for charge storage.
Lança, C. M., E. R. Neagu, C. Dias, and J. Marat-Mendes,
Dielectric spectra of natural cork and derivatives,
, vol. 356, pp. 763-767, Jan, 2010.
Abstractn/a
Lanca, M. C., I. Cunha, J. P. Marques, E. R. Neagu, L. Gil, C. J. Dias, and J. N. Marat-Mendes,
"Water Content Control To Improve Space Charge Storage in a Cork Derivative",
Advanced Materials Forum Vi, Pts 1 and 2, vol. 730-732, pp. 395-400, 2013.
Abstractn/a
Lanca, M. C., J. N. Marat-Mendes, and L. A. Dissado,
"The fractal analysis of water trees - An estimate of the fractal dimension",
Ieee Transactions on Dielectrics and Electrical Insulation, vol. 8, no. 5, pp. 838-844, 2001.
AbstractWater trees result from ac electrical aging of the polymeric insulation of medium and HV power cables in a humid or wet environment. As suggested by their name, they arise from penetration of water in the polymer. Visual observation with the help of an optical microscope shows tree (bush) type structures. This suggests that water trees might be fractal objects. Calculation of the fractal dimension from experimental samples may confirm the fractal characteristics and also give information on the damage caused to the polymer. In this work images of water trees taken under the optical microscope, dyed by methylene blue and etched for scanning electron microscopy (SEM), were studied in order to estimate the fractal dimension using a box-counting algorithm. The photographs, made using an optical microscope (scale of 100 mum), of the dyed samples were obtained from laboratory-aged low-density polyethylene (LDPE) specimens using accelerated techniques. Different field amplitude and frequency and also time of aging were used and the dimension values were compared. SEM images resulting from aged cross-linked polyethylene (XLPE) cables revealed a structure at a different scale (similar to 3 mum). Each photograph was analyzed to compare regions with and without water trees.
Lanca, M. C., E. R. Neagu, and J. N. Marat-Mendes,
"Combined isothermal and non-isothermal current measurements applied to space charge studies in low-density polyethylene",
Journal of Physics D-Applied Physics, vol. 35, no. 8, pp. L29-L32, 2002.
AbstractA new experimental procedure combining usual isothermal DC charging and discharging with non-isothermal current measurements has been recently proposed. It is mainly suitable for very high insulating polymers and it was successfully applied to the study of space charge trapping and transport in low-density polyethylene. The analysis of the isothermal currents revealed the presence of different traps whose characteristic (de)trapping times can be deduced. The isothermal procedures allowed the selective charging of the sample. By choosing the charging field and the ratio of charge/discharge times, non-isothermal analysis permitted the differentiation of three or four peaks (at approximate to50degreesC, approximate to65degreesC, approximate to70degreesC and approximate to85degreesC) associated with charge detrapping from surface or near-surface (<20 mum) traps. These traps have activation energies between 0.21 and 1.54 eV. The mobility at 30degreesC is around 5 x 10(-16) m(2) V-1 s(-1). Samples had to be conditioned before each experiment in order to obtain reproducible results.
Lanca, M. C., and J. Marat-Mendes,
"Dielectric breakdown statistics of polyethylene for progressively-censored data",
Advanced Materials Forum Ii, vol. 455-456, pp. 602-605, 2004.
AbstractThe dielectric breakdown of thin films of low-density polyethylene (LDPE) electrically aged in an aqueous solution of NaCl under an AC electric field was investigated. A two-parameter Weibull function was used for the dielectric breakdown time to failure. The probability of failure for a sample was obtained by the White method for progressively censored data. Samples aged at different temperatures were compared. The results show that initially the samples aged at lower temperature (approximate to25degreesC) are more prone to fail, while those aged at higher temperature (50degreesC) fail at longer times. This was attributed to a competition between oxidation and diffusion.
Lanca, C. M., S. Peuckert, E. R. Neagu, L. Gil, P. C. Silva, J. Marat-Mendes, A. T. Marques, A. F. Silva, A. P. M. Baptista, C. Sa, F. J. L. A. Alves, L. F. Malheiros, and M. Vieira,
"Electrical Properties Studies of a Cork/TetraPak (R)/Paraffin Wax Composite",
Advanced Materials Forum Iv, vol. 587-588, pp. 613-617, 2008.
Abstractn/a
Lanca, M. C., S. Peuckert, E. R. Neagu, L. Gil, P. C. Silva, and J. Marat-Mendes,
"Electrical Properties Studies of a Cork/TetraPak (R)/Paraffin Wax Composite",
Advanced Materials Forum Iv, vol. 587-588, pp. 613-617, 2008.
AbstractLately the electrical and dielectric properties of cork and some cork-based materials (commercial and non-commercial) have been studied in order to understand their ability to store electrical charge. The main problem found so far is related to the water content in cork, only of a few % weight. but large enough to influence greatly the conductivity of cork and, consequently, the charge storage capability. To overcome this problem cork has been combined with hydrophobic materials. In this work a commercial wax (paraffin wax) was used to produce a cork/paraffin composite by hot pressing. After milled and mixed natural cork. TetraPak (R) containers waste and paraffin were pressed to make plaques of a new composite. Different concentrations of cork. TetraPak (R) and paraffin, different granules sire, different temperature and pressure were used to produce the samples. The electrical properties of the new composite were measured by the isothermal charging and discharging current method and the results compared to previously ones obtained for natural cork and other derivative products. The new composite has shown to have lower conductivity than the commercial agglomerate. which makes it a better material for charge storage.
Lanca, M. C., C. J. Dias, D. K. Dasgupta, and J. Marat-Mendes,
"Dielectric properties of electrically aged low density polyethylene",
Advanced Materials Forum I, vol. 230-2, pp. 396-399, 2002.
AbstractLow density polyethylene (LDPE) films kept in a sodium chloride aqueous solution, were aged under a high AC electrical field. The films were prepared from press moulding of LDPE pellets with small amounts of antioxidants. The dielectric spectra at 30 degreesC in the range of 10(-5) Hz to 105 Hz were obtained prior and after ageing. Three different experimental techniques were used to obtain the full spectrum. For the low frequency (LF) region (10(-5) Hz to 10(-1) Hz) the time domain technique was used (charge and discharge currents were also measured). The measuring device used for the 10(-1) Hz to 10(1) Hz medium frequency (MF) region was a lock-in amplifier. While for the high frequency (HF), 10(-1) Hz to 10(5) Hz, RLC bridge measurements were performed. Differences can be seen between aged and unaged PE. The region showing less changes with ageing is the MF region where the peak of the unaged samples seems to become less defined with ageing time. This peak is probably due to additives and impurities (such as antioxidants) that will tend to slowly diffuse out with time. The LF peak is a broad peak related to localised space charge injection driven by the electric field. This peak increases in an earlier stage of ageing decreasing afterwards possibly when the polymer becomes more conductive. Finally the HF shows the beginning of a peak due to gamma and beta transitions. The later is related to dipolar rotation of carbonyl groups in amorphous polymer regions, while the former is associated to crankshaft motions in the main polymer chain. This peak decreases with ageing disappearing for the most aged samples. This could also be explained if the sample becomes more conductive.
Lanca, M. C., E. R. Neagu, R. M. Neagu, C. J. Dias, J. N. Marat-Mendes, and D. K. Das-Gupta,
"Space charge studies in LDPE using combined isothermal and non-isothermal current measurements",
Ieee Transactions on Dielectrics and Electrical Insulation, vol. 11, no. 1, pp. 25-34, 2004.
AbstractUsing a recently developed procedure combining isothermal and non-isothermal current measurements space charge trapping and transport in LDPE was successfully studied. Unaged, thermally and electrically aged samples were investigated. The samples were conditioned before each measurement in order to obtain reproducible results. In the non-isothermal measurements appeared a broad peak (40degreesC to 50degreesC) that was possible to decompose into two or three peaks (35, 45 and 65degreesC). At even higher temperature another peak was sometimes present (85degreesC) depending on the prior sample conditioning. The space charge is trapped near the surface in deep traps (maximum depth of approximate to 15 mum). Relaxation times, mobilities and activation energies have been calculated for different charging/discharging conditions. For unaged samples the reproducibility of the results was poor while for the aged polyethylene it was quite good, meaning that aging helps conditioning. In the electrically aged LDPE there is a decrease of conductivity and the broad peak of the non-isothermal spectra shows a slight shift towards higher temperatures when compared with the data found in the thermally aged polymer.
Lanca, M. C., E. R. Neagu, R. M. Neagu, C. J. Dias, J. N. Marat-Mendes, and D. K. Das-Gupta,
"Space charge studies in LDPE using combined isothermal and non-isothermal current measurements",
Ieee Transactions on Dielectrics and Electrical Insulation, vol. 11, no. 1, pp. 25-34, 2004.
AbstractUsing a recently developed procedure combining isothermal and non-isothermal current measurements space charge trapping and transport in LDPE was successfully studied. Unaged, thermally and electrically aged samples were investigated. The samples were conditioned before each measurement in order to obtain reproducible results. In the non-isothermal measurements appeared a broad peak (40degreesC to 50degreesC) that was possible to decompose into two or three peaks (35, 45 and 65degreesC). At even higher temperature another peak was sometimes present (85degreesC) depending on the prior sample conditioning. The space charge is trapped near the surface in deep traps (maximum depth of approximate to 15 mum). Relaxation times, mobilities and activation energies have been calculated for different charging/discharging conditions. For unaged samples the reproducibility of the results was poor while for the aged polyethylene it was quite good, meaning that aging helps conditioning. In the electrically aged LDPE there is a decrease of conductivity and the broad peak of the non-isothermal spectra shows a slight shift towards higher temperatures when compared with the data found in the thermally aged polymer.
Lanca, M. C., E. R. Neagu, and J. N. Marat-Mendes,
"Combined isothermal and non-isothermal current measurements applied to space charge studies in low-density polyethylene",
Journal of Physics D-Applied Physics, vol. 35, no. 8, pp. L29-L32, 2002.
AbstractA new experimental procedure combining usual isothermal DC charging and discharging with non-isothermal current measurements has been recently proposed. It is mainly suitable for very high insulating polymers and it was successfully applied to the study of space charge trapping and transport in low-density polyethylene. The analysis of the isothermal currents revealed the presence of different traps whose characteristic (de)trapping times can be deduced. The isothermal procedures allowed the selective charging of the sample. By choosing the charging field and the ratio of charge/discharge times, non-isothermal analysis permitted the differentiation of three or four peaks (at approximate to50degreesC, approximate to65degreesC, approximate to70degreesC and approximate to85degreesC) associated with charge detrapping from surface or near-surface (<20 mum) traps. These traps have activation energies between 0.21 and 1.54 eV. The mobility at 30degreesC is around 5 x 10(-16) m(2) V-1 s(-1). Samples had to be conditioned before each experiment in order to obtain reproducible results.