Carmo Lança

Thin films of low density polyethylene (LDPE) and crosslinked polyethylene (XLPE) were aged under an AC electric field while kept in sodium chloride aqueous solution. After aging the samples showed water trees (localized damaged with the appearance of hydrophilic ramified structures whose size ranges from a few microns to I mm). Some of the samples suffered dielectric breakdown showing small channels (1-2 mm. diameter) crossing the film and sometimes also signs of carbonization. In order to identify the oxidation mechanisms contributing to aging, FTIR was used to analyze both unaged and aged specimens. Comparing between unaged and aged LDPE an increase in the FTIR spectrum for bands at 1720 cm(-1), 1640 cm(-1) and 1590 cm(-1) was visible for the aged samples. The first region corresponds to carbonyl groups (C=O bonds) resulting from oxidation (most probably ketones). While the second one is related to carbon double bonds formed due to chain scission. Finally the third one is due to carboxylates. For the XLPE the analysis is more difficult. Besides aging it needs to be taken into account the by-products of crosslinking that will tend also to diffuse out with time. The main effect of aging is an increase in the concentration of 1640 cm(-1) band (C=C bonds). For the water treed regions dry and wet samples were compared. In the wet ones the absorbance is larger for the 3380 cm(-1) exhibiting, as expected, water absorption in the water treed regions (hydrophilic characteristics were increased).

Space charge in electrically aged LDPE was studied using a recently developed technique combining isothermal charging and discharging with non-isothermal measurements. Samples were aged in a NaCl aqueous solution at 40degreesC for 1500h under an AC field of 6MV/m (50Hz). The samples were then isothermally DC charged and discharged (both currents recorded). Next a non-isothermal experiment with constant heating rate was performed. Finally the sample was kept at the highest temperature and the final isothermal discharge current registered. The last step has to be carried on for long time to ensure an almost complete discharge of the remnant charge so that results become reproducible and possible to analyze. Selective charging (careful choice of the field, temperature and the ratio of charging/discharging times) revealed the presence of different trapping sites. From the analysis of the isothermal and non-isothermal data the relaxation times and activation energies could be obtained.

Space charge in electrically aged LDPE was studied using a recently developed technique combining isothermal charging and discharging with non-isothermal measurements. Samples were aged in a NaCl aqueous solution at 40degreesC for 1500h under an AC field of 6MV/m (50Hz). The samples were then isothermally DC charged and discharged (both currents recorded). Next a non-isothermal experiment with constant heating rate was performed. Finally the sample was kept at the highest temperature and the final isothermal discharge current registered. The last step has to be carried on for long time to ensure an almost complete discharge of the remnant charge so that results become reproducible and possible to analyze. Selective charging (careful choice of the field, temperature and the ratio of charging/discharging times) revealed the presence of different trapping sites. From the analysis of the isothermal and non-isothermal data the relaxation times and activation energies could be obtained.

Water 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.

Water 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.

In PE films aged under electric field the crystallisation of water (and melting of ice) has been studied by quadrupolar NMR, this technique allows one to determine the concentration of water as low as 10(-4). It is shown that the pore dimensions of the tracks forming the water trees of the order of 2.5 nm, are independent of the ageing time. The mobility of water in these water trees and in porous glass, of similar pore dimensions, are compared. (C) 2000 Elsevier Science Ltd. All rights reserved.

In PE films aged under electric field the crystallisation of water (and melting of ice) has been studied by quadrupolar NMR, this technique allows one to determine the concentration of water as low as 10(-4). It is shown that the pore dimensions of the tracks forming the water trees of the order of 2.5 nm, are independent of the ageing time. The mobility of water in these water trees and in porous glass, of similar pore dimensions, are compared. (C) 2000 Elsevier Science Ltd. All rights reserved.