Carmo Lança

Polyethylene is one of the most widely used polymeric insulators in medium and high voltage power cables. However the importance of space charge distribution and its influence on the electrical aging in this polymer is not fully understood. The very good insulating properties of the material implying very long relaxation times (few days and even longer are usual) and low currents (few pA or below) make individual measurements of isothermal charge/discharge currents and thermostimulated currents difficult to analyze and reproduce. A single type of measurements does not take into account the space charge that remains trapped for long times. A combined procedure of isothermal and non-isothermal current measurements developed for high insulating polymers was used for low density polyethylene (LDPE) and crosslinked polyethylene (XLPE) films electrically aged. The press-molded LDPE and XLPE films were electrically aged under similar conditions using an AC electric field while immersed in a sodium chloride aqueous solution at constant temperature (electro-thermal aging). The use of the combined procedure for current measurement allowed obtaining information about space charge traps, activation energies and relaxation times for both LDPE and XLPE. This data was used to compare electrical aging under similar conditions for the two types of polyethylene.

Using 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.

Using 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.

A 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.

A 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.

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.