Tesis doctoral de Jordi Orrit Prat
Since cross-linked polyethylene (xlpe) started to be used as electrical insulation for power cables, much research has been focused on polyethylene dielectric properties and the effect of the space charge on its behavior. In this sense, thermally stimulated depolarization currents (tsdc) have been widely used to study space charge relaxation. This technique has proved to have enough resolution to determine differences in charge trapping properties among xlpe insulations with different composition and manufacturing processes. In this thesis work, the conduction mechanisms of medium voltage (mv) cable xlpe insulation have been studied by tsdc and several complementary techniques, such as dynamic electrical analysis (dea), absorption/resorption currents (arc), pulsed electroacoustic (pea) and isothermal depolarization currents (idc). Other techniques, like fourier transform infrared (ftir) spectroscopy or x-ray diffractometry, have been used to characterize the material. Tsdc spectra have been obtained for different cable samples, which in service conditions work under ac voltages ranging from 12 to 20kv and at temperatures around 90ºc. On the other hand, conductivity measurements by arc and dea have been performed in cable samples and in xlpe cylinders, as well as xlpe films. Measurements have been carried out at temperatures within the melting temperature range of xlpe (50¿110ºc) on samples annealed at such temperatures during several annealing times. Results show significant differences in the behavior of the conductive properties of xlpe cable samples with semiconducting (sc) screens and without them (xlpe cylinders). The observed behavior has been explained by the coexistence of two conduction mechanisms. Diffusion of impurities from sc screens determines the medium and long-term behavior of one of these contributions and, hence, of cable conductivity. Ftir results are consistent with this model. With respect to microstructure, dsc and x-ray diffractometry results show that recrystallization processes exist when samples are annealed in the melting range of temperatures. Electrets formed by means of the windowing polarization method (wp) showed a tsdc discharge with a wide heteropolar peak in the melting temperature range, and with the maximum at about 105ºc. This peak was associated with the melting of the crystalline fraction in previous works. However, in spite of decreasing with the temperature of polarization, an optimal polarization temperature around 90¿95ºc is found. This behavior has been explained by taking into account recrystallization processes when the insulation is isothermally polarized. During recrystallization, the new crystalline fraction grows in a polarized state due to the applied electric field, and it causes the depolarization current when it melts during the tsdc measurement. Results obtained from different experiments are consistent with this assumption. With the aim to find out the origin of other tsdc peaks present in the spectrum of xlpe cable samples, idc has been used as complementary technique. Idc currents obtained from as-received cable samples at temperatures close to service conditions can be considered as the combination of two different contributions: a power law current and a stretched exponential contribution. The last relaxation process has been successfully associated with with a tsdc peak found at 95ºc. By this way the dipolar origin of the peak has been determined. Finally, pea measurements have provided the space charge distribution profiles of polarized samples with different annealings. A transient behavior has been observed in both pea and tsdc measurements. However, no straightforward relation between tsdc discharges and space charge detected by pea can be established. Therefore, an explanation for tsdc curves has been proposed, which considers mechanisms that are not noticeable in charge profiles obtained by pea.
Datos académicos de la tesis doctoral «Study on conduction mechanisms of medium voltage cable xlpe insulation in the melting range of temperatures«
- Título de la tesis: Study on conduction mechanisms of medium voltage cable xlpe insulation in the melting range of temperatures
- Autor: Jordi Orrit Prat
- Universidad: Politécnica de catalunya
- Fecha de lectura de la tesis: 07/03/2012
Dirección y tribunal
- Director de la tesis
- Juan Belana Punseti
- Tribunal
- Presidente del tribunal: gilbert Teyssédre
- jordi García ojalvo (vocal)
- fabián Frutos rayego (vocal)
- Juan Jorge Martinez vega (vocal)
