Study on conduction mechanismes of mediun voltage cable XLPE insulation in the melting range of temperatures.

D’ençà que el polietilè reticulat (XLPE) es va començar a utilitzar com aïllament elèctric per cables de subministrament elèctric, s’han destinat molts esforços a l’estudi de les propietats dielèctriques del polietilè i l’efecte que la càrrega d’espai té sobre el seu comportament. En aquest sentit, les corrents de despolarització estimulades tèrmicament (TSDC) s’han utilitzat extensament per estudiar les relaxacions de càrrega d’espai. Aquesta tècnica ha demostrat tenir prou resolució per distingir diferències en aïllaments de XLPE amb composicions o processos de fabricació diferents. En aquesta tesi, els mecanismes de conducció dels aïllaments XLPE de cables de mitjana tensió (MV) han estat estudiats per TSDC i diverses tècniques complementàries, com l’anàlisi dinàmica elèctrica (DEA), les corrents d’absorció/resorció (ARC), el pols electroacústic (PEA) i les corrents de despolarització isotèrmiques (IDC). Altres tècniques, com l’espectroscòpia d’infrarojos (FTIR) o la difracció de raigs X, han estat també utilitzades per caracteritzar el material. S’han obtingut espectres TSDC per diferents mostres de cable, les quals en condicions de servei treballen en un rang de tensió AC de 12 a 20kV i a temperatures al voltant dels 90ºC. D’altra banda, s’han realitzat mesures de la conductivitat per ARC i DEA en mostres de cable, en cilindres de XLPE i en films. Les mesures s’han dut a terme a temperatures pertanyents al rang de fusió del XLPE (50–110ºC), en mostres sotmeses a aquestes temperatures durant diferents períodes de temps. Els resultats mostren diferències importants entre el comportament de les propietats conductives de les mostres de cable amb pantalles semiconductores (SC) i sense (cilindres de XLPE). El comportament observat ha estat explicat mitjançant la coexistència de dos mecanismes de conducció. La difusió d’impureses des de les pantalles SC determina el comportament d’una d’aquestes contribucions a mig i llarg termini. Els resultats obtinguts per FTIR són consistents amb aquest model. Respecte la microestructura, tant les mesures DSC com la difracció per raigs X mostren que existeixen processos de recristal•lització quan les mostres són sotmeses a temperatures ubicades dins del rang de fusió. Els electrets formats mitjançant el mètode de la polarització per finestres (WP) mostren una descàrrega TSDC amb un ample pic heteropolar en el rang de fusió, amb el màxim al voltant dels 105ºC. En treballs previs, aquest pic es va associar a la fusió de la fracció cristal•lina. Tanmateix, en lloc de decréixer quan la temperatura de polarització augmenta, el pic presenta una temperatura de polarització òptima al voltant de 90-95ºC. Aquest comportament ha estat explicat tenint en compte els processos de recristal•lització que es produeixen quan el material es polaritza isotèrmicament. Durant la recristal•lització, una nova fracció cristal•lina creix en un estat polaritzat degut el camp aplicat, i origina una corrent de despolarització quan es fon durant la mesura TSDC. Amb l’objectiu de determinar l’origen d’altres pics que apareixen en l’espectre TSDC del XLPE, s’han emprat les IDC com a tècnica complementària. Les corrents IDC obtingudes de mostres no tractades es poden representar com la combinació de dues contribucions diferenciades: un terme que és una funció potencial del temps i un d’exponencial. La segona relaxació es correspon amb un pic TSDC que apareix a 95ºC. D’aquesta manera s’ha pogut determinar l’origen dipolar del pic. Finalment, mitjançant la tècnica PEA s’ha obtingut la distribució de la càrrega d’espai en mostres polaritzades que havien estat sotmeses a diferents tractaments tèrmics. S’ha observat un comportament transitori tant per PEA com per TSDC. Tanmateix no s’ha pogut establir cap relació directa entre les descàrregues TSDC i mesures PEA. En conseqüència, s’ha proposat una explicació per les corbes TSDC que considera mecanismes que no són detectables en els perfils de càrrega obtinguts per PEASince 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.Postprint (published version

A model to explain the TSDC spectrum of XLPE insulation in mid voltage cables based on a virtual electrode

The analysis by TSDC of a large variety of cables subjected to long annealing times, has resulted in spectra that sometimes show an oscillation of the current peak observed at 99°C. This process has been observed in some cables only and it is scarcely reproducible. We present an explanation of this behavior based on the formation of charge packets and some other assumptions: charge injection occurs mainly from the negative electrode, and chemical components that act as trapping centers are diffused continously from the semiconducting layers into the bulk during annealing. When the field is applied the negative electrode injects charge that get trapped in these centers, so that a virtual electrode is formed. The electric field created by this layer results in a decrease of the field in the gap between the injecting and virtual electrodes, preventing new charge injection. On the other hand, during polarization the virtual electrode shifts through the insulation bulk towards the opposite electrode. This layer that acts as a virtual electrode becomes neutral by means of recombination when it reaches the opposite electrode, so that the electric field in the region between the electrodes recovers its initial value and the whole process happens again cyclically. This model can explain the observed TSDC oscillating spectra of annealed samples. It can also explain why the oscillations can not be observed in most cases.Postprint (published version

Características dieléctricas, térmicas y mecánicas de la poliamida (PA) reforzada con neumáticos fuera de uso (GTR)

El presente trabajo tiene como objetivo obtener materiales adecuados para la industria eléctrica a partir de la mezcla de poliamida 6 (PA) con neumáticos antiguos utilizados (GTR), con la premisa que sus costes sean mínimos en el proceso del reciclado. Para conseguirlo se ha utilizado GTR sin desvulcanizar y sin ningún pretratamiento previo, pero en cambio, se ha actuado sobre el tamaño de las partículas con un simple y económico tamizado. Una novedad que ofrece este estudio pasa por el gran número de compuestos analizados, así como por los análisis profundos a los que han sido sometidos (dieléctrico, mecánico y térmico), lo que ha permitido obtener un gran número de variables para cada prueba.Peer ReviewedPostprint (published version

Improved adhesion of cathodic arc PVD AlCrSiN coating on ion-implanted WC-Co substrates

Ion implantation has been shown to improve adhesion strength of AlCrSiN coatings due to a synergic enhancement on fracture toughness and load bearing capability of the substrate that can potentially increase the in-service efficiency of coated cutting tools. In this work, AlCrSiN coatings deposited by PVD on WC-Co substrates implanted with Ti, Cr and N ion species have been processed. The mechanical properties and adhesion have been characterized by contact techniques and the residual stress of the coatings and substrates have been evaluated using FIB-DIC technique and Vickers indentation tests, respectively. An improvement of adhesion strength is obtained for treated substrates, especially for those implanted with titanium and chromium ions. This improvement is attributed to the introduction of residual stresses in the substrate, which increases its fracture toughness and enhances its load bearing capability.Work funded through The Spanish Ministry of Science, Innovation and Universities through grants PGC-2018-096855-B-C41, PGC-2018-096855-B-C42 and PGC-2018-096855-A-C4

The development and use of a high-temperature triaxial cell to measure the workability of rolled asphalt

SIGLEAvailable from British Library Document Supply Centre- DSC:DX89744 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

General analysis of the three-phase asynchronous motor with spiral sheet rotor: operation, parameters and characteristic values

The objectives of this article arise through the investigation of a motor with a high starting torque although with limited intensities, weak intensity variations depending on the load, controlled losses in no-load tests, and an acceptable performance in steady state and with a good response in transient periods. All these requirements would have to be obtained without the help of power electronics, in this way the motor would be more robust, longer lasting and without the need for auxiliary equipment. In this article, the operation principle, the characteristic parameters and the obtained results for the spiral sheet motor are analyzed for three speed regimes 1000, 1500 and 3000 rpm. Also, at the end of the paper the spiral sheet motor is compared with the squirrel cage rotor, the solid rotor with rings, the wound rotor with additional resistances in the rotor circuit, and the hollow diamagnetic rotor.Peer Reviewe

Space charge measurements in XLPE insulated mid-voltage cable: correlation with cable performance

Three experimental midvoltage XLPE cables, C2, C3 and C4, rated respectively as 'good', 'very good' and 'bad' in perforation tests, are studied. All these cables have been systematically measured by PEA as produced and after being annealed at 90ºC and 120ºC up to 672 hours. Measured internal charge of cable C4 at least doubles that of cables C2 and C3. Evolution with annealing show as well differences in the reticulation process carried out during cable C2 and C3 preparation, which can explain why cable C3 performs better than C2 in perforation tests. Infrared spectroscopy measurements (IR) showed differences in components injected from the external semiconducting layer (SC) into the isolation during annealing. These results explain other observed differences in PEA measurements with regard of the SC type of each cable. To sum up, combination of PEA and IR measurements are a useful tool in understanding charge relaxation processes and XLPE cable performance.Peer ReviewedPostprint (published version

Method to distinguish between space-charge and dipolar relaxation in the TSDC spectra of polyethylene electrical insulation

Medium-voltage cross-linked polyethylene (MV-XLPE) cables have an important role in the electrical power distribution system. For this reason, the study of XLPE insulation may lead to improve cable features and lifetime. Although relaxational analysis yield a lot of information about XLPE properties, sometimes their results are difficult to interpret. To overcome this handicap, we have used a combination of thermally stimulated depolarization currents (TSDC) and isothermal depolarization currents (IDC) techniques. In order to discard spurious effects from the semiconductor interfaces, preliminary measurements have been done on specially prepared cables. TSDC have been performed using conventional poling between 140°C and 40°C. IDC measurements also have been carried out at temperatures between 90°C and 110°C in 2°C steps. The TSDC spectra are dominated by a broad peak of uncertain origin. On the other hand, IDC show a combination of power and exponential currents. Exponential currents are fitted to a KWW model. The relaxation times obtained from the model present an Arrhenius behavior with Ea = 1.32 eV and τ0 = 3.29×10-16 s. The KWW parameter obtained is β = 0.8. The calculated depolarization current given by the exponential relaxation matches the predominant peak of TSDC spectra. Therefore, we conclude that in the MV cables studied the most visible peak of the TSDC spectrum has a dipolar origin.Peer Reviewe