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Whereas synthetic materials have been used for long in the insulation of HVAC cables, their use in the case for HVDC applications is still pending due to a lack of knowledge on the behaviour of space charge and on their impact on the reliability of cables. As volatile cross-linking by-products are known to have an impact on space charge accumulatio...
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... charge measurements have been performed using the Pulsed electroacoustic (PEA) method. The method consists in detecting and analyzing acoustic waves generated by the interaction between the space charge in the material and an applied electric pulse – see [7-8] for further information about the technique. A schematics of the set-up used for the current measurements is provided in Fig. 1. The PEA test cell was provided by TechImp, Italy. All PEA results presented herein were obtained at 25°C. Polarization/depolarization cycles were applied consecutively on the same sample. Applied fields were in the range 10 to 40kV/mm as specified in the following. Voltages ramps-up and ramp-down were 1kV/mm/s. PEA profiles were recorded for 2h at each voltage step (including volt-off measurements), averaging acoustic signals to provide a time resolution of 1 profile/min. Rough products were LDPE –low density polyethylene – and semiconducting –SC- materials (LDPE + carbon black), in the form of granules, both being cross-linkable, i.e. containing peroxide. Plaques of LDPE and SC materials were processed by press-molding at 120°C. Then, disks of SC material, 12mm in diameter, were cut. The final samples were obtained, with structure as described in Fig.2, by cross-linking the 3 layers altogether at 180°C for 15 min under a press. The upper semiconducting electrode is thicker than the lower one to attenuate possible reflections that may occur at the interfaces SC/Insulation. The insulation layer was limited to 500μm in order to prevent too important attenuation and dispersion of the acoustic waves. Since we intend to evaluate the impact of degassing on space charge built-up, 5 samples have been thermally conditioned in an oven in air to achieve 5 degassing levels (see Table 1). PEA measurements have been carried out immediately after conditioning. PEA results are represented by means of cartography of space charges. The time is represented along the horizontal and the position in the sample along the vertical axis. Charge density is represented according to a color code. This kind of representation enables a global overview of the spatiotemporal evolution of space charge within the material. Once manufactured, the sample was placed in the PEA test cell. As shown in Fig.3, negative charges appear to be injected from the cathode for the first voltage step. Only a small quantity of heterocharges is observed on the cartography for 10 to 30 kV/mm applied ...
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Citations
... In literature, several works focused on the additives and byproducts effect are present. In [13][14][15][16][17] it was found that cumyl alcohol is responsible for homocharge in proximity of electrodes, acetophenone is responsible for heterocharge formation and -methylstyrene promotes the charge trapping. In this work, due to the fact that the specimens have been provided by a cables industry, it has not been possible to mention the kind of additives added in the samples as well as further treatment useful to reduce the space charge formation and accumulation. ...
The aim of the present work is to evaluate the behavior, from the space charge accumulation phenomenon point of view, of three different XLPE specimens used as insulation materials in high voltage systems. Considering that under DC stress the charge accumulation is more relevant as compared to the AC case, in the specimens for DC applications additives are typically added. In this work, three different specimens are analyzed. One is a free additive sample used conventionally under AC stress and two others include different additives that are used under DC stress. In order to measure the space charge accumulation, the Pulsed Electro Acoustic (PEA) method has been employed. For each sample, the three-dimensional space charge patterns and the electric field distributions have been acquired and analyzed. Finally, a comparison among the tested specimens has been made.
... In our case we have focused our work on studying the interface and chemistry influence because in other works it has been shown that the external interface is more significant than the internal interface (Vissouvanadin et al, 2009 t between 0 and 672 hours, and the charge distribution profile and the diffusion of chemical components from the semiconducting external shield into the XLPE isolation is measured. ...
Three experimental mid-voltage cables , C 2, C3 and C4 with cross-linked polyethylene (XLPE) isolation, rated respectively as Medium, High and Low in perforation tests over 150KV, are studied. All these cables have been systematically measured by Electroacoustic Pulse (PEA) and Thermally Stimulated with Depolarization Currents (TSDC), using 120KV of polarization voltage in samples as received, annealed at 90kV and 120ºC up to 672 hours. Measured internal charge of cable C4 at least doubles that of cables C2 and C3. The interfacial phenomenon has been studied by Infrared spectroscopy measurements Attenuated total reflectance (ATR) that show chemical components that migrate, spread, and transport from the external semiconductor layer to the cross-linked polyethene isolation during the thermic treatment of the cable. The PEA measurements using 120 kV/mm electric field showed the formation and propagation of space charge packet from SC to isolation. These results are coherent with TSDC measurements that also show differences in the areas under the curve of current as a function of temperature for each cable type C 2, C3and C 4. This is a result of the amount of charge accumulated. To sum up, combination of PEA, TSDC and ATR measurements is a useful tool in understanding charge relaxation processes and XLPE cable performance.
... In our case we have focused our work on studying the interface and chemistry influence because in other works it has been shown that the external interface is more significant than the internal interface (Vissouvanadin et al, 2009 t between 0 and 672 hours, and the charge distribution profile and the diffusion of chemical components from the semiconducting external shield into the XLPE isolation is measured. ...
Se estudiaron tres cables experimentales de media tensión: C2, C3 y C4; con aislamiento de polietileno reticulado (XLPE) denominados: Medio, Alto y Bajo en los test de perforación con tensiones sobre 150 kV. Los tres cables han sido medidos sistemáticamente mediante las técnicas del Pulso Electroacústico (PEA) y de las Corrientes de Despolarización Estimuladas Térmicamente (TSDC) usando un voltaje de polarización de 120 kV en muestras tal y como se reciben, tratadas térmicamente a 90 oC y 120 oC durante 672 horas. Las medidas de carga interna en el cable C4 son de un orden del doble que en los cables C2 y C3. El fenómeno de carga interfacial ha sido estudiado por espectroscopia infrarroja mediante la técnica de Reflectancia Total Atenuada (ATR) y muestra que hay componentes que migran, se difunden y se trasportan desde la capa semiconductora externa hacia el aislamiento de polietileno reticulado durante el tratamiento térmico del cable. Las medidas por PEA usando un campo eléctrico de 120 KV/mm muestran la formación y propagación de paquetes de carga de espacio desde el semiconductor (SC) hacia el aislamiento. Estos resultados son coherentes con las medidas mediante TSDC que muestran diferencias entre las áreas bajo la curva de corriente en función de la temperatura para los tres cables C2, C3 y C4 que es el resultado de la carga acumulada. Para resumir, la combinación de las mediciones PEA, TSDC y ATR son herramientas útiles para la comprensión de los procesos de relajación de carga de espacio y la eficiencia de los cables con aislamiento de XLPE.
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