Electrónica Nova Scientia Study of Space Charge in SC Shield / XLPE Interface and Mid-Voltage Cable Perfomance

Three experimental mid-voltage cables ,C2,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 C o 120 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 mm kV 120 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 C2,C3 and C4 . 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.

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.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 2 C , 3 C and 4 C .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.

Introduction
The presence of space charge in XLPE in dielectrics material not only affects the value of its electric field and its conductive properties but also plays an important role in aging processes (Dissado, 1998).From an applied point of view, for mid-voltage cable, the study of space charge formation and relaxation processes is especially interesting as they condition cable lifetime.
Polyethylene (PE) is a linear semi-crystalline non polar polymer that is used in such cable insulation and the conduction processes observed can be associated with free charge (Tamayo, 2003).In industrial applications, PE is commonly cross-linked (XLPE) by the addition of some chemical additives (cross-linking agents) and heating, up to a temperature higher than C o 200 to produce a vulcanization reaction.This process introduces several cross-linking by products that influence materials' conductive properties.
The development of new techniques for determining space charge profiles in insulators (Sessler, 1999) has renewed the interest in the study of space charge in dielectric materials in recent years.These studies have shown that the formation of space charge profiles in XLPE, used in cable insulation, is greatly influenced by several factors, including the cross-linking byproducts, antioxidants, oxidation of the material, temperature, electrode type, and other circumstances of the manufacturing process that must be taken into account (Hozumi, 1996;Bambery, 1996;and Suzuoky et al, 1991).
Previous works showed that cable semiconducting shields (SC) play an important role in charge trapping processes in XLPE.The charge that arises was previously trapped in defects that diffuse from the SC layers.Infrared spectroscopy (IR) analysis showed that this diffusion of defects into the XLPE bulk takes place in a continuous way when the sample is annealed at temperatures above C o 80 (Tamayo et al, 2004).Diffused defects act as traps for the charge that is injected from the electrodes if a polarizing electric field is present.There are numerous works dealing with the formation and propagation of space charge packet in the XLPE bulk, the influence of antioxidant (Kon et al, 1999), by-products cross-linked, acetophenone concentration (Tamayo, 2011).
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).In this work, three experimental cables supplied by General Cable S.A., each one with quite different perforation test results, are systematically measured.

TSDC measurements
The samples that were polarized during the measurements, using the PEA technique, were cut without altering the zone where there were PEA measures and got the download details TSDC

ATR measurements
Attenuated Total Reflectance (ATR) measurements were performed with a 'Nicolet 510M .

PEA measurement as received cables
The Figure 2 show measurements of received cables at mm .500 depth in XLPE insulating from the outer semiconductor shield.We can observe clear differences between them.Cable 4 C shows much higher internal charge than the other two cables.However, cable 3 C that performs better than 2 C in perforation tests, shows higher internal charge as well.In all cases a distribution of trapped charge, injected from the external semiconductor electrode, can be observed.With the goal of clarifying the results obtained in the measures technique on the cable 3 C , we have proceeded to study using the thermal aging in the cable samples because of the perforation test we carried at  To speed up the effect of the temperature we proceeded to anneal the cables for 672 hours at 120 ºC and repeat the measures by PEA and TSDC.In figures 4 c) and 5 c) we finally see that cable In figure 6 a) we show the spectra ATR for the layer of XLPE in contact with the semiconductor screens of cables 2, 3 and 4 without heat treatment.We can clearly see that in the XLPE cables 2 and 4 there is band absorption from1557 , 1638 , 1742, 1 3300  cm that is not of the XLPE.The bands1557 , 1638 and 1 3300  cm are of the azodicarbonamide ADA, and the band In the external interface there is a more significant presence of charge and is due to the external semiconductor shield structures.It has to be manipulated and flexible for their application, and the additives are necessary.The semiconductor shield of the cable 3 C does not have azodicarbonamide; this contribution had an improved performance before the perforation test.
We initially detected more charge by means of the techniques PEA and TSDC, but the heat treatment improved the behavior with regard to the cables 2 C and 4 C .This may be due to the fact that the cable 3 C contains a resin commercial basis (LDPE-2) different from 2 C and 4 C .The influence of temperature can cause a higher degree of cross-linking and the elimination of by-products of the reaction of reticulated.
main goal to achieve is to obtain a correlation between cable characteristics and measured results.Following this purpose, several samples were annealed at C 672 hours, and the charge distribution profile and the diffusion of chemical components from the semiconducting external shield into the XLPE isolation is measured.In the PEA measures, the charge accumulated near outer semiconducting electrode is related with the charge obtained by TSCD.Finally, we have obtained by PEA technique that the chemical composition of the semiconductor shield affect the formation of space charge packet and you propagation velocity in Low Density Polyethylene LDPE.The anterior results they are in accordance with the perforation test on 2 C , 3 C and 4 C cables.Experimental Samples Cable samples were supplied by General Cable S.A Figure 1 and consisted of a cylindrical insulating XLPE (3) layer with mm .based on EVA and Carbon Black (2,4) in contact with the inner and external surfaces of XLPE.The aluminium conductor (1) has been used as a positive electrode, while outer semiconductor (4) as a negative electrode, the copper screen (5) and the PVC cover (6) were removed during the sample manufacturing.
the intensity of the download stream as a function of temperature.The area under the curve Intensity vs Temperature is the amount of charge involved in the depolarization current.
that the 4 C cable has more charge accumulated in comparison with 3 C and 2 C .These results are consistent with the findings by TSDC (figure 5 b).We can observe that the three cables, also after annealing internal charge distribution in all cables, follow similar relation as in the received samples but with less internal charge (figure 4 a) and a decrease of the charge with the PEA measurements.However, in 3 C there is still more charge accumulated than in cable 2 C .a) As received b) Annealed at 90 ºC c) Annealed at 120 ºC

3
C has less charge than cables 2 C and 4 C .While the charge in cable 3 C tend to decrease, in cables 2 C and 4 C it tends to increase.The above associates the differences in the process of cross-linking these cables.ATR spectra in xlpe layer in touch with the semiconductor 2 C , 3 C and 4 C .

distribution measurements in cables and LDPE films in touch with the
). Cables were evaluated by perforation tests at General Cable S.A., delivering quite different results.The following table summarizes each cable properties.
LowCharge C were cut and used as electrodes during the measurement through the technical PEA applying an electric field of mm