In the present work, a XLPE mid-voltage cable
from General Cable CO. has been studied by Thermally
stimulated depolarization currents. Systematic
measurements have been carried out in order to compare
the conductive processes in this cable with previous results.
Depolarization current as a function of thermal
annealing, thermal history, polarizing field and polarizing
time and temperature has been obtained. The results
show the presence of a broad and complex heteropolar
process between 60 and 120°C as expected. Annealing
of the sample at temperatures above SOT develops an homopolar
contribution associated to chemical components
diffused from the cable semiconducting layers into the
XLPE bulk. For annealing times of 60min at 140T and
2 days at 90°C, the homoplar current intensity reaches
a maximum, decreasing and recovering the heteropolar
sign with further annealing. Experiments performed with
different polarizing times and temperatures show as well
the presence of an homopolar contribution, overlapped
to the heteropolar behavior, that increases continuously
with polarizing time. These results indicate that conductive
processes within the XLPE are probably responsible
of homopolar charge injection

Belana, J.

Cañadas, J.C.

Diego, J.A.

Frutos Rayego, Fabián

Orrit, J.

Sellarés, J.

idUS. Depósito de Investigación Universidad de Sevilla

2004 Intenmrioiid Colrfewnce on Solid Dielecrrics, Toulouse, Frunce, July 5-9, 2004 Space Charge Studies on Mid-voltage Cable by Thermally Stimulated Depolarization Currents in the Melting Temperature Range. J.A Diego". J.C. Caiadas, J.Belana, J. Sellads, I. Orrit. F. Frutos' 'Dept. de Fisica i EN..  ETSEIT, Universitat PoliBcnica de Catalunya. CIColom I I, 08222 Terrassa, Spain 'Dept. de Fisica Aplicada I, ETSII, Universidad de Sevilla. Avda. Reina de las Mercedes s/n, 41012 Sevilla, Spain 'E-mail : jose.antonio.diego@upc.es Abstract: In the present work, a XLPE mid-voltage ca- ble from General Cable CO. has been studied by Ther- mally stimulated depolarization currents. Systematic measurements have been carried out in order to compare the conductive processes in this cable with previous re- sults. Depolarization current as a function of thermal annealing, thermal history, polarizing field and polariz- ing time and temperature has been obtained. The results show the presence of a broad and complex heteropolar process between 60 and 120°C as expected. Annealing of the sample at temperatures above SOT develops an ho- mopolar contribution associated to chemical components diffused from the cable semiconducting layers into the XLPE bulk. For annealing times of 60min at 1 4 0 T  and 2 days at 90°C, the homoplar current intensity reaches a maximum, decreasing and recovering the heteropolar sign with further annealing. Experiments performed with different polarizing times and temperatures show as well the presence of an homopolar contribution, overlapped to the heteropolar behavior, that increases continuously with polarizing time. These results indicate that conduc- tive processes within the XLPE are probably responsible of homopolar charge injection. INTRODUCTION The presence of space charge in a dielectric material not only affects the value of the electric field and its conduc- tive properties, but also plays an important role in aging processes. From an applied point of view, in the case of mid-voltage cable, the study of space charge formation and relaxation processes is specially interesting as they condition cable lifetime. The working temperature of this cables, around 9OoC, is located in the melting temperature range, so the conduction processes observed are probably associated with free charge. Thermally stimulated depo- larization cuments (TSDC) has been widely used to study relaxation processes in polymers and bas become a use- ful technique to determine the origin of these charges in cable [I]. Previous works in mid-voltage cable XLPE insulation by TSDC, have shown a rather complex behavior with three, highly overlapped, heteropolar current peaks lo- cated approximately at 80, 95 and 105°C f21. Together with these heteropolar peaks, an homopolar peak was de- tected at 98'C approximately, that is highly affected by ~ . 7 P n l . P l A P . h l ~ A l B ? n  Wl IF)7nnA lPP0  the annealing condition of the material. This behavior, and specially the evolution of the homopolar contribu- tion observed, was explained in terms of two separated processes responsible of the TSDC spectrum. One broad heteropolar contribution located between 60 and 120"C, associated to the material melt, and a second process of homopolar character located at approximately 98°C that increases with annealing [3]. IR-FTIR measurements in cable showed that the evolu- tion of the homopolar contribution was probably related to the diffusion of some chemical components from the isolation surface into the bulk [4]. During annealing at temperatures above S O T ,  an important fraction of the material is melt, and diffusion of these components can take place. The growth of the homopolar current was ex- plained assuming that some of these components remain located in the surface of the crystalline spherulites and act as trapping centers during polarization. In any case, the complex behavior of the TSDC spectrum evidenced the need of more data and a systematic study in order to establish the origin of the peaks firmly. Study of the TSDC spectrum in the melting temperature range with conventional polarization methods presents an important drawback. The whole thermal steps of the po- larization process will affect the fusion and solidification processes of the material in different ways. To overcome *this drawback, in this paper the study was also made by non isothermal windowing polarization (NIW). In this new polarization method, the sample is cooled down from the melt always at the same cooling rate, and the electric field is applied at the desired temperature interval. By this way we can analyze the TSDC response in different polarization conditions with identical thermal history in all cases. In the present work we focus our attention in the study of a new mid-voltage cable from General Cable CO. Several base components vary between this cable and the ones used in previous works. Systematic measurements have been carried out in order to compare the conductive pro- cesses in this cable with previous results and verify the validity of the stated model. TSDC response as a function of thermal annealing, thermal history, polarizing field and polarizing time and temperature has been obtained. EXPERIMENTAL Cable samples were supplied by General Cable and consisted of a cylindrical insulating XLPE layer, with 5.5 mm and 10 mm of inner and outer radii respectively. The samples had two 1 mm thickness semiconductor lay- ers (SC), in contact with the inner and the external sUr- faces of XLPE. The SC layer composition and insulating geometry vary between the present cable and the ones used in previous works. Seven centimeters long samples were cut from cable for TSDC measurements. The semiconductor layers were used as electrodes. To avoid shoncircuits, the external layer was partially removed from the sample ends, leav- ing a 2 cm wide semiconducting strip centered in the sam- ple. The experimental setup for these measurements is described in detail elsewhere [2]. 2"C/min heating and cooling rate were used for all measurements. Sample polarization was performed by two different methods. In the first method (windowing polarization method, WPj, the potential V, is applied at a polarization temperature t, for a polarization time tp  and, afterward, the sample is cooled down to a temperature To switching off the electric field just before the sample cooling begins. This polarization method allows one to resolve complex relaxations into its elementary components, determining the structure of complex relaxation processes. To avoid the different thermal history effects in the fusion and so- lidification processes of the material, the non isothermal windowing polarization method (NIW) was also used. In this polarization method the sample is cooled down from the melt always at the same cooling rate, and the electric field is applied at the desired temperature interval. RESULTS AND DISCUSSION TSDC results First, we explored the behavior of the TSCD spectrum by WE' as we set larger polarization temperatures. The samples were cooled down from the melt (140'C) to the polarization temperature and then polarized for tP=5min and cooled down afterward without applied field. The results (Fig. 1 j demonstrate the presence of a broad spectrum composed by at least two overlapped peaks at 80 and 110°C (curves for Tp<800C in Fig. 1). In the case of curve obtained for TP=7O0C, a weak current peak at 900C can be also observed. The intensity of the higher temperature peak increases as T, increases at the time the lower temperature peaks disappear for T,> 100°C. As stated in previous works for similar cables [ 2 ] ,  this spectrum must be associated to the melting process of crystalline spherulites within the material. This melt- ing process takes place between 60 and 115"C with a Fig. 1: WP-TSDC curves obtained cooling down from the melt to different Tp: o 55°C. 0 65OC. 0 70% A 7YC. V 85°C. x 105°C. (processes: Ti=140°C => Tn (V,=lOkV. t ,=S min) => T,=5OoC ( 5  min) => TSDC). 3e~LLl 40 60 80 LW 120 TIT) Fig. 2: WP-TSDC curves obtained heating up from r w m  temperature to different Tp: o SYC, 0 60°C. 0 80°C. A 90T. V IoO°C, x llO°C. (processes: Ti=35"C => Tp (+lOkV, t,=S min) => T,=50"C (5  min) => TSDC). maximum around 1 0 5 T  as indicate differential scanning calorimetry results (DSC). As the material is progres- sively melt, polar charges induced during polarization in the crystal phase or in the interphases will release devel- oping the observed discharge current. Similar measurements were carried out by WP but in this case heating up the sample from room temperature to T,. polarizing for t,=5min and cooling down afterward with- out applied field. In this case (Fig. 2) a rather anomalous behavior is obtained, with much higher depolarization current intensity than in the previous case. A maximum in polarization is obtained as well for Tp=9O0C. These remarkable results can be explained in terms of recrys- tallization processes that take place when the material is p h a l l y  melted and annealed at temperatures within the melting range. More detailed study of this behavior will be the subject of future work. Fig. 3: NIW-TSDC curves obtaioed with samples starting polarization at different Tp: o 60°C, 0 80°C. 0 90°C, A IoO"C, V IIOT. (processes: To=3S0C => 140T => Tp (VplOkV) => T,=5OoC (cooling with applied field) => TSDC). To overcome this drawback related to the thermal history of each sample, a study of the spectrum was also made by the NIW polarization method (Fig. 3). In this case the samples were cooled down from 1400C to 500C at 2"C/min in all cases. The electric field was applied during cooling at Tp (60, 80, 90, and 1 10°C respectively) and maintained until the sample reaches SOOC. By this way all cables will have identical thermal history previous to measurement. Obtained results demonstrate in this case as well the pres- ence of a broad spectrum that extends along all the melt- ing temperature range of XLPE. Only two overlapped peaks were detected in this case at 80 and 1 IOOC, indicat- ing that the weak current peak detected at 90°C (Tp=700C in Fig. I )  is probably originated by the thermal process carried out during polarization. Evolution of TSDC discharges with thermal annealing We have annealed samples at 90 and 1400C for annealing times between 0 and 600 hours. These samples were then polarized by the NIW method, between T,=llO0C and 500C, and the TSDC discharges measured afterward. Fig. 4 show the TSDC discharges obtained with the sam- ple annealed at 9OoC. As an overall trend we can observe, in the curve corresponding to the low annealed sample, the presence of two heteropolar peaks at 85 and 115°C. With further annealing the 85'C peak shifts up to 95°C. An homopolar peak appears after one hour annealing at this temperature, and its height decreases again with fur- ther annealing, disappearing after l day annealing. The same evolution hut at a slower rate is observed in sam- ples annealed at 9OoC up to 192 hours (not plotted). The homopolar peak appears after two days annealing at this temperature, and its height decreases again with further annealing. In this case, however, the homopolar current A I I I T ("c) 60 80 1W 120 140 Fig. 4: NIW-TSDC curves obtained with samples aged at 140T for different times: o 5 min. 0 15 min. 0 30 min. A 1 hour, V 6 hours, x I day, + 2 days. (processes: To=3S0C => 140°C => T,=llOT (li,=lOkV) => T,=50°C (cooling with applied field) => TSDC). does not fully disappears after 8 days annealing. The oh- served behavior is in good agreement with previous re- sults obtained in other cables [4]. The generation of an homopolar current with annealing of the cable samples, and the subsequent described evo- lution, can he conveniently explained in the frame work of the model stated in previous works: IR-FTIR measure- ments showed that the evolution of the homopolar contri- hution is probably related to the diffusion of some chem- ical components from the isolation surface into the bulk [4]. This chemical components are mainly present in the surface of the XLPE isolation after manufacture. During annealing at temperatures above 80°C, an important frac- tion of the material is melt, and diffusion of these com- ponents into the hulk can take place. The appearance of the homopolar contribution was re- lated to the creation of trapping centers in the hulk of the XLPE. The chemical components diffused from the surface into the isolation remain probably located in the surface of the spherulites and act as trapping centers dur- ing polarization. When the trapping center density is high (in samples annealed for long times) the polarizing field limits the homopolar contribution, that tends to satura- tion, and a maximum in the homoplar current is observed. Besides the described process that lead to the homopo- lar contribution, free charge is generated continuously in the material. This generation of free caniers predomi- nates for long annealing times promoting the observed heteropolar current increase. Evolution of TSDC discharges with polarization time To analyze the effect of the electric field in the TSDC spectrum, cable samples were polaryzed according to the following process; the samples were cooled down from 1 4 0 T  to the polarization temperature Tp and polaryzed Fig. 5: TSDC curves obtained polarizing at ?',=SST for different polarization times t,: o S min. 0 30 min. 0 60 min. A I20min. (processes: To=14OoC => T,=5S°C (V'=IOkV, to)  => T,=SO"C (cooling with applied field) => TSDC). afterward for different times t,. The sample was then cooled down to 50°C without switching off the electric field, and the the TSDC curves were measured. We can observe in figures 5 and 6 the curves obtained for TP=55'C and TP=900C. In both cases the two over- lapped relaxation processes at 80 and 1 lWC are detected, however they present rather different evolution with po- larization time t,. The 80°C peak show a continuous in- crease as higher polarization times are applied. This in- crease, coherent with the polarization process canied out, contrast with the observed evolution in the I lWC peak. In this case the intensity current grows initially, but with further increase in t,, it decreases again obtaining a mm- imum in discharge current for t,=3Cmin and t,=6Omin when TP=55"C and TP=9WC respectively. This evolution is better seen for TP=55"C, as for T,=90"C the intensity of the heteropolar 1 10°C peak is high and interfere its observation. This behavior indicates that the injection of charge in the material takes place in a continuous way during polariza- tion, probably associated to a conducting process. Even more remarkable is the fact that this behavior is observed in not annealed samples indicating that some diffusion of chemical components from the surface towards the bulk of XLPE had already occurred during the manufacture process. CONCLUSIONS A rather complex TSDC spectrum is obtained in XLPE mid voltage cable isolation between 50 and 120°C, which shows the development of an homopolar current contribu- tion located around 1 10°C. The evolution of this homopo- lar current is probably related to the diffusion of some chemical components from the isolation surface into the bulk during annealing at temperatures above X O T .  At Fig. 6:  TSDC curves for Tp=90'C and different polarization times t,: o 5 min, 0 30 min, 0 60 min, A 90 min. v 120 min. (processes: T0=140"C => T,=90°C (V,=lOkV, tp) => T,=SO"C (cooling with applied field) => TSDC). this temperatures an important fraction of the material is melt, and diffusion of these components into the bulk can take place. The initial increase of the homopolar current is explained in terms of a continuous increase of tapping centers as the components diffused into the insulator bulk. When the trap density is large enough the applied field determines the charge injection process and the largest homopolar peak is observed. Results ob- tained with different polarization times indicate that in- jected charge is not corona originated, but penetrates in the material probably due to a conduction process. The increase of the hetoropolar contribution with further an- nealing indicates that free carriers are produced contin- uously in the material. These results are in good agree- ment with the modelization of the conducting processes propounded in previous works. REFERENCES [I]  R. Gerhard-Multhanpt, Electrets, third edition, Val 2, (chapter 9). Morgan Hill. California, USA: Laplacian Press, 1999. [21 L T m y o ,  J.Belana, J.C.Caiiadas, J.A.Diego, M.Mudarra, JSellarks. J.Pol.Sci. Part B, Pol Phvs m. vol41,pp 1412-1421, 1993. [31 I. Tamayo, M. Mudarra, 1.A. Diego, J.C. Caiiadas, J. Sellah,  J. Belana, in APTDAM2001 Proceedings, (Wroclaw, Poland), pp. 84-87.2001. [41 1. Tamayo, 1. Belana, J.A. Diego. J.C. Caiiadas, M. Mudarra and J. Sellares, Space charge studies in XLPE mid-voltage cable isolatiorz by TSDC. IR-FTIR and SEM, submitted to Journal of Polymers Science Piut B: Polymer Physics. Acknowledgments: Present work is financed by ID project Ref MAT 2001-2338-CO2-01 (MCT Spain). The authors wish to thank General Cable for their conlribution to the work (cable samples and useful suggestions). 

Space Charge Studies on Mid-voltage Cable by Thermally Stimulated Depolarization Currents in the Melting Temperature Range.

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Space Charge Studies on Mid-voltage Cable by Thermally Stimulated Depolarization Currents in the Melting Temperature Range.

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