Dielectric Characterization of Thermally Aged XLPE High Voltage Cable Insulation

Dielectric Characterization of Thermally Aged XLPE High Voltage Cable Insulation

A. Hedir

, M. Moudoud

, N. Benamrouche

, F. Bellabas

1

LATAGE Laboratory, Mouloud Mammeri University, BP 17 RP 15000 Tizi-Ouzou, Algeria

2

Physics Laboratory, Electro-Industries Company, Tizi Ouzou, Algeria

(*)

E-mail: abdallahhedir@yahoo.fr

Abstract—The widespread use of crosslinked polyethylene (XLPE) as insulation in high voltage power cables may be attributed to its excellent electrical, thermal and mechanical characteristics. However, this insulation may undergo crucial degradations when exposed to the various constraints, which lead to its general weakness. During its operating under service conditions, the cable is in permanence subjected to thermal aging which can provokes an irreversible alteration of the cable insulation properties. This work presents an investigation on the changes induced in the dielectric and electrical properties of a thermally aged XLPE cables insulation.

Keywords— Crosslinked polyethylene (XLPE), insulation, thermal aging, dielectric properties.

I. INTRODUCTION

Nowadays, polymers are commonly used in the electrical insulation field. Since last 50 years, the old insulation materials, like impregnated paper, have been replaced by synthetic polymers, especially polyethylenes [1]. In industrial applications, polyethylenes are often used as electrical insulation for medium and high voltage power cables [2].

Cross-linked polyethylene (XLPE) is widely used as an insulating material [3]. It is gradually replacing the others polyethylenes in the manufacturing of medium and high voltage cables thanks to its outstanding electrical, mechanical and thermal properties [4].On the other hand, for maintaining the dielectric properties of the original polymer (low density polyethylene)[5]. However, it is well known that regardless of its good characteristics, XLPE may undergo dramatic effects on its properties when exposed to stress conditions.

Degradation under service conditions is the major problem in the use of XLPE as cables insulation [5]. In consequence, several of its qualities may alter.

Temperature is one of the most aggressive constraints which can affect the basic characteristics of the insulation.

Thus, under thermal aging, an irreversible chemical and/or morphological change may take place and reduce strongly the properties of XLPE [6]. These alterations may produce deleterious changes in the material which affect its serviceability, i.e. its ability to satisfy requested performances [7], and may reduce the effective service lifetime of the power cable [8]-[9].

The aim of this work is to characterize the degradation of XLPE insulation subjected to thermal aging. Evolutions of dielectric properties such as dielectric permittivity, dissipation factor and AC volume resistivity; and DC surface resistivity are investigated and considered as possible methods to detect degradation due to thermal aging.

This paper is organized as follows: Section II presents the experimental setup. In Section III, the obtained experimental results are illustrated and discussed. Section IV concludes this paper.

II. EXPERIMENTALPART A. Samples Preparation

Square plates of 130 mm x 130 mm with 2±0.2 mm have been obtained from granules containing dicumyl peroxide with concentration of 2% to generate cross-links. The granules were preheated for 5 min to a temperature of 130 °C, at different pressures and for different time durations. After that they were crosslinked at a high pressure of 300 bars and temperature of 180 °C. The preheating/crosslinking process was followed by cooling at 45°C.

B. Thermal Aging

Thermal aging experiments were carried out in a thermo- ventilated oven that could maintain the average ±2 °C. The temperature of aging was 100°C (higher than the operating temperature which is 90°C). Samples of 60mm×60mm cut from the obtained plates were also vertically suspended and exposed to the thermal constraint during 2400 hours inside the oven. After each 240 hours, two samples were removed and subjected to the different tests.

C. Dielectric Characterization

The dielectric measurements (dielectric permittivity (ε^’), dissipation factor (tanδ) and AC volume resistivity (ρ)) were performed with an LCR-meter (Instek-LCR 817 type); able of measuring the materials properties at frequencies ranging from 12 to 1000 Hz, the measurement voltage of the apparatus did not exceed 2V. The dissipation factor is obtained by direct lecture on the apparatus, while dielectric permittivity and AC volume resistivity were calculated using the formulas:

' 0

. . C e

 S

 (1)

R S.

  e (2)

whereCis the capacitance of the sample sandwiched between electrodes,eis the spacing between electrodes which is equal to the sample thickness,Sis the electrode area, ε0=8.85×10^-12 F.m^-1 is the vacuum permittivity respectively and R is the resistance given by the measuring apparatus.

D. DC Surface Resistivity

DC surface resistivity is a versatile tool for aging characterization. The evolution of this property gives information about the physico-chemical alterations arising in the material [10].

Surface resistance was measured according to IEC 60093 recommendations. It was performed using the three electrodes method under DC voltage of 500 V. Surface resistivity(ρS) is related to the configurations and dimensions of the used electrodes. It is calculated by:

 

s P g Rs

  (3)

WherePis the effective perimeter (P= π (D1+ D2) / 2) of the guarded electrode, g ((D2-D1) / 2)) is the distance between the guarded and ring electrode andRsis the measured resistance in ohm.

III. RESULTS ANDANALYSIS A. Dielectric Properties

1) Dielectric Permittivity

The relative permittivityε’as a function of aging time and frequency variation is shown respectively in Fig.1 and Fig.2.

As can be seen in Fig.1, ε’ has non-monotonic variation with aging time. Relaxations peaks are also observed, their appearance is due to the structural changes induced by heat.

Prolonged exposure of the XLPE to thermal aging induces a material shrinkage, consequently an increase of capacity and thus of the dielectric permittivity [11].

0 500 1000 1500 2000 2500

2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8

Aging time (h)

Dielectric permittivity

5 kHz 10 kHz

Fig.1. Plot of dielectric permittivity versus aging time for different frequencies Fig.2 shows that ε’ presents monotonous variation according to frequency. This variation is characterized by an accentuated decrease in low frequencies and relative stability

in high frequencies. This behavior can be attributed to polarization and relaxation phenomena arising in the material, governed by the number of orientable dipoles present in the system and their ability to orient themselves under an applied electric field [12]-[13].

10⁰ 10¹

2.2 2.3 2.4 2.5 2.6 2.7 2.8

Frequency (kHz)

Dielectric constant

Unaged Aged for 2000 hours

Fig.2. Plot of dielectric permittivity versus frequency 2) Dissipation Factor

The evolution of the dissipation factor tanδ versus aging time and frequency is shown respectively in Fig.3 and Fig.4.

0 500 1000 1500 2000 2500

0 0.005 0.01 0.015 0.02

Aging time (h)

Dissipation factor

5 kHz 10 kHz

Fig.3. Plot of dissipation factor versus aging time for different frequencies

10⁰ 10¹

0 0.01 0.02 0.03 0.04 0.05

Frequency (kHz)

Dissipation factor

Unaged Aged for 2000 hours

Fig.4. Plot of dissipation factor versus frequency

The dissipation factor measurement is an important tool for testing the reliability of high voltage cable insulation. It can serve as a representative indicator of other insulating material properties relevant to the aging phenomena [14]. In Fig.3 we note thattanδdoes not present significant variations according to aging time. The appearance of occasional peaks

can be explained by probable movements of segments of the XLPE macromolecules chains [15]. Fig.4 shows that tanδ decreases according to frequency. This behavior can be assigned to the polarization, namely space charge polarization, caused by charges’ accumulation on the dielectric interface / electrodes. Indeed, at low frequencies, the dipoles’

polarization is so stronger that it leads to an increase in conductivity [16] and hence in dissipation factor. At high frequenciestanδremains constant because of the space charge polarization disappearance.

3) AC Volume Resistivity

The evolution of AC volume resistivity ρ versus aging time and frequency is shown in Fig.5 and Fig.6, respectively.

In Fig.5, it can be seen thatρdoes not present significant evolutions according tothermal aging time. Furthermore, we mention that the evolution is identical for the two frequencies (5 kHz and 10 kHz). As in the case of the others dielectric properties studied earlier, it can be seen in fig 6 that this parameter decreases with increasing frequency. This decrease is more pronounced at low frequencies.

B. DC Surface Resistivity

As can be seen in this Fig.7, ρS shows variations with respect to aging time. This behaviour is assigned to the thermooxidation phenomenon, which leads to the apparition and accumulation of charge carriers. These carriers induce an increase in surface conductivity (decrease in the electric resistivity) [17].

0 500 1000 1500 2000 2500

2 3 4 5 6 7 8 9 10x 10⁷

Aging time (h)

AC volume resistivity (ohm.cm)

5 kHz 10 kHz

Fig.5. Plot of AC volume resistivity versus aging times for different frequencies

10⁰ 10¹

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5x 10⁹

Frequency (kHz)

AC voume resistivity (ohm.cm)

Unaged

Aged for 2000 hours

Fig.6. Plot of AC volume resistivity versus frequency

0 500 1000 1500 2000 2500

10¹² 10¹³ 10¹⁴ 10¹⁵ 10¹⁶ 10¹⁷

Aging time (h)

DC surface resistivity (ohm)

Fig.7. Plot of DC resistivity versus aging time IV. CONCLUSION

The experiment presented was focused on the investigation of the thermal aging effects on dielectric and electrical properties of XLPE insulation. It was shown that all the properties are with different degrees sensitive to thermal aging; and that they can be considered as useful diagnostic parameters for estimating the degree of deterioration of the thermally aged XLPE cables.

According to the investigation, it was found that dielectric permittivity, dissipation factor and AC volume resistivity; and DC surface resistivity change according to aging time and frequency of the applied field. These changes will lead automatically to the attenuation of other characteristics such as physical, chemical and mechanical properties.

ACKNOWLEDGMENT

The authors wish to thank Enicab Biskra for helping to get XLPE plates.

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