The Effect of Accelerated Water Tree Ageing on the ... - IEEE Xplore

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J. Li et al.: The Effect of Accelerated Water Tree Ageing on the Properties of XLPE Cable Insulation

The Effect of Accelerated Water Tree Ageing on the Properties of XLPE Cable Insulation Jianying Li, Xuetong Zhao, Guilai Yin, Shengtao Li State Key Laboratory of Electrical Insulation and Power Equipment Xi’an Jiaotong University, Xi’an, Shaanxi, 710049, China

Jiankang Zhao and Benhong Ouyang State Grid Electric Power Research Institute, Wuhan, 430074, China ABSTRACT The influence of accelerated water treeing test (AWTT) on the properties of 10 kV cross-linked polyethylene (XLPE) cable insulating materials was investigated in this paper. The dielectric and physicochemical properties of both aged samples and unaged samples were tested. Dielectric property investigation found a new peak of the dielectric loss tangent with an activation of 0.14 eV in the low-frequency domain for aged samples. With increased ageing time, the conductivity in low frequency region of less than 50 Hz increases obviously, while it nearly keeps unchanged in the frequency above 50 Hz. Physicochemical investigation of FTIR, DSC, XRD and density measurement results indicated that AWTT ageing of XLPE insulated cables will lead to decreased crystallinity and density, and methyl group at the outer insulation layer cannot be found in aged samples. The melting temperature (Tm) was also decreased with ageing time. It was suggested that water was introduced from the outer layer of insulation and the degradation was developed by a combination of mechanical forces and electric force. During the ageing process, water will choose a path between surface of lamella and amorphous region, which consequently lead to the increase of low frequency conductivity and decrease of lamella thickness. Furthermore, a model was proposed to explain how the microstructure of XLPE cable insulation will change during AWTT ageing process. It is shown that XLPE cable insulation will be changed both in dielectric and physicochemical properties before any water tree was found, and a combination of dielectric and physicochemical methods is effective to detect the degradation of the XLPE cable insulation materials.

Index Terms — XLPE cable, water tree, dielectric property, cable insulation.

1 INTRODUCTION CROSS-LINKED polyethylene (XLPE) has been extensively used as an electrical insulating material for medium and high voltage power distribution cables because of its outstanding physical, chemical and electrical properties. The electrical qualities of most presently available materials are quite satisfactory for the insulation of cables when they are kept in dry environment. However, in a wet environment, XLPE based materials are subject to water treeing which is still an important cause of cable failures [1-2]. Water treeing is a prebreakdown phenomenon associated with dielectric cable failure. It is currently suspected that buried cables throughout the country are laced with these defects which will inevitably produce an accelerating failure rate [3]. The existence of water in cable insulations (XLPE) is required for water tree formation. Water can be absorbed Manuscript received on 1 November 2010, in final form 22 May 2011.

into the insulation at the time of cable manufacturing, stocking and installation. In practical service condition, water may come into insulation from the end or terminal of the cables. But, this could be avoided. The last and most important process is that water enters through the cable insulation from outside environment during service of installed cables [4]. During the operating service, the cable is permanently subjected to thermal ageing, which can cause an irreversible damage of the cable insulation. The normal operating temperature of the XLPE cable insulation caused by the large ampacity in steady conditions is considered to be 90 oC and the maximum temperature up to 150 oC is allowed in overload or short-circuit conditions, even if for short times [5-6]. XLPE cable ageing has been studied all over the world for nearly 40 years and many methods such as breakdown strength, space charge distribution and so on have been proposed to evaluate the properties of XLPE, [7] however, there still remains some debates about which of the probable factors observed after hydrothermal ageing is

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IEEE Transactions on Dielectrics and Electrical Insulation

Vol. 18, No. 5; October 2011

the most influential to the water treeing behavior. When aged in hydrothermal condition for a period of time, the mechanical and electrical properties of the XLPE insulation may degrade, which may lead to an apparent deterioration in dielectric performances such as electric conductivity, permittivity and dielectric loss [8]. Also, the chemical structure and physical form of XLPE cables will be altered. Especially, thermal oxidative degradation produces low molecular weight and oxygenated products. During thermal ageing, several structural changes occur such as variation in crystallinity, chain scission and variation in heat of fusion and in melting point [9]. So it’s essential to investigate the ageing process of cable insulation with the dielectric and physicochemical measurement methods. In previous work, a large number of studies mainly focused on the formation and growth mechanism of the water tree in the XLPE cable [10]. In addition, many models of water tree were also proposed, however, the early stage of cable insulation ageing such as at the degradation stage before the water tree growth is relatively less concerned. Therefore, the change in properties of XLPE cables insulating materials at early ageing stage is imperative to further study. In this research，in order to perfectly simulate the service conditions and obtain the early ageing state during the XLPE cables operating, the accelerated water treeing test (AWTT) was conducted in this research. Samples with different ageing were also obtained from 10 kV XLPE cables. In the present paper, the dielectric results demonstrated a new loss peak emerges at about 10 Hz and an obvious increasing conductivity in low frequency for aged samples. The physicochemical properties, for example, FTIR, DSC, XRD and density of the samples with different ageing were tested. It manifests the decreasing melting temperature, crystallinity and density, and some methyl groups cannot be found for aged samples. It is clear that early AWTT ageing stage of XLPE cable insulation can be shown by means of dielectric and physicochemical methods.

2 EXPERIMENTAL AND TEST RESULT 2.1 SAMPLE PREPARATION Accelerated water treeing test on 10 kV cables were carried out as shown in Figure 1. The test condition is as follows:  Cables were placed in tubes filled with tap water;  Cables were heated by inductive current generated in the conductor loop for 8 h and a temperature between 95 oC and 100 oC was reached, and then ambient cooling for 16 h was carried out;  5 days thermal load and 2 days non-thermal load to form a 7 days load circle;  A voltage of 27 ±1 kV at 50 Hz was applied in the whole ageing process. The cable B and C were aged for 120 days and 180 days, respectively, while cable A is unaged for comparison. The XLPE samples which were cut from

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outer, middle and inner layers of the cable insulating materials by using J/Q XLPE cable slicer, were named 1, 2 and 3, respectively. Thus sample A1 means outer layer insulation of cable A while C3 means inner layer of cable C. Altogether 9 samples were achieved: A1, A2, A3, B1, B2, B3, C1, C2, C3.

Figure 1. AWTT process for 10 kV XLPE cable.

Figure 2. The XLPE insulation at different layers of the 10 kV cable.

Figure 2 shows the 10 kV cable with a diameter of 20 mm. The diameter of inside conductor is 10 mm. The outer, middle and inner layer locates at the diameter of about 1718 mm, 14-15 mm and 12-13 mm respectively. In order to conveniently measure the dielectric properties, the larger samples with diameter of 13 mm and thickness of 0.9 mm from inner layer of cable insulation were also prepared. Then, gold were sputtered as electrodes at both sides of samples. At least 5 samples of each type were tested to ensure the reproducibility. 2.2 DIELECTRIC PROPERTIES MEASUREMENT The dielectric properties of the XLPE samples were measured using Novocontrol broadband dielectric spectrometer made in Germany in the frequency range from 10-1 Hz to 4.5×104 Hz and in the temperature range from 40 °C to 20 °C with an interval of 20 °C. The dielectric spectra of sample A3, B3, and C3 at different temperatures are shown in Figure 3, respectively. A multi-peak resolution method for the dielectric spectra has been applied to aged sample B3 and C3. The curve

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tested at -20 °C was taken for example just as shown in the inset of Figures 3b and 3c. For unaged sample, the dielectric loss is very low and only one loss peak was found at 103 Hz. However, for AWTT aged samples B3 and C3, a new loss peak was found at low frequency of about 10 Hz, which lead to a great increase of dielectric loss, especially at low frequency region. 0.04

Sample A3 -40oC -20oC 0 oC 20 oC

tan

0.03

0.02

FTIR spectra of both aged and unaged samples are shown in Figure 4. It is generally accepted that in the FTIR spectra of XLPE insulating materials, methyl group absorption give peaks at 1377 cm-1, 2150 cm-1, 2632 cm-1 and 2953 cm-1 [11-13]. In outer layer of Figure 4a, it is evident that after AWTT ageing of XLPE insulated cables, the methyl groups (2632 cm-1) in outer layer cannot be found. Nevertheless, in middle layer of Figure 4b and inner layer of Figure 4c, the methyl groups results are similar no matter the cable is aged or not.

peak 

0.01

0.00

-1

0

1

2

0.04

tan

0.03

tan

0.00

0.00

-1

0

-1

(a)

peak 

peak 

Sample B3 o -40 C o -20 C o 0 C o 20 C

5

Sample B3 o -20 C



0.02 0.01

0.01

4

0.04 0.03

0.02

3

logf (Hz) (a)

0

1

2

3

4

5

logf (Hz)

1

2

3

4

5

logf (Hz) (b) 0.04

0.04

Sample C3 o -20 C

tan

0.03

tan

0.03



0.02

peak 

0.01

0.02 Sample C3 -40oC 0.01 -20oC 0 oC 20oC 0.00 -1 0

0.00

1

(b)

peak 

-1

2

logf (Hz)

0

1

2

3

logf (Hz)

3

4

4

5

5

(c) Figure 3. Dielectric spectra of samples A3, B3 and C3 at different temperatures (a)  A3; (b)  B3; (c)  C3. (c)

2.3 FTIR RESULTS The structural changes in XLPE during ageing were analyzed with FTIR spectrometer (model IR Prestige-21). Each sample mentioned above was tested in the wave number range of 500-4000 cm-1. One computer was connected to the spectrometer in order to receive and analyze data.

Figure 4. The FTIR spectra of the different aged level samples (a)  outer layer; (b)  middle layer; (c)inner. layer

This result of Figure 4 strongly suggests that the microstructure change of cable insulation during AWTT process may be initiated in the outer layer.



2.4 XRD RESULTS AND CRYSTALLINITY To clarify any changes in the crystalline phase of XLPE during ageing, X-ray diffraction was carried out by a DX1000 X-ray diffractometer. The operational interval of Bragg angle was 2θ=10 o -55o by a step of 0.3o/min. 2500

Intensity(a.u.)

Gaussian functions as shown in Figure 6 using ORIGIN 7.5 software and the crystallinity is given by the following relationship: area2  area3 (1)  100% χ(%)  area1  area2  area3 where: χ(%): crystallinity percentage. area 1: area under the amorphous halo. area 2: area under the principal crystalline peak at about 2θ=22°. area 3: area under the secondary crystalline peak at about 2θ=24° [16].

A1 B1 C1

2000

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1500 1000

The result in Figure 7 shows that the crystallinity of the samples decreases progressively with the increasing ageing time.

500 0 10

15

20 25 2degree

30

35

Figure 5. The X-ray spectra of different aged samples

The inner, middle and outer samples with the same ageing time have nearly the same X-ray spectra. So Figure 5 only shows the X-ray spectra of the outer layer samples with the different ageing. In order to clearly show the two main crystallization peaks which are observed respectively at about 2θ=22° and 2θ=24°, the curves are only given in the range from 2θ=10° to 2θ=32° [14-15].

2.5 DENSITY RESULT The density was measured with suspension method. Firstly, the mass of the volumetric flask with specific volume was measured using photoelectric analytical balance model AG135 FR from Switzerland METTLER TOLEDO. Secondly, proper volume of mixed liquid of water and pure alcohol was filled into volumetric flask to keep XLPE fragments suspending. Finally, the density of mixed liquids can be calculated which is equal to that of the XLPE fragments. 0.91

3

Density(g/mm )

A1

Figure 6. The Gaussian Fitting for the X-ray spectra of the crystalline regions and the amorphous regions of the samples.

C rystallinity (% )

34.00%

B1 C1

0.906

0.904 Samples Figure 8. The comparison of the density of the different ageing samples.

38.00% 36.00%

0.908

A1 B1 C1

32.00%

It was presented in Figure 8 that the averaged density of the outer samples decreases with the ageing time, which were shown in middle and inner samples as well. This result indicates low density area formed in the XLPE cable insulation during the AWTT process.

30.00% 28.00% Samples

Figure 7. The crystallinity of the samples with the different ageing.

The crystallinity of XLPE according to the ageing time was calculated using Hinrichsen’s method [16]. The method is based on fitting the diffractogram into three

2.6 DSC RESULT Thermal properties of the XLPE samples were studied using Differential scanning calorimetry (DSC) 822e from Switzerland METTLER TOLEDO with computer data system. Measurement results achieved from the fresh and aged cables were analyzed by heating and cooling at a constant rate of 10 °C/min under nitrogen atmosphere. The heating interval was 60-160 °C.

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As can be seen in DSC curves (Figure 9), there are some changes in the heat flow peaks for different samples. The column diagrams clearly reveal that the peak temperature decreased with the increase of ageing time. In addition, by comparison of the result of the enlarged DSC curves in the in inset of Figure 9, a shoulder can be found in the aged sample C1. The shoulder reflects the thermal history or the characteristics of the samples resulting from the degradation and give indication on the stress formed and morphological changes occurring during degradation [16]. 30

12

o

-1.8 -2.4 78

84

6

111 110 109 108 107

A1

B1 C1

-12 100 120 140 o Temperature ( C)

160

Figure 9. The differential scanning calorimetry (DSC) curve of the same layer samples with different aged degrees.

The values of the temperature of melting temperature (Tm), melting range (ΔTm), crystallization temperature (Tc), subcold temperature (ΔTc=Tm-Tc) and full width at half maximum of melt peaks (D) are shown in Table 1. From Figure 9 and Table 1, it can be seen the melting temperature (Tm) has decreased from 109.33 oC to 108.83 oC and 107.83 o C with the increase of ageing time (A1> B1 > C1). Table 1. Parameters obtained from DSC measurement.

Sample A1

o

Tm/ C 109.33

o

ΔTm/ C 20.53

and

 g

(2)

 0  r

where γ is the conductivity of the dielectric, g is equivalent conductivity of relaxation polarization loss, ε0 is permittivity of vacuum and εr is the relative permittivity of the dielectric, respectively. Therefore, it can be proposed from Equation (2) that the increase of dielectric loss for samples B and C may attribute to a combination of conductivity and low frequency relaxation, which can be clearly shown in Figure 3 that new loss peak α and enlarged conductivity γ appears for aged samples.

A1 B1 C1

-6

80

tanδ

1 Samples

90 96 Enlarge

0

60

tan  

Conductivity (S/cm)

18

-1.2 T( C)

Heat flow(mW)

24

The relation between dielectric loss conductivity can be expressed as follows [17]:

o

Tc/ C 80.46

o

ΔTc/ C 28.87

o

D/ C 9.98

B1

108.83

19.66

81.13

27.7

10.62

C1

107.83

20.61

81.80

26.03

10.76

3 DISCUSSIONS 3.1 INFLUENCE OF AWTT ON DIELECTRIC BEHAVIOR OF XLPE CABLE INSULATION It was shown in Figure 3 that the dielectric loss will increase after the ageing of XLPE insulation. For sample A3, only one loss peak β can be observed at between 102 Hz and 103 Hz. However, for sample B3 and C3, an additional new loss peak α at 10 Hz was found, which shifts to higher frequency with increasing temperature and represents a typical Debye relaxation process. This relaxation process was expressed by multi-peaks Gaussian fitting technology. It can be considered that the dielectric loss tanδ is contributed by conductivity process and two relaxation processes.

10

-11

10

-12

10

-13

10

-14

10

-15

10

-16

0.1Hz 1Hz 50Hz 0

30

60

90

120

150

180

Ageing (days)

Figure 10. The relation between electrical conductivity and ageing.

Figure 10 illustrates a markedly enlarged conductivity γ in low frequency region with increased ageing time. It should be noticed that the variation of conductivity will be decreased at higher frequency, which suggest that very low frequency conductivity may be used as an effective parameter to show the ageing behavior of cable insulation. It is known that free radical, which consists of unpaired electron, exists in the ageing of XLPE and plays an important role in the creation of terminal group and fracture of polymer chains. Furthermore, the free radical, terminal group and fracture of polymer can all create charge traps [18]. The role of charge trapping in treeing process is also proposed by Kao [19]. The conduction band width of polyethylene is only about 0.1 eV. The carriers such as the injected electron or some charged small chain segments which usually exist in amorphous area and the interface between crystalline and amorphous area may easily fall into the trap after its several scatter in conduction band and become the trapped charge [20]. In the deep trap the carriers cannot return to conduction band easily because the energy is not high enough, however, in the low frequency range, the carriers have enough time to achieve a high energy to detrap from the shallow trap. The carriers motion is in a capturing, releasing, migrating and re-capturing process [18, 21]. Non-polar organic dielectrics, such as polyethylene (PE) with neither polar groups nor weak link ions, contain only electron polarization and slight impurity conduction loss



[17]. The loss peak β shown in Figure 3 is observed in the dielectric spectra of both the aged and unaged samples. It may be an intrinsic loss peak and relate to the additives or the new generated polar groups during the PE crosslinking process. It can be thus suggested that the loss peak α at 10 Hz represents a typical Debye relaxation process. In Figure 11, the activation energy for the Debye relaxation of sample C3 was calculated according to Arrhenius equation:

f p (T )  f 0 e

 E / kT

(3)

Where fp is the frequency corresponding to loss peak, f0 is a constant, T is absolute temperature, k is Boltzmann’s constant, E is the activation energy for the Debye relaxation. It can be deduced from physical and chemical test results that AWTT ageing process will lead to lower density which means some defects such as microvoids in the XLPE cable insulation materials. Meanwhile, increased conductivity and additional low frequency relaxation with activation energy of 0.14 eV shown in Figure 11 were found for aged samples. Thus it can be concluded that AWTT will lead to great change of dielectric property and microstructure before some obvious treeing behavior was found.

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melting point, the smaller chain segments and some dangling bond will be generated due to the bond scission in the backbone of the XLPE macromolecules. Chain scission is favored by oxidation process [23]. It is normally accepted that the degradation, though taking place mainly in the amorphous phase, may also occur at the lamellar fold surfaces, and causes an increase in the surface free energy of the crystals resulting in reduced melting temperature [24]. Due to continuous heating, the crystalline parts of XLPE become amorphous which make oxygen diffuse easily and therefore undergo thermo-oxidative chain scission readily. The same mechanism can be proposed also for thermal ageing temperature lower than the XLPE melting point, however, at these temperatures thermal degradation phenomena take place at lower rates [16]. Therefore, the decrease of the XLPE crystallinity and the XLPE melting point which are shown in Figure 7 and in Figure 9 respectively can attribute to the thermal-oxidation degradation in AWTT process. The changes of melting temperature of the XLPE after ageing correspond to the modification of the lamellar thickness distribution. The Thomson-Gibbs equation gives the relationship between the lamella thickness L and the melting temperature of lamellae of thickness L:

T m  T m（ 0 1  3.0

-ln fp

2.4 2.1

0.14eV 1.8 1.5 0.0036

(2)

where Tm is the melting temperature (K) of lamellae of thickness L; Tm0 is the equilibrium melting temperature of an infinite crystal; σe is the surface free energy per unit area of the basal plane; ΔHm is the enthalpy of fusion per unit volume; and L is the lamellae thickness. The used values for calculation were as follows [25]: Tm0= 414.6 K, ΔHm= 2.88×108 J·m-3 and σe= 60.9×10-3 J·m-2. The slight variation of the average lamella thickness according to the ageing time can be observed in Table 2.

2.7

0.0034

2 e ) H m L

0.0038

0.0040 -1

0.0042

0.0044

1/T (K ) Figure 11. The relationship between -lnfp and 1/T.

Table 2. Lamellar thickness at the main endothermic peak.

Sample -10

Mean lamellar thickness (×10 ) (m)

It is believed that the charge carriers may accumulate at the interface of the defect areas [22] or in the deep traps, and thus the interfacial polarization may occur at some low frequency region. Generally the bond gap of the polymer is about 8.5 eV and most of C-H or C-C bond energy in polymer is about 4.0 eV [19]. It is clear that the activation energy of peak α 0.14 eV shown in Figure 11 is much smaller than that bond energy of 4.0 eV. Therefore, it is most possible that the new low frequency relaxation for aged samples is introduced by interfacial polarization relaxation of defects formed during AWTT process. 3.2 A NEW MODEL PROPOSED FOR AWTT AGEING PROCESS In general, the chemical degradation can be considered to be a secondary role because the temperature in AWTT is not high enough. At temperatures higher than the XLPE

A1

B1

C1

54.34

53.51

51.92

After AWTT ageing for 120 days and 180 days the average lamellar thickness of samples decreases with 1.53% and 2.66% respectively, which can further explains the decreased crystallinity and chain scission degradation of the insulating materials in AWTT. In addition, when the location changed from outer layer to middle layer and inner layer, the lamellar thickness of sample C will be increased from 51.92×10-10 m to 52.18×10-10 m and 54.07×10-10 m, respectively. According to above experimental results, it is manifested that dielectric and physicochemical performance can be greatly changed due to AWTT ageing, although there is no water tree found. Therefore, a model displaying the change of macroscopic and microscopic structure at the early stage of AWTT for cable insulation was suggested and shown in Figure 12.

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Figure 12. A new model for AWTT aged cable insulation (a) macroscopic model; (b) microscopic model.

Three regions, named A, B and C respectively in Figure 12a, were suggested to represent macroscopic ageing state of sample A, B and C. The black dots stand for submicrovoids which are introduced in the manufacture process and exists in original fresh cable samples, while ellipse symbols stand for microvoids which are caused by water and stress during AWTT process. It is recognized that new generated microvoids is a development of original submicrovoids under ageing condition [17]. Tanaka et al suggested that stress will lead to the formation of end radicals which are an evidence for nucleation of submicrovoids [4]. Since the development of submicrovoids to microvoids caused by water is similar in both region B and region C, only region B was enlarged in Figure 12b. It is known that XLPE is composed of crystalline phase and amorphous phase. Generally interface is regarded as weak point of insulation system, thus defects such as submicrovoids and microvoids in XLPE may be formed and developed at the interface of crystalline and amorphous area. In Figure 12, the introduction of water entering cable insulating materials in AWTT process is illustrated for different aged samples. During ageing process, the water immersed into the submicrovoids and microvoids of the insulation materials will be under the combination of two forces: thermal expansion force σ1 due to selective heating and dielectrophoresis forces σ2, as shown in the Figure 12b. Consequently, more submicrovoids and microvoids will be generated with the AWTT ageing process as shown in sector parts A and B in the model, which may cause the low density area formed in the insulation materials. Dissado et al [20] considered that extended states of carriers such as mobile electrons lie in between the chains in the crystals or in region of unfulfilled lattice sites in the amorphous region, which indicates that the carriers in these regions may mainly contribute to the increasing conductivity in the ageing samples. He also suggested that if the detected trap depth is less than 0.3 eV, the relaxation process can be supposed due to chain bends on lamella surface. In this work a new relaxation with activation energy of 0.14 eV was found for aged samples, which is less than 0.3 eV. By the comparison of two activation energies it can be verified that the relaxation is caused by interface polarization of new generated defects shown in Figure 12. During this ageing process, the new generated carriers also lead to great increase of low frequency conductivity as shown in Figure 10. Since tap water was used in the ageing process and oxidation may happen in the ageing process, some metal ions and carboxylates may work as carriers in the interface [26]. The interface between amorphous and lamellar zones was noticed and reported by several researchers in the research of

water tree [27] and electrical tree [28-29]. It was found [28-29] by TEM observation that lamellar near the semiconducting layer/insulation interface were almost perpendicular to the interface and the initial electrical tree grew along the lamellar layer. These reported results support our suggestion that lamellar/amorphous interface at the outer layer of insulation may work as channel of water during AWTT ageing and consequently lead to the change of physical, chemical and electrical performance shown in this work. It is clear that polymer morphology do show great influence on the water tree behavior of XLPE cable and more detailed investigation need to be carried out in the future.

4 CONCLUSIONS 1) In the dielectric spectra of AWTT aged samples, the lowfrequency conductivity γ increases obviously and a new loss peak α with an activation energy of about 0.14 eV is observed at about 10 Hz. It is suggested that the increased dielectric loss is due to increased conductivity and new Debye relaxation process in low frequency region. Furthermore, it is found that low-frequency conductivity is sensitive to AWTT ageing process and consequently it can be used as a parameter to show the ageing state of cable samples effectively. 2) The results of physicochemical measurement show that the methyl absorption peak at 2632 cm-1 was not found in the FTIR spectra of the outer layer of aged samples. In addition, the crystallinity and density of samples decreased with the increased AWTT ageing time. It is suggested that ageing process in AWTT starts from outer layer and develops to inner layer by a combination of water, electric field and temperature. 3) A new model for AWTT aged cable insulation is proposed to illustrate the development of macroscopic and microscopic defects. It is suggested that water may move along the interface of crystalline phase and amorphous phase. Thus new microvoids and low density can be achieved and the methyl absorption peak cannot be found in FTIR results any more for aged samples. Since additional charge carriers and new microvoids were generated in the ageing process, the low frequency conductivity will be increased and a new relaxation process will be found, as shown in the dielectric measurement results. This new model can well explain the results of both dielectric and physicochemical measurement.

ACKNOWLEDGMENT The authors are grateful to Dr. G. Chen of the University of Southampton for his helpful comments and discussion on this paper. The authors wish to express their thanks for the financial support from National Natural Science Foundation of China (Grant No. 50977071) and the science and technology project of state grid corporation of China (Grant No. SG0866).

REFERENCES [1] [2]

S.V. Nikolajevic, “Investigation of water effects on degradation of crosslinked polyethylene insulation”, IEEE Trans. Power Delivery, Vol. 8, pp. 1682-1688, 1993. L. Castellani, F. Peruzzotti, A. Zaopo, P.L. Cinquemani, S. Foulger, J.C. Filippini and V. Lachevre, “Water Treeing Retardant Materials for Cable Insulators”, IEEE Conf. Electr. Insul. Dielectr. Phenomena (CEIDP), Minneapolis, USA, Vol. 1, pp. 312-316, 1997.

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[28] T. Okamoto, M. Ishida and N. Hozumi, “Dielectric breakdown strength affected by the lamellar configuration in XLPE insulation at a semiconducting interface”, IEEE Trans. Dielectr. Electr. Insul., Vol. 24, pp. 599-607, 1989. [29] N.Hozumi, T. Okamoto and H. Fukagawa, “TEM observation of electrical tree paths and microstructures in polyethylene”, IEEE Int’l. Sympos. Electr. Insul. (ISEI), Boston, MA, USA, pp. 331-334, 1988. Jianying Li was born in Shaanxi, China in 1972. He received the B.S, M.S and Ph.D. degrees in electrical engineering from Xi'an Jiaotong University, China in 1993, 1996 and 1999, respectively. He is now a professor at Xi'an Jiaotong University. His major research fields are high voltage insulation and dielectrics.

Xuetong Zhao was born in Henan, China in 1984. He received the B.S in College of Electrical Engineering from Henan Polytechnic University. Currently, he is a graduate student of high voltage and insulation technology in Xi’an Jiaotong University. His main reserch field is insulation material and zinc oxide varistor ageing.

Guilai Yin was born in Jiangxi, China in 1983. He received the B.S in mechanical engineering from Center South University. Currently, he is a graduate student of high voltage and insulation technology in Xi’an Jiaotong University. His main research field is organic nanocomposites and environmentally friendly insulation.

Shengtao Li, (M’96), was born in Sichuan, China, in February 1963. He received the B.Sc., M.Sc. and Ph.D. degrees in electrical engineering, from Xi’an Jiaotong University in 1983, 1986, and 1990, respectively. Currently, he is a professor at the State Key Laboratory of Electrical Insulation and Power Equipment in Xi’an Jiaotong University. His research interests include electronic ceramics and devices, insulating materials and insulation system, electrical treeing in polymers. Benhong Ouyang was born in Hubei, China, in 1980. He received the B.Eng. and M.Sc. degrees in electrical engineering from Xi’an Jiaotong University in 2002 and 2005, respectively. Currently, he is studying on the testing and operation technology of power cable.

Jiankang Zhao was born in Shaanxi, China, in 1963. He received the B.Eng. degree in electrical engineering from Xi’an Jiaotong University in 1983. Currently, he is the Vice President of High Voltage Institute in State Grid Electric Power Research Institute. He is presently working on his Ph.D. degree in Wuhan University. His research interests include operation and standardization of power cable.