2018 IEEE Conference on Electrical Insulation and Dielectric Phenomena – Cancun – Mexico
Chemical Structure and Breakdown Behaviors of a New AC 500kV XLPE Submarine Cable Insulation with Different Thermal Aging Conditions Xiaojian Wang1, Zhen Gao2, Zhiqian Liu3, Zhigang Zhu1, Shiqiang Li2, Jian Hao3*, Zhipeng Ma3 2
1 State Grid Zhejiang Electric Power Supply Company, Hangzhou, 316000, China Zhoushan Power Supply Company of State Grid Zhejiang Electric Power Supply Company, Zhoushan, 316021, China 3 State Key Laboratory of Power Transmission Equipment & System Security and New Technology Chongqing University, Chongqing, 400044, China
[email protected] Differential scanning calorimetry (DSC) was used to characterize changes in crystallinity [6]. In terms of electrical properties, Broadband frequency dielectric spectroscopy (FDS) was used to measure the relative permittivity and loss factor of the sample under different frequency conditions [7-8]. In terms of microstructure, Fourier transform infrared (FTIR) and microspectrophotometry were used to analyze changes in the chemical structure of materials, including carbonyl peaks, spherulite changes and material surface changes [9-10]. In recent years, China has vigorously developed marine power transmission technology. The 500KV power interconnection project in Zhoushan is currently the world's highest voltage and longest routing XLPE cable project. This paper studied the chemical and electrical characteristics of a new AC 500kV XLPE submarine cable insulation with different thermal aging conditions. The changes of the color, the microstructure changes and the AC breakdown properties of the cable insulation dielectrics at different aging conditions were analyzed.
Abstract- With the rapid rise of new energy industry and the recent development of offshore wind power market, there is a great market for submarine cable products. In recent years, China has vigorously developed marine power transmission technology. The 500kV power interconnection project in Zhoushan is currently the world's highest voltage and longest routing XLPE cable project. This paper studied the chemical and electrical characteristics of a new AC 500kV XLPE submarine cable insulation with different thermal aging condition. Results show that the aging rate is greatly accelerated after the occurrence of white opaque patches. In the thermal aging process, the carbonyl peak at 1710 cm-1 gradually increased. A new peak appeared at 1170 cm-1, which was the stretching vibration peak of -C=O-. The carbonyl index increases continuously. XRD results show that the height of the diffraction peak significantly decreases and only one diffraction peak remains after 160 days aging. As the aging time increases, the α relaxation peaks of the XLPE samples have a tendency to shift to lower temperature. The crystallinity of the sample firstly increases and then decreases obviously after aging 110 days. Compared with the fresh sample, the breakdown voltage of the 160-day aged samples declined more than 40%.
II. EXPERIMENTS
I. INTRODUCTION
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Sample Preparation and Thermal Aging The 500 kV submarine XLPE cable insulation material was provided by the Zhoushan Power Supply Company. Air heat aging chamber was prepared for accelerated thermal aging, and the aging temperature was controlled at 130℃. The 1 mm thick rectangular piece samples were taken out at 0-day, 80day, 110-day, 130-day and 160-day for the analysis of X-ray diffraction (XRD), FTIR, Dynamic mechanical analysis (DMA) and DSC. 0.5 mm thick rectangular piece samples were taken out at 0-day, 10-day, 17-day, 44-day and 70-day for short-time AC breakdown test. Aged samples were placed in a plastic bag with desiccant and stored away from direct sunlight during the process of 24 h cooling.
Cross-linked polyethylene (XLPE) has been widely used as cable insulation material for its excellent physical, chemical, and electrical properties [1]. During the service of cable, XLPE will suffer from various stresses (electrical, thermal, mechanical, etc.) which result in the changes of its chemical composition and physical morphology experience [2]. The degradation of XLPE is irreversible which will result in the reduction of the lifetime and the failure of insulation,. It is in desperate need to master the structural changes and electrical insulation properties of the cable insulation with different ageing conditions. The study of polymeric insulating materials is mainly conducted in terms of mechanical properties, thermal properties, electrical properties and microstructure. The thermal aging degree of cable insulation materials is measured by the retention rate of elongation at break [3-4] and the retention rate of elongation at 50% is regarded as the end life point of the XLPE material [5]. In terms of thermal properties,
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B. Chemical Structure and Thermal Performance Test The Nicolet iS5 FT-IR infrared spectrometer manufactured by Nicolet Inc. was used to measure the infrared spectrum of the samples with different thermal aging conditions. The spectrum range was 4000-500 cm-1 and the resolution was 4 cm-1. The XRD patterns was obtained by an X-ray
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diffractometer (PANalytical Empyrea, Almelo, Netherlands) whose scan rate was 10°/min and a scan angle 2θ in the range of 5°to 90°. A longitudinal dynamic tensile test was carried out on a rectangular XLPE sample (10 mm×5 mm×1 mm) with a Q800 dynamic thermal mechanical analyzer from TA Instruments of the United States. The vibrational frequency in the test is 1 Hz. The test ambient temperature is linearly increased from 20°C to 120°C, and the increasing rate of temperature is 3°C/min. The United States TA Instruments Q200 DSC was used to test the XLPE samples of different aging stages by a heating-cooling-heating process. The melting curves of the samples in different aging stages were obtained. The temperature was increased from 20°C to 120°C.During the experiment,the temperature was firstly lowered to -40°C and then heated to 120°C to eliminate the effect of the thermal history of the sample on the test results. The increasing and cooling rate of temperature was 10°C/min.
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Fig.2 FTIR spectra of XLPE cable aging at 130 ℃
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C. AC Breakdown Test For the short-time AC breakdown test, five samples were parepared for each aging condition to ensure repeatability.The electrode used is a plate-plate electrode. The diameter of the electrode is 25 mm and the voltage boost rate is 1 kV/s.
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III. RESULTS AND DISCUSSIONS
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A. Color Changes in the Aging Process The surface color and surface topography of the aging samples are shown in Fig.1. a) 0-day→80-day. At this stage, the color of the sample surface gradually changes from white to light yellow. The color gradually deepens as the aging time increases. b) 80-day→110-day. White opaque patches appear on the surface of the sample at this stage, and the patches have a tendency to grow outwards. Other parts change from pale yellow to dark yellow. c) 110-day→130-day→160-day. At this stage, the patches change from white to brown and air bubbles appear on the surface of the sample. The change part becomes hard and brittle, and it is easily broken. By 160-day, the sample has completely turned dark brown.
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Fig.3 the relationship between carbonyl index and aging time of XLPE cable at 130 °C
The color changes and white patches suggest that the chemical structure may change during the thermal aging process. Therefore, the changes in chemical structure of the samples with different aging conditions were investigated by FTIR and XRD. The FTIR results of the samples with different aging conditions are shown in Fig.2. The significant absorption peaks at wavelength of 2915 cm-1, 2847 cm-1, 1470 cm-1, and 718 cm-1 are characteristic absorption peaks for CH2- in the polyethylene carbon chain [5]. The intensity of these peaks is slightly weakened during the thermal aging. The carbonyl peak of 0-day and 80-day sample is not obvious. With the progress of thermal aging, the carbonyl peak at 1710 cm-1 gradually increased. Besides, a new peak appeared at 1170 cm-1, which was the stretching vibration peak of -C=O，indicating the production of free radicals. Thermal oxidation of XLPE cable insulation samples can be evaluated by the carbonyl index (CI), the expression for the CI is: CI I 1710
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Where I1710 is the intensity of the absorption peak at 1710 cm-1, I2915 is the intensity of the absorption peak at 2915 cm -1. Fig.3 shows the relationship between CI and aging progress of XLPE cable at 130 °C. The CI of XLPE slowly increased before 80-day and then increased sharply after 110-day, indicating that the aging process was greatly accelerated. This
Fig.1 1mm samples at different aging stages under 130°C
B. FTIR and XRD
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is because the high temperature accelerates the changes of the molecular chain structure of the insulating material and exacerbates the reaction of O2 with the molecular chain, resulting in the dramatical increase of oxygen-containing group in the molecular chain and the carbonyl index.
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The aging samples of the five periods shown in Fig.1 were subjected to the XRD test. The 0-day, 80-day and 160-day sample comparison charts are shown in Fig.4. It can be seen that the shape of the XRD curve of the XLPE cable insulation is basically unchanged during the aging time while the height of the diffraction peak continues to decrease. However, it can be seen from the XRD curve of the 160-day sample, the height of the diffraction peak significantly decreases and only one diffraction peak remains. The full-width at half-maximum (FWHM) of the diffraction peak was calculated from the XRD curves to characterize the interplanar spacing and changes in grain size [11]. From Fig.5, as the aging time increases, the FWHM increases. According to Scherrer's formula, the grain size is inversely proportional to the FWHM [12]. Therefore, it can be judged that with the aging time increasing, the grain size decreased significantly and more small crystals were produced.
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influence of the XLPE structure. In the DMA test, 130-day and 160-day samples were destroyed due to excessive clamp stress. Other curves of damping loss factor in Fig.6 show a peak called α relaxation peak, which can be used to characterize the relaxation process of the side groups of the molecular chains in the lamellae. As the aging time increases, the α relaxation peaks of the XLPE samples have a tendency to shift to the left which means the required temperature for the corresponding groups to produce the motion relaxation process is lower. This phenomenon indicates the number of broken chains of XLPE increases, and the relaxation movement of the side chain groups becomes easier. XLPE will undergo a series of physical and chemical reactions such as melting, crystallization and thermal decomposition under thermal stress. Fig.7 is the DSC curve of XLPE cable aged at 130°C. Table Ⅰ shows the results of DSC. The expression for the crystallinity in Table Ⅰ is: Xc H (2) H 0 Where Xc is the degree of crystallinity, H is a melting enthalpy, H 0 is a melting enthalpy with a crystallinity of
Fig.4 XRD curves of XLPE samples aged for different periods
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TABLE I. RESULT OF DSC FOR SAMPLES 0 80 110 60.40 59.79 60.94
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Fig.5 FWHM changes for different aging time
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DMA and DSC The DMA test can obtain parameters such as storage modulus, loss modulus and damping loss factor of the tested sample. The damping loss factor is the tangent of the lag angle between the strain and the external force caused by the
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100%, H 0 387.3J g . The DSC results show that when the aging reaches a certain level (110-day), the melting peak of XLPE deviates seriously, the peak shape broadens and the melting enthalpy decreases, indicating that the molecular structure of XLPE has been seriously deteriorated. On the other hand, the glass transition temperature of XLPE shifts toward the lower temperature, indicating that XLPE will be less heat-resistant after thermal aging and XLPE will more easily change from a glassy state to a highly elastic state.
diffraction peak remains after 160 days aging while the FWHM increases. As the aging time increases, the α relaxation peaks of the XLPE samples have a tendency to shift to lower temperature. The crystallinity of the sample firstly increases and then decreases obviously after aging 110 days. The AC breakdown voltage of the samples deteriorates seriously in the thermal aging process. Compared with the fresh sample, the decline of the AC breakdown voltage for the sample thermally aged for 160 days is more than 40%.
D. AC Breakdown Experiment In this paper, 0.5 mm thick samples were aged and taken for short-time AC breakdown experiment. Compared to the 1 mm thick sample, the 0.5 mm thick sample aged more rapidly which has completely turned dark brown at 70-day as shown in Fig.8. However, the color changes and the appearance of air bubbles during the aging of both samples are the same. Fig.9 is the AC breakdown of the samples. As can be seen from the Fig.9, as the aging process progresses, the AC breakdown electrical field strength gradually decreases.
ACKNOWLEDGMENT The authors thank the supports by Project of the State Grid Corporation of China (Key technology and application research on 500kV cross-linked polyethylene (XLPE) cable. NO.52110417000N) REFERENCES [1]
Geng, Pulong, et al. "Influence of thermal aging on AC leakage current in XLPE insulation." Aip Advances 8.2(2018):025115. [2] Jakub Souček, Pavel Trnka, and Jaroslav Hornak. "Proposal of Physical-Statistical Model of Thermal Aging Respecting Threshold Value." Energies 10.8(2017):1120. [3] Wang Hexun, Study of the Rule of Ship Cable Insulation Aging and Rapid Detection Methods [D], Dalian Maritime University,2014(in Chinese). [4] Wang Z Q,Zhou C L,Li W W, et al. "Residual life assessment of butyl rubber insulated cables in shipboard"Proceedings of the CSEE ， 2012,32( 34) :189-195. [5] Hang Wang,Guoxin Tan,Ying Tan,Lei Zhou1,Fan Zhou,Gang Liu,Ying Lu. "Analysis of Thermal Aging Life and Physicochemical Properties of Crosslinked Polyethylene Seabed Cable Insulation"Polymer Materials Science and Engineer 31.3(2015). [6] Qian, Yihua, et al. "Lifetime Prediction and Aging Behaviors of Nitrile Butadiene Rubber under Operating Environment of Transformer." Journal of Electrical Engineering & Technology 13.2(2018):918-927. [7] Nedjar, M. "Effect of thermal aging on the electrical properties of crosslinked polyethylene." Journal of Applied Polymer Science 111.4(2010):1985-1990. [8] Kim, Chonung, et al. "Investigation of dielectric behavior of thermally aged XLPE cable in the high-frequency range." Polymer Testing 25.4(2006):553-561. [9] Boukezzi, Larbi, et al. "Thermal aging of cross-linked polyethylene." Annales De Chimie Science Des Matériaux 31.5(2006):561-569. [10] Groeger, Joseph H. "The analysis of polyethylene cable insulation using microspectrophotometry." IEEE International Conference on Electrical Insulation IEEE, 2016:204-207. [11] ZHAN Weipeng, CHU Xuelai, SHEN Zuojia, LUO Zhiyi, CHEN Tengbiao, ZHANG Xu. "Study on Aggregation Structure and Dielectric Strength of XLPE Cable Insulation in Accelerated Thermal-oxidative Aging" Proceedings of the CSEE, 2016, 36(17):4770-4778.(in Chinese) [12] Drits, Victor. "XRD Measurement of Mean Crystallite Thickness of Illite and Illite/Smectite: Reappraisal of the Kubler Index and the Scherrer Equation." Clays & Clay Minerals 45.3(1997):461-475.
Fig.8 0.5mm samples at different aging stages under 130°C
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Fig.9 AC breakdown of the samples with different ageing time
IV. CONCLUSIONS The microstructure changes and the AC breakdown properties of the 500kV XLPE cable insulation during the thermal aging process were tested in detail. The conclusions could be drown as follows: The color of the sample is continuously deepened, and the aging rate is greatly accelerated after the occurrence of white patches. With the progress of thermal aging, FITR results show that the carbonyl peak at 1710 cm-1 gradually increased. A new peak appeared at 1170 cm-1, which was the stretching vibration peak of -C=O-. The carbonyl index increases continuously. XRD results show that the height of the diffraction peak significantly decreases and only one
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