This paper reveals a correlation between morphology and thermal parameters on cross-linked polyethylene (XLPE) cable with different insulating states. Several cables were selected to detect the physicochemical and thermal parameters of the XLPE. The results show that the cable ampacity is determined by the thermal parameters, which are deeply subjected to the morphology of the XLPE. The molecular chain and crystal structure of the XLPE have a close connection with the thermal resistivity. The physicochemical parameters of carbonyl index (CI) and unsaturated band index (UBI) from Fourier transform infrared spectrum (FTIR) and melting range (R m) from differential scanning calorimetry (DSC) can be the indicator to evaluate the diversity of the thermal resistivity. The change of thermal capacity is governed by the crystal distribution of the XLPE. The physicochemical parameters of crystallinity (χ) and lamellar thickness (L) from DSC can be the indicator to evaluate the change of the thermal capacity. In addition, FWHM of the crystallization peak W , crystalline rate index (T 0-T P) and cross-linking degree (G) can also be the indicator of the thermal parameters. Finally, this paper proposes a more accurate on-line monitoring method for electric power industry by detecting thermal parameters to diagnose the operating cables in the practical application.

In this paper, the performance of several annealing methods on three retired cables and the annealing effects on the improvement in the thermal and electrical properties of cross-linked polyethylene (XLPE) insulation were discussed. The cable insulation layer was peeled, and the peels near the inner semi-conductive layer were used as the test samples. Isothermal treatment and heat recycling treatment were performed at temperatures of 85, 90, 95, and 100 • C, and the temperatures were held in the heat recycling treatment for 8, 16, and 24 h, respectively. Each heat recycling treatment was repeated 20 times, and the duration of the isothermal treatment was the same as that for the heat recycling treatment with a 24 h temperature holding hour. Then, Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) were performed, and the dielectric spectrum, DC conduction current, and dielectric breakdown strength E B were measured. The results showed that damage involving molecular changes is linearly related to the cable service year. As the annealing temperature increases, the melting range and electrical conductivity decrease; the melting point, crystallinity, lamellar thickness, and dielectric breakdown strength increase; and the optimal values appear for the samples annealed at 95 • C. With an increased temperature holding hour, the peels annealed at 95 • C exhibit a decreased melting range and electrical conductivity and an increased crystallinity, melting point, lamellar thickness, and dielectric breakdown strength. As a result, the two different treatments are verified to effectively improve the thermal and electrical properties for the XLPE as early research on cable rejuvenation by heat treatment.

This paper verifies the fluctuation on thermal parameters and ampacity of the high-voltage cross-linked polyethylene (XLPE) cables with different insulation conditions and describes the results of a thermal aging experiment on the XLPE insulation with different operating years in different laying modes guided by Comsol Multiphysics modeling software. The thermal parameters of the cables applied on the models are detected by thermal parameter detection control platform and differential scanning calorimetry (DSC) measurement to assure the effectivity of the simulation. Several diagnostic measurements including Fourier infrared spectroscopy (FTIR), DSC, X-ray diffraction (XRD), and breakdown field strength were conducted on the treated and untreated specimens in order to reveal the changes of properties and the relationship between the thermal effect and the cable ampacity. Moreover, a new estimation on cable ampacity from the perspective on XLPE insulation itself has been proposed in this paper, which is also a possible way to judge the insulation condition of the cable with specific aging degree in specific laying mode for a period of time.Energies 2019, 12, 2994 3 of 22 insulation endured the most severe electrical and thermal stresses. These obtained specimens were all cleaned by alcohol to remove the surface impurities.

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In this work, heat treatment was performed on three retired 110 kV AC cables with service years of 0, 15 and 30, and the effects on the thermal and electrical performance of the cable insulation were investigated. First, each cable with a length approximately of 5 m was prepared and cut into five equal segments. Four segments of each cable were annealed at a temperature of 90, 95, 100, or 105 • C by building a small circuit to simulate cable operation. Then, a section at the middle of each segment was cut out, and the insulation layer was peeled away. The peeled-off layers from the inner, middle and outer positions were selected as the test samples. Subsequently, Fourier transform infrared spectrometry (FTIR), and differential scanning calorimetry (DSC) were performed, and the DC conduction current and dielectric breakdown strength were measured. The best thermal properties of the highest melting point, crystallinity, and smallest melting range were found when the cables were annealed at 100, 105, and 105 • C for the inner, middle, and outer positions, respectively. The highest dielectric breakdown strength and lowest electrical conductivity were found at temperatures of 95 or 100 • C for the inner and middle positions, respectively, and at 105 • C for the outer position. The FTIR results showed that thermal annealing for hundreds of hours did not adversely affect the molecular chains. It is found that this type of heat treatment could be a feasible method to rejuvenate retired cables, and a conservative temperature of 95 • C is regarded as the optimum annealing temperature. INDEX TERMS Cable, DSC, melting point, crystallinity, electrical conductivity, dielectric breakdown strength.

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Further research on retired cable rejuvenation via heat treatment was investigated in this paper. Three retired cables with service year of 7, 15 and 30 years were used and annealed at different temperatures following two different heat treatment methods. In the first test, two short cables with a length of 5 m were prepared and cut into five equal segments. Four segments of each cable were annealed at 90, 95, 100, and 105 • C to simulate the cable operation. Then, the cross-linked polyethylene (XLPE) from the cable insulation was peeled, and the peels from the inner, middle and outer positions were selected and subjected to the differential scanning calorimetry (DSC), DC conduction current, and dielectric breakdown strength measurements. The thermal and electrical performance of these samples were analyzed, and the optimum annealing temperature was determined. In the second test, three identical long cables were connected to a transformer, joints, and terminals to build two experimental circuits. The cables were annealed at the optimum temperature determined in the first test. The same measurements were performed on XLPE samples taken from the same positions as those in the first test. The results showed that the short cables exhibited the best thermal and electrical performance when annealed at 95 • C. The three long cables annealed at 95 • C verified that the thermal and electrical properties of the cable insulation were significantly improved. The results of the two different annealing methods, which closely represent the actual cable operations, verified the feasibility that old cables without overheating history could be rejuvenated via heat treatment. INDEX TERMS Cable, DSC, melting point, crystallinity, electrical conductivity, dielectric breakdown strength.