Annealing Effects on XLPE Insulation of Retired High-Voltage Cable

Abstract

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.

Enhancement of Service Life and Electrical Insulation Properties of Polymeric Cables With the Optimum Content of Aromatic Voltage Stabilizer

“…Hence, there is direct correlation between the electrical breakdown strength and the lamellar thickness. Similarly, the enhancement in the treeing resistance, lower permittivity, and reduced DC conductivity due to the higher value of the crystallinity are reported elsewhere .…”

Thermal Effect of Different Laying Modes on Cross-Linked Polyethylene (XLPE) Insulation and a New Estimation on Cable Ampacity

“…Therefore, we may admit the crystal structure changes on XLPE should be in the light of the features of different laying modes. For more precision on the objective to analyze the crystal structure changes among the specimens, the crystallinity percentage and the grain size are introduced by Formulas (5) and (6). From Figure 10, it can be noticed remarkably that a turnover phenomenon occurs in the thermal aging of tunnel mode, which is reflected by the fact that the crystal of the (110) lattice plane modifies into the (200) lattice plane.…”

“…Therefore, we may admit the crystal structure changes on XLPE should be in the light of the features of different laying modes. For more precision on the objective to analyze the crystal structure changes among the specimens, the crystallinity percentage and the grain size are introduced by Formulas (5) and (6).…”

“…It is obvious that the electric properties were improved in varying degrees among XLPE-0 specimens after the different thermal effects. This phenomenon is probably attributed to the flexible movement of the molecular chain which activates the development of crystalline order and the annealing effect [6].…”

“…XLPE is a high-molecular polymer with crystal and amorphous phase, which is subjected mostly to the thermal effect because the crystal structure of XLPE is prone to be affected by the melting and 2 of 22 cooling process [5]. It is well evidenced that long-term thermal effect can change the morphology of the insulation distinctly and lead to degradation on the insulation [6,7]. In fact, the influence of thermal effect on polymers is a complicated process and the insulation properties can be improved in some certain situations [8].…”