119 / 2018-08-24 21:53:56

Study on Ablation between Metal Sheath and Buffer Layer of High Voltage XLPE Insulated Power Cable

XLPE insulated cable,metal sheath,buffer layer,ablation point,electric discharge

In electric power system, metal sheath discharge and ablation occur frequently in the faults of high voltage cross-linked polyethylene (XLPE) insulated cable. In recent years, there are many studies on this problem, but most of them only optimize the structure of metal sheaths and buffer layer with simulation methods instead of basing on real cables. In order to provide some effective references for the solving approaches of the metal sheath discharge and ablation, some main factors of the ablation are studied in this paper according to an actual model with wrinkled aluminum from faulty cables. First, we applied finite element simulation (Ansys) to calculate high voltage cable electric field, magnetic field and thermal field distribution. Afterward, an actual model is built by installing wrinkled aluminum on the common electrode structure and using buffer layer as the sample between the electrodes. As wrinkled aluminum and buffer layer are obtained from the faulty cables, the actual operating conditions of the metal sheath can be simulated. Then, some comparative experiments are carried out to analyze the influence of different variables on the ablation of metal sheath. The variables are introduced according to the investigation of the faulty cables. In the process of disintegration for the faulty cables, it is found that the ablation points are mostly located in the close contact position between the metal sheath and the buffer layer, thus the distance between those two layers can be chosen as a variable. Moreover, the morphology of ablation point on the metal sheath in the faulty cables is similar to that caused by electric arc discharge. In order to explore the relationship between electric arc and ablation, different waveform voltages are set (AC, high repeat frequency pulsed voltage, etc.) as variables. Meanwhile, some other factors such as moisture content, temperature and defects on buffer layer also need to be taken into account, for they may increase the formation of ablation point and ablation velocity. Through these experiments, some artificially ablated samples can be obtained. Subsequently, the residues around the ablated points on the buffer layer are sampled and tested by using FTIR, XRD and XRF methods. Then, the test results are compared with the chemical products collected from the ablation point in the fault cables to get the differences between them in the aspects of absorption peak and element content. The result of comparative experiment demonstrates that high repeat frequency pulsed voltage requires the shortest time in the process of forming ablation compared with other waveforms. While the source of high repeat frequency pulsed voltage can be attributed to the operating overvoltage of the electric power system in actual working conditions. Furthermore, the experimental data indicates that moisture, temperature and the distance between sheath and buffer layer can accelerate the formation and development of ablated points. In conclusion, operating overvoltage in electric power system, cable operating temperature, mechanical collision, drying degree in cable will lead to ablation of the buffer layer and metal sheath. Therefore, these factors should be avoided during the laying and operation of the cable.