93 / 2018-08-24 15:17:40

Thermal Effect of Different Laying Modes on XLPE Insulation and a New Estimation on Cable Ampacity

XLPE Cable; Thermal Effect; Different Laying Modes; Cable Ampacity; Comsol Multiphysics.

Although the factor of the different laying modes have been taken into account in the IEC and IEEE standards on the calculation of cable ampacity, the influence of cable ageing was rarely concerned. It is universal to calculate XLPE cable ampacity by thermal-circuit method and simulation softwares, which follow the basic principle of calculating the maximum current through the conductor corresponding to the steady-state temperature of 90 °C in different environment. To a large extent, these methods do not consider the change of XLPE insulation itself because of the different ageing factor and the influence of the heating and cooling process to the material during the cable operating condition, which would totally change the morphology of the XLPE. It can be inferred that the thermal resistance and thermal capacity of XLPE are changeble for the reason that the microstructure of XLPE insulation are changing with many factors, such as heat and electricity. Furtherly, it can be considered that the cable ampacity is fluctuating value rather than a constant as to the same specification XLPE cable in the same environment with the passage of time.
In this paper, three cables were selected including two retired cables which have operated for 15 and 30 years, and a spare cable was located in the same period. Importantly, no over heated operation was reported for the cables above, that means the temperature in insulation layer was below the 90 ℃, while the maximum temperature in the insulation layer was unknown. For the two retired cables, no physical damage in the cable and no water was detected. Firstly, Comsol Multiphysics was adopted to construct the cable models under different laying modes as the experimental guide. In order to verify the reliability of the models, the cable ampacity under the air laying mode was tested and applied current was 1200 A, approaching to the air laying model which was constructed in Comsol Multiphysics. We define cable ampacity in air laying mode as reference cable ampacity IR. Secondly, the 1.2IR was applied in the cables, which were laid under ground, in pipeline, and in tunnel. The heating and cooling curves from 30 °C to 90 °C and 90 °C to 30°C were obtained through the Comsol Multiphysics. Thirdly, the thermostat was used to stimulate the ageing process based on the heating and cooling process from results of the formal Comsol Multiphysics models. the heating and cooling process fitting the simulative curves and add 2 hours constant temperture phase of 90 °C between the heating and cooling phase among three cable samples with different operating years. The 3 phases had been repeated 80 times in order to enlarge the differences among the samples. Finally, several diagnostic measurements including differential scanning calorimetry (DSC), fourier infrared spectroscopy (FTIR) and breakdown field strength were conducted on the treated and untreated samples. In the measurement of differential scanning calorimetry (DSC), XLPE peels with the weigth of 5 mg were produced to measure the morphological changes including the melting point, crystallinity and lamellar thickness under the procedure of heating from 25 to 140 °C at the rate of 10 °C /min, holding at 140 °C for 5 minute and cooling from 140 to 25 °C at the rate of 10 °C /min. In the measurement of fourier infrared spectroscopy (FTIR), each sample was performed at 32 scans in the range 400- 4000 cm-1 with a resolution of 4 cm-1 to measure the carbonyl index I1741/1471 and double band index I1635/1471. In the measurement of breakdown field strength, 30*30*5mm XLPE peels were picked to measure the breakdown field strength in the transformer oil with DC electric field.
The results show that the significant differences in the heat dissipation efficiency among 3 laying modes were observed. The highest value in the Tunnel laying mode, while the lowest value in the pipeline laying mode. Similarlly, the aging of XLPE in different laying modes also shows different characteristics from the physicochemical and electrical performance measurements. It can be analysed the treated XLPE sample properties by contrast the untreated ones. Therefore, it can be realized to regulate the cable ampacity based on the reference cable ampacity IR, which means the IR can be rised when the properties remain as usual or are better, should be declined when the properties are worse deeply as to different cables in different conditions. This is a new estimation on cable ampacity based on the XLPE insulation itself, which is a possible way to judge the insulation condition about the cable with specifical ageing degree in specifical laying mode.