Investigation on Insulation Material Properties of 220 kV XLPECables after Pre-Qualification Test
Hou Shuai1, Fan Zhenyou2, Xu Man2, Zhan Yunpeng1, Fu Mingli1
1. CSG Electric Power Research Institute Guangzhou 510663 China; 2. State Key Laboratory of Electrical Insulation and Power Equipment Xi' an Jiaotong University Xi'an 710049 China
Abstract：In order to improve the technology of domestic electrical materials for high-voltage cables, some domestic enterprises had successively established cross-linked polyethylene (XLPE) production lines for 110 kV cables. And these cables had more than 10 years of operating experience. However, the long-term aging performance of 220 kV cross-linked polyethylene insulated cables based on domestic insulating materials was still insufficiently studied. In order to evaluate the long-term operational reliability of domestic 220 kV cross-linked polyethylene insulated cables, this work researches the long-term aging performance of the insulation material of domestic 220 kV XLPE cables after the pre-qualification test (PQ-test). The 220 kV cross-linked polyethylene insulating materials produced by two domestic enterprises were used by five cable enterprises to manufacture five batches of 220 kV cables. PQ-test were carried out on the cables in accordance with GB/T 18890.1—2015, in which the thermal cycle was effectively carried out 186 times at a voltage of 216 kV, with the maximum temperature of the conductor core at 90℃. There was no breakdown of the circuit in 9 000 hours. All cables passed positive and negative polarity 1 050 kV lightning impulse tests 10 times each. Taking the new cables of the same batch reserved before the PQ-test as reference, the insulating layer of the cables after the PQ-test was sampled in inner, middle and outer layers. The performance of the specimens before and after the PQ-test was systematically characterized. Firstly, the change mechanisms of thermal oxidation products and cross-linking by-products were analyzed by infrared spectroscopy, and the activation energy of thermal decomposition of the materials was characterized by thermogravimetric analysis. Afterwards, differential scanning calorimetry, gel content, mechanical properties and electrical properties were investigated. The results of infrared spectroscopy showed that small molecule cross-linking by-products diffused and escaped under the thermal effect of the PQ-test, the aromatic ketone carbonyl content of XLPE decreased, whose distribution tended to be the same in the radial direction due to sufficiently long diffusion times. The oxidative chain breakage of XLPE led to an increase in the oxidation product saturated fatty acid lipid/ketone carbonyl content. The residual cross-linking agent prompted XLPE to continue to undergo cross-linking reaction in the pre-identification test, and the degree of cross-linking increased slightly. The thermal history results of differential scanning calorimetry experiments showed that the temperature of the inner, middle and outer layers of the insulation decreased about 3~5℃ sequentially during the PQ-test. The thermal stress of the PQ-test promoted the crystal refinement and the increase of the material crystallinity. Limited by the increased cross-linking density, the size of the newly formed microcrystals was smaller, and the crystals were less spatially confined to each other, favoring an increase in the number of nuclei. After the PQ-test, the mechamical properties of the main insulation material of domestic 220 kV cross-linked polyethylene cables showed little change, and the AC breakdown strength meet the standard limits.
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