从缺陷影响看高压XLPE电缆绝缘减薄的必要性

Necessity of Reducing the Insulation Thickness of HV XLPE Cables from the Defect Point of View

传统观念认为增厚的绝缘可以提高高压XLPE电缆的安全裕度与运行可靠性,但近年来大量的理论研究与实际运行结果都对这个观点提出了质疑。为此,利用极不均匀电场中的应力集中理论以及固体电介质的弱点击穿理论,研究了绝缘与半导电屏蔽层交界面上的凸起引起的应力集中以及绝缘本体存在的杂质、微孔等缺陷导致的弱点击穿现象。结果表明,绝缘的增厚对减小凸起处电场强度的作用不大,反而降低了电缆绝缘的最小击穿电场强度,并大大增加了成本,导致了电缆在制造、运输、敷设中的一系列问题。采用高性能原材料、改进生产加工工艺才是提高电缆性能行之有效的根本措施。

From the point of view of Chinese cable industry, it is sure that the thicker insulation can serve as a complementation for the inferior material quality and lower technological level, so as to improve the safety margin and operation reliability of cross-linked polyethylene （XLPE） cables, especially for high voltage and large capacity ones. As a result of this, the cable insulation layers of HV and EHV cables in China are always much thicker than those of other developed countries, where researchers and manufacturers have being trying to reduce the insulation thickness. In recent years however, the validity of this view has been called in question by results from theoretical researches and practical services. Based on the stress concentration theory of the non-uniform field, along with the weak point breakdown theory of solid dielectrics, what have been studied in this paper are the stress concentrations of the protrusions at the interface of the insulation layer and the semi-conductive shield, as well as the weak point breakdowns resulted by impurities and rnicropores in insulation bulk. It is found that thick insulation does little to ameliorate electrical fields at the protrusions, on the contrary, it reduces the minimum breakdown fields of the cables. This result is in opposition to the traditional standpoint, in which thicker insulation can reduce the highest electric field of cables, so as to achieve a much safer operation life. Besides this, as we all know that, if the XLPE insulation is thickened, the manufacturing cost will rise remarkably, along with problems in production, transportation and installation. Thus rather than adopting a much thicker insulation to improve the cable performance, what is really effective is to use high performance materials, such as the ultra-pure insulation and screen raw materials, to improve the production technology, and to strictly control the production circumstance and procedures.