XLPE交流电缆线路改为直流运行的载流量设计与分析

在现役XLPE交流电缆线路的直流改造中,载流量的合理设计是关键问题之一,决定了改造线路的传输容量和运行可靠性。文中对同一线路在交、直流电压下运行时的等效热路模型及载流量解析算法进行了对比,分析了造成交、直流线路载流量差异的关键因素,并以空气敷设的三芯10 kV XLPE电缆为例,进行了同一线路在相同敷设条件下的交、直流载流量模拟试验。研究发现,目前直流改造所涉及的现役XLPE交流电缆线路,在进行直流载流量评估时,绝缘温差要求不成为限制条件,仅需考虑线芯温度限制、按照IEC60287-2017推荐方法进行计算;在线芯电阻、金属护套损耗、载流芯数、环境热阻及线芯允许长期工作温度等影响因素中,交流电缆改为直流运行后线芯允许工作温度由90℃下降为70℃,在很大程度上抵消了其他因素对载流量的有利贡献;10 kV XLPE电缆载流量模拟试验数据和解析计算结果吻合,偏差很小,验证了解析计算方法的有效性。对10、35 kV三芯和110 kV单芯电缆在不同典型敷设情况下的交、直流载流量计算显示,改为直流运行后,三芯电缆的载流量略有增加,单芯电缆稍有下降,变化幅度均未超过6.5%。

Design and Analysis of Current Ratings of XLPE Cable AC Lines Transformed into DC Operation

In the transformation of an existing XLPE cable line from AC operation into DC,the design of DC current rating is one of the key issues which determines the transmission capacity and operation reliability of the transformed line.In this paper,the equivalent thermal circuits and analytical calculation formulas for the same cable line running under AC and DC are compared,the main factors resulting in the difference between AC and DC current rating of a cable line are analyzed,and the AC and DC current ratings of a 10 kV three-core cable in air are tested under same conditions.It is found that for XLPE cables involved in the AC-to-DC transformation,the insulation temperature gradient will not be a limiting condition,and the DC current rating can be calculated according to IEC 60287-2017 based on the requirement for the permitted conductor temperature.Among the main influencing factors,which are the resistance of conductor,the loss of metallic sheath,the number of cores with current carrying,the thermal resistance of surrounding circumstance and the permitted maximum temperature of conductor,the decrease of the conductor temperature from 90℃under AC to 70℃under DC is the most unfavorable condition for the cable line,which largely offsets the improving effect of other factors to the DC current rating.A good correspondence is found between the current rating results tested and c alculated,which verifies the validity of the analytical calculation method.According to the AC and DC current ratings analytically calculated for the 10 kV and 35 kV three-core as well as 110 kV single-core cables under typical installation conditions,the DC ratings of the three-core cables increase slightly while those of the single-core cables decrease compared with the AC ones,with the amplitudes of variation no more than 6.5%.