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Current Situation Analysis
Why do relevant departments at home and abroad widely recommend using AC withstand voltage instead of DC withstand voltage? With the development of the national economy and the increase in the voltage level of urban power supply, cross-linked polyethylene insulated power cables (XLPE) have been increasingly widely used at home and abroad due to their reasonable process and structure, excellent electrical performance, and safe and reliable operation characteristics. Especially in the field of high-voltage power transmission, great progress has been made. Compared with oil-filled cables, cross-linked cables are easy to lay and install, simple to operate and maintain, and have no oil dripping problem. However, in recent years, operation and research have shown that the insulation of cross-linked polyethylene cables is prone to tree-like discharge during operation, causing insulation aging and damage, which seriously affects the service life of cross-linked polyethylene insulated power cables. Therefore, fully understanding the insulation characteristics of cross-linked cables and timely and effectively detecting and preventing certain defects in the insulation are of great significance for ensuring the safe operation of equipment and even the system. This paper elaborates on the main factors affecting the insulation of cross-linked cables and the principle of cable handover tests, and believes that it is necessary and feasible to implement AC withstand voltage tests on cross-linked cables on site. To ensure the safe and reliable operation of cables, relevant international standards have clearly stipulated various tests for cables. The main test items include: measuring insulation resistance, DC withstand voltage, and leakage current. Among them, measuring insulation resistance is mainly to check whether the cable insulation is aging, damp, and whether there are insulation defects exposed during the withstand voltage test. DC withstand voltage and leakage current tests are carried out simultaneously, aiming to discover defects in the insulation. However, in recent years, test and operation experience at home and abroad has proven that DC withstand voltage tests cannot effectively detect insulation defects in cross-linked cables, and may even cause hidden dangers to cable insulation. In Germany, Sechiswag Company conducted tests on 41 circuits of 10 kV XLPE cables from 1978 to 1980, with 87 failures occurring; in Sweden, more than 9,000 km of 3 kV to 24.5 kV XLPE cables were put into operation, with 107 failures occurring. Domestically, cable accidents have also occurred many times, and a considerable number of cable faults are caused by the negative effects of frequent DC withstand voltage tests. Therefore, relevant departments at home and abroad widely recommend using AC withstand voltage to replace traditional DC withstand voltage. From the 1980s to the mid-1990s, advanced countries such as Canada, Germany, and the United States formulated corresponding AC withstand voltage test standards and promoted their application. Since the late 1990s, Guangdong, Beijing, Shanghai, Zhejiang, Shandong and other places in China have issued interim regulations for AC tests on XLPE cables. The national standard GB50150-2006 "Standard for Handover Test of Electrical Equipment Installation Engineering" recommends the use of variable frequency series resonance withstand voltage method.
