New after-laying test methods for XLPE cable lines

An after-laying test is required to demonstrate the soundness of XLPE underground cable lines with joints. AC voltage application is desirable for the test; however, the equipment for ac application must be very large because it must supply large capacitive current for cables. DC voltage is normally applied, which can be supplied with small equipment; however, the effectiveness of the de application is questioned because the electrical stress distribution is quite different from that with ac application and, moreover, de application could damage insulation properties of cables. The authors targeted the Oscillating Wave (OSW) method and the Very Low-Frequency (VLF) method as alternative after-laying tests. They first developed modeling methods of various defects on underground cable lines and applied OSW and VLF to these defect models. They found that OSW and VLF methods have higher fault detection capability than de voltage application. They also demonstrated that the defects which can be easily detected with the OSW method are difficult to detect with the VLF method and vice versa. They showed that serial application of both methods highly improve the fault detection ability for XLPE underground cable lines.