The Dielectric Properties in Frequency Domain of the 22 kV Underground Cables with Defective Joint

The failure that occurs in the underground cable systems leads to losses in many aspects, not only in the power system but also in the commercial and business. The most failure location in medium voltage underground cable systems is cable joint and cable termination, where the defects occur from poor workmanship. This paper represents the mean to investigate the dielectric properties of the 22 kV XLPE underground cable system to assess the insulation's integrity of the underground cable system. In the experiments, the underground cable system specimens were prepared by simulating the defects in cable joints, i.e., the iron powder on the XLPE surface and the incision on the XLPE surface; moreover, the underground cable system with the healthy condition joint was prepared to represent the new (or unused) underground cable system. To compare the dielectric properties in the frequency domain of the 22 kV underground cable systems with defective joints and the healthy condition joint, the frequency domain spectroscopy (FDS), the off-line non-destructive test, was performed in this research. From the test results, the dielectric dissipation factor shows a significantly different value in the case of iron powder when compared with the healthy condition joint. It reveals the loss peak on the curve at a frequency of 1 Hz. However, the case of incision on the XLPE surface is almost similar to the healthy condition case. The real part of the complex capacitance of all three cases has a constant value in the measured frequency ranges using the log-log scale. However, the imaginary part capacitance shows the distinct value of all three cases, of which the trends are the same as the dielectric dissipation factor curve.