This paper presents the characteristics of the contaminated underground cable joints tested by a very low frequency voltage source. The underground cable system used in the experiment was comprised of a 12/20(24) kV underground cable with a cross-section area of 240 mm(2), and the length of 6 m. Four different case studies were simulated on the cable joint, i.e., a healthy heat shrink cable joint (case 1), a cable joint with an incision on XLPE surface(case 2), a cable joint contaminated with 20-micron iron powder on XLPE surface(case 3), and a cable joint contaminated with 20-micron copper powder on XLPE surface(case 4). The dielectric dissipation factor, also known as tan delta at very low frequency (0.1Hz) of such underground cable systems, was examined at 0.5 U-0, 1 U-0, and 1.5 U-0, respectively. The very low frequency(VLF) test voltage was applied to the cable specimen, according to IEEE 400.2(2013). The test voltage was recorded and analyzed. It was found that the cable joint contaminated with 20-micron iron powder on XLPE surface had a maximum tan delta. The tan d value decreased respectively for the cable joint with an incision on XLPE surface and the cable joint contaminated with 20-micron copper powder on XLPE surface. The healthy heat shrink cable joint showed the lowest tan delta. It was clearly shown that the contaminated cable joint caused higher very low frequency tan d compared with the healthy cable joint.
