A Comparison of VLF and 50 Hz Field Testing of Medium Voltage Cables
A series of PD and tan δ tests were conducted on 11 kV cables with both a VLF and a 50 Hz HV source. This research was aimed at illustrating the difference between the two methods and assessing the feasibility of the partial core resonant transformer as a portable 50 Hz test kit. The cables had hybrid PILCA/XLPE insulation systems and were up to 2 km in length. Tan δ values measured at VLF were an order of magnitude higher than those at 50 Hz. The VLF test displayed a negative DDF whilst the 50 Hz test showed a positive tip up tan δ characteristic. The 50 Hz test showed more PD than the VLF test which was expected but no consistent difference was observed between the PDIVs. The PCRTX proved to be suitable for field testing cables. All the equipment was light enough to be carried in and small enough to be set up in a confined space. With a trained technician, the tuning procedure was accomplished relatively quickly. With coarse tuning, a 2km, 0.76 μF, 11 kV cable was energised to 2U0 at 50 Hz and drew only 13 A from the supply. The VA supplied to the load exceeded the VA drawn from the supply by a factor of 20 to 30.