Partial discharge measurements in XLPE cables with misplaced grading system under different applied voltage frequencies

Partial discharge (PD) test is an important methods to detect cable defect under HVAC. For HVDC, since PD tests under DC are difficult to carry out, partial discharge (PD) testing under AC is still used. However, doubts arise concerning the feasibility of this approach, because PD tests using AC waveforms might highlight the presence of defects that do not affect the reliability of the cable system in operation. On the other hand, PD tests might miss defects that have a negative influence on cable reliability. This paper reports results of an investigation on the meaning of testing HVDC cables using AC and DC voltages. HVDC XLPE Cable models with artificial defects are tested under AC (50 Hz and 0.1 Hz) and DC, at room temperature. To carry out this investigation, a technique for measuring PD with high sensitivity under DC has been devised. PD measurement results on a cable with misplaced grading system (a typical defect due to bad assembly in the field) indicate that AC tests provide conservative result (PDIV is lower under AC compared to DC). VLF (0.1 Hz) provides results (PDIV and PD magnitude) that are closer to those observed in DC. The behavior of PDIV is confirmed by FEM simulations.