Evaluation of Abrupt Grounding as Quality Control Method for HVDC Extruded Cables

This paper presents results from laboratory experiments performed in order to evaluate different methods for quality control of polymeric insulated HVDC cables. In this work, we have investigated the effect of short term (24 h) DC stress followed by rapid grounding of the cable. This is comparable to a voltage step of -U₀ at the point of grounding ,and depending upon the cable length, a step of up to -2 U₀ at the far end. Laboratory experiments were performed on a 200 m long 12 kV XLPE cable with a metallic needle inserted 2.6 mm into the 4 mm thick insulation at one end of the cable. The cable has no outer metallic screen and was wound on a cable drum. Aluminium foil was used as screen between the layers. The far end of the cable was either an open end or terminated with the wave impedance of the cable. The rate of voltage reduction was measured using an RC filter with a voltage divider. The time variation of voltage along the cable after grounding was calculated using an EMTP model. Tests were performed at different DC voltage levels in the range of 55-85 kV. In the case of 70 kV DC pre-stress for 24 hours, 260 groundings were needed for breakdown with a needle placed close to the short circuiting site. With a needle at the far end 500 abrupt groundings were needed. Even though the far end was exposed to oscillations of up to twice the applied voltage due to reflections, approximately twice the number of groundings was needed before electric breakdown occurred. Close to the grounding, the voltage slope was measured to 260 kV/mus. At the far end, the voltage slope was 107 kV/mus. Thus a rapid voltage variation appears to be more critical for breakdown initiation than the amplitude of the transients.