Extruded HVDC cables experience during a VSC ageing program with online monitoring techniques

Extruded cable insulation, as opposed to mass impregnated paper, is becoming the preferred material for HVDC cables especially for solutions that utilise Voltage Source Converters (VSC) [1]. Over 4000km of extruded cables are deployed to date for VSC point to point schemes. Ensuring that the cable's performance is maintained over the lifetime of service (40 years) is critical to successful schemes. A 360 day VSC ageing program as recommended by CIGRE TB 496 has been carried out successfully at the GE's Grid Solutions HVDC cable ageing facility [2]. Two full size 200 kV cables have been tested in parallel under identical electrical stress but different thermal stress. One of the cables is a land cable supplied by Nexans and the other cable is a submarine cable supplied by FURUKAWA Electric. The aim of the ageing program was to develop a better understanding of the ageing mechanisms of extruded cable insulation and space charge behaviour. Thus, the ageing program employed online monitoring techniques with high sensitivity to detect ageing of polymeric materials and monitoring of space charge evolution. The four online monitoring techniques were; DC conductivity measurements; time domain dielectric spectroscopy; the pulsed electroacoustic technique (PEA) [3] and the thermal step method (TSM) [4]. The first two techniques were intended to monitor the electrical conductivity and polarisation changes to the cable insulation whilst the latter two techniques were employed to probe the distribution of space charge within the cable insulation.