Partial discharge in mass-impregnated, lapped PPLP cables

The work presented here tackles three key objectives: Firstly, a method of producing realistic models of lapped HVDC cables is devised and implemented. Secondly, the properties of poly propylene laminated paper (PPLP) as used in mass impregnated (MI), HVDC cables are investigated. Finally, partial discharge (PD) measurements are made under DC conditions in order to assess the suitability of this method for online condition monitoring of HVDC assets. In-house fabrication of model HVDC cables makes it possible to deliberately insert defects into the samples during manufacture. The PD behavior of mass impregnated samples is thereby compared with dry lapped ones and it is shown that dry samples (or dry regions within a cable) exhibit higher rates of PD but with decreased average PD charge. In addition, DC PD is measured for specimens that are subjected to bending. In this study no clear change in DC PD is observed due to bending (even up to the point of failure due to breakdown). Dissection of a sample that has broken down reveals the extensive damage caused by the bending. It is concluded that DC PD tests are relatively insensitive to cable bending defects and that fluid impregnant may further mask these. DC PD is, however, quite sensitive to incomplete impregnation and may be well suited to continuous monitoring where large data sets are available.