Early detection of faults in underground distribution cable joints by partial discharge measurements

Underground power distribution cable joints, which showed detectable levels of partial discharges (PD) in service, were removed and submitted to an accelerated ageing test in a laboratory. Prior to, and after ageing, the joints were examined by X-rays to check for any cavity or air gap that could be the site of PD generation. During ageing, the joints were simultaneously monitored with an infrared camera to locate any hot spot, and with a portable partial discharge detector, to locate precisely the areas of PD generation. The ageing test comprises overstressing at 3 p.u. and thermal cycling at 130°C and lasts until joint failure. The aim is to attempt to define a critical PD threshold (as measured by the portable instrument) at which failure is imminent