Off-Line PD Cable Mapping

Partial Discharges in the insulation are the main cause of ageing in high voltage paper insulated cables. The discharges are responsible for much of the deterioration seen in the insulation wall, such as pinholing and electrical treeing, and eventually lead to the breakdown of insulation.

Tianu provides Off-Line Cable Mapping which cable is isolated and removed from the service before performing PD mapping test. The technique uses a Very Low Frequency (VLF) power supply to energise long lengths of cable from a standard single phase mains outlet. The 40kV peak VLF power supply is van mounted and has a fully variable voltage and frequency output allowing testing of HV cables up to 33kV with 30% overvoltage.


Typical test criteria

  • Cable isolated and open circuit at remote end
  • Can be tested from switchgear spouts, cable box or pole termination in yard
  • PILC circuits up to 3km can be tested, usually to 20% overvoltage
  • Ideal for commissioning new XLPE circuits, 8km successfully mapped, 30% overvoltage
  • Test frequency of 0.1, 0.2, 0.5Hz normally used dependant upon cable length


Sample Maps


  • Discharge occurring from half way and confined to within 10-20% of Cable length, in this case 37-74m, damage most likely caused during installation or by a 3rd party contractor.
  • Note the low inception voltage indicating that discharge is normally always present, and the relatively high levels recorded for each phase both average and maximum values.
  • Finally, a failure was reported nearly 2 years after test as shown on this map demonstrating the unpredictable nature of Partial Discharge and its damaging influence on cable systems.

  • The most obvious feature here is the 200m section of discharge approx 100m out from End 1, particularly on Blue phase.
  • Note also the high number of joints in this location indicating continual failure / ineffective repairs.
  • This circuits reliability can be greatly improved with the replacement of this small damaged section of cable while removing the number of potentially problematic joints at the same time