On-site diagnosis of power cables using damped AC voltages.

The importance of high reliability of distribution and transmision power cables increases in times of higher competition between utilities and due regulator requirements. Customers minutes lost has to be monitored and the asset management departments are requesting clear indication about the condition of MV and HV cables and their accessories. It is known, that the insulation failures in a cable network may be caused by lower dielectric strength due to aging processes and by internal defects in the components of the cable systems. A huge amount of failures in distribution cables is related to defects which show as typical symptom partial discharges. These defects are mainly located in the accessories. From several failure statistics it can be stated that more and more insulation failures in old paper insulated cables (PILC) reducing the reliability in distribution networks. Not only PILC cables are reducing the reliability in distribution networks, also newly installed polymer cables (XLPE) are adding to the reduction in reliability. These failures in polymer cables are mostly related to bad-workmanship, which is often a result of the competition between external contractors. A way of detecting bad-workmanship or any other weakspots in cables, joints or terminations is by means of Off-line PD-diagnosis. The cable system will be temporarily energized with an attenuated alternating voltage, so called Damped AC-voltage (DAC), during this temporarily energizing partial discharge data will be stored, which will provide enough information to assess the condition of the insulating system and support asset management decisions for necessary repair or replacement activities. PD diagnosis is a well established technology for cost effective condition based maintenance.