Stress conditions in HVDC equipment and routes to in service failure

High voltage equipment designed for DC applications is stressed differently from ac equipment. Most notably, the load conditions of the equipment play a major role in determining the electric field distribution in insulation under dc, due to the temperature dependence of conductivity. Additionally, polarization and space charge phenomena impose a time dependent behavior on the stress distribution. This has major consequences for the way in which degradation of the insulation under dc stress takes place. The nature of the stresses under dc may lead to accelerated degradation processes which could result in early breakdown of the insulation. This paper provides an overview of the stress conditions specific for dc application and highlights via which routes they may lead to failure. The stress distributions in converter transformers and dc cable systems will serve as typical examples.