Time and space transition characteristics of DC electric field stress in oil‐pressboard composite insulation systems

In AC/DC converter transformers under DC application, the electric field distributions are distorted due to the accumulated charge on pressboard. However, the time‐dependent characteristics of DC electric field stress in oil has not yet been clarified in detail. In this paper, we investigated the time and electric field space transition characteristics of DC electric field distributions in oil‐pressboard composite insulation systems. As a result, we quantitatively clarified that DC electric field stress strongly depends on the elapsed time and the location in the electric field space in oil. In particular, it was found that the most severe electric field stress in oil may emerge during the time transition process.