Electric field distributions to analyze possible manufacturing defects of a Preventive Autotransformer

In this contribution, a possible and interesting manufacturing defect related to a clamping bolt endangered by lightning surge is simulated. The electric field stress distributions in the oil region around a defective clamping bolt used in the limbs of a Preventive Autotransformer (PA) is computed using 2D Finite Element (FE)-based software. Two scenarios of bolt were analyzed in this paper: 1) long tip metallic bolt, 2) metallic bolt without tip. The maximum field stress was computed in the oil ducts of the PA. The simulated results were compared with the critical field stress calculated from an analytical formula. The long tip metallic bolt revealed a possible dielectric breakdown in the oil ducts. To reduce the risk of potential failures, thermoplastic bolts were used instead. A thermoplastic bolt was analyzed and the results compared with those of the defective metallic bolt. It was shown that, the appropriate selection of clamping bolt materials in transformers can help avoiding/reducing the impact of manufacturing errors on the serviceability of power equipment.