Breakdown Characteristics of Unequal Sphere–Sphere Electrode Configuration Under DC Stress

Failure of air gap insulation is one of the prominent issues in insulation coordination for outdoor applications. Though uniform electric field distribution is desirable, the difficulty in achieving it often makes insulation engineers settle for weakly non-uniform fields. One of the electrode systems known for its weakly non-uniform field is sphere gap, which is reliable due to its standardized breakdown characteristics. Though the breakdown characteristics of spheres with the same diameter are widely studied and standardized, spheres with unequal diameters have received minimal attention. In this paper, an attempt is made to study the breakdown characteristics of unequal spheres under DC stress in atmospheric air. The experimental breakdown studies were conducted for different spacings of spheres with unequal diameters of 100 mm, 50 mm, and 20 mm. The electric field variation for the experimental combination of sphere gaps and their corresponding utilization factors were computed using ANSYS 2024 R1. The results obtained were compared with the standard sphere gap. An unequal sphere gap has a non-uniform electric field distribution and a lower utilization factor compared to the standard sphere gap. It appears that the larger sphere experiences the maximum electric field, regardless of whether it is high-voltage or ground electrode. However, its breakdown characteristics are found to be comparable with standard sphere gap up to certain gap spacing under DC voltage.