The onset voltage of negative corona discharge on high-voltage conductors is strongly influenced by surface irregularities and electric field non-uniformity. Previous studies have investigated corona inception on stranded conductors and the effect of conducting protrusions on electric field enhancement; however, their combined influence has not been adequately addressed. In this paper, a three-dimensional numerical model is developed to investigate the onset of negative corona on a stranded conductor with a conducting protrusion in the presence of a grounded plane. The electric field distribution is computed using an extended charge simulation technique, in which the stranded conductor is represented by helical line charges, while the protrusion is modeled using ring and point charges. Image charges are implemented to simulate the grounded plane. To ensure high computational accuracy, the boundary conditions were evaluated, demonstrating that for all investigated protrusion geometries, the maximum percentage potential error remained below 1.06 %. The proposed algorithm is tested against previous experimental work, and excellent agreement is reached. The results show that a conducting protrusion dramatically increases the local electric field compared to a stranded conductor without protrusions, which in turn decreases the corona onset voltage. The proposed method is implemented to study the effect of different dimensions of protrusions on the surface electric field and the corona onset voltage, aiding the performance analysis of irregular practical stranded conductors.
