  Direct current (DC) gas insulated transmission lines (GILs) have been widely
used in power transmission, but might be threatened by partial discharge due to
the presence of floating impurities (e.g., dust and metal particles) inside the
sealed chamber. In this letter, by using a 2D fluid model we characterize the
microscopic properties of the partial discharge induced by a floating linear
metal particle in SF6 (both the discharge propagation and interaction between
space charge and metal particle) under negative high voltage direct current
(HVDC) conditions. Due to the strong electronegativity of SF6, the
spatiotemporal distributions of the charged species (electrons, positive and
negative ions), space charge, and reduced electric field are rather different
from those in air. Notably, a negative ion region is observed around the top
tip of the metal particle, and it plays an important role in the generation and
propagation of primary and secondary streamers in SF6, which may lead to severe
motion characteristics of the particle and aliasing of partial discharge
signals. Additionally, we analyze the charging process and electric force
reversal phenomenon, which may provide a more precise understanding of the
underlying mechanisms of the firefly motion previously reported for DC GILs.
