  In recent years, the buffer layer ablation failures of high voltage cables
are frequently reported by the power systems. Previous studies have dominantly
regarded the buffer layer as the continuous homogeneous medium, whereas
neglects its microstructures. In this paper, the current distribution within
the random fiber networks of buffer layer are investigated. Experiment results
of our self-designed platform revealed an uneven current distribution in buffer
layer at the moment of bearing current. This phenomenon is named as the
intrinsic current concentration where the current density concentrates at
certain sites inner the buffer layer. And the degree of current concentration
will be suppressed by compressing the sample. Then, a 2D simulation model of
the random fiber networks was constructed based on the Mikado model. The
simulation results also presented an uneven current distribution in the
networks whose every fiber can be viewed as a micro-resistor. Two types of
dimensionless current concentration factors were defined to describe the degree
of current concentration, finding their values decreasing with the rise of
fiber density. Meanwhile, it is equivalent of compressing the buffer layer and
increasing the fiber density of model. We believe that the intrinsic current
concentration phenomenon is mainly related with the inhomogeneity of geometry
structure of buffer layer. The ablation traces and fracture fibers observed by
the X-ray micro-computed tomography test supported this point. In addition, the
non-ideal surface of sample can also induce this phenomenon. The intrinsic
current concentration can aggravate the degree of originally existed
macroscopic current concentration in cables, thus causing the ablation failure.
Our work may unveil a deeper understanding on the cable ablation failure and
the electrical response of the similar fibrous materials.
