Three-Dimensional Nanometer Features of Direct Current Electrical Trees in Low-Density Polyethylene

Electrical trees
are one reason for the breakdown of insulating materials in electrical
power systems. An understanding of the growth of electrical trees
plays a crucial role in the development of reliable high voltage direct
current (HVDC) power grid systems with transmission voltages up to
1 MV. A section that contained an electrical tree in low-density polyethylene
(LDPE) has been visualized in three dimensions (3D) with a resolution
of 92 nm by X-ray ptychographic tomography. The 3D imaging revealed
prechannel-formations with a lower density with the width of a couple
of hundred nanometers formed around the main branch of the electrical
tree. The prechannel structures were partially connected with the
main tree via paths through material with a lower density, proving
that the tree had grown in a step-by-step manner via the prestep structures
formed in front of the main channels. All the prechannel structures
had a size well below the limit of the Paschen law and were thus not
formed by partial discharges. Instead, it is suggested that the prechannel
structures were formed by electro-mechanical stress and impact ionization,
where the former was confirmed by simulations to be a potential explanation
with electro-mechanical stress tensors being almost of the same order
of magnitude as the short-term modulus of low-density polyethylene.