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.
School of Chemical Science and Engineering, Fibre and Polymer Technology, KTH Royal Institute of Technology , SE-100 44 Stockholm, Sweden.
Love K H
LK
Pallon
School of Chemical Science and Engineering, Fibre and Polymer Technology, KTH Royal Institute of Technology , SE-100 44 Stockholm, Sweden.
Fritjof
F
Nilsson
School of Chemical Science and Engineering, Fibre and Polymer Technology, KTH Royal Institute of Technology , SE-100 44 Stockholm, Sweden.
Shun
0000-0002-9663-7705
S
Yu
School of Chemical Science and Engineering, Fibre and Polymer Technology, KTH Royal Institute of Technology , SE-100 44 Stockholm, Sweden.
Dongming
D
Liu
Paul Scherrer Institute , 5232 Villigen PSI, Switzerland.
Ana
A
Diaz
Paul Scherrer Institute , 5232 Villigen PSI, Switzerland.
Mirko
M
Holler
Department of Materials and Manufacturing Technology, High Voltage Engineering, Chalmers University of Technology , SE-412 96 Göteborg, Sweden.
Xiangrong R
XR
Chen
Department of Materials and Manufacturing Technology, High Voltage Engineering, Chalmers University of Technology , SE-412 96 Göteborg, Sweden.
Stanislaw
S
Gubanski
School of Chemical Science and Engineering, Fibre and Polymer Technology, KTH Royal Institute of Technology , SE-100 44 Stockholm, Sweden.
Mikael S
MS
Hedenqvist
School of Chemical Science and Engineering, Fibre and Polymer Technology, KTH Royal Institute of Technology , SE-100 44 Stockholm, Sweden.
Richard T
RT
Olsson
School of Chemical Science and Engineering, Fibre and Polymer Technology, KTH Royal Institute of Technology , SE-100 44 Stockholm, Sweden.
Ulf W
0000-0001-5133-4877
UW
Gedde
DC-tree; Electrical tree; HVDC; polyethylene; ptychography