159 / 2018-08-25 18:19:36

Comparative Study of Aging and New-made LSR Materials Used for HVDC Converter Substation

LSR,aging,external layer,internal bulk,macroscopic,Microscopic mechanism

Liquid silicone rubber(LSR) is often used as external insulation material for important equipments in substations due to its excellent performance. However, a part of LSR sheds used for 500kV wall bushings and arresters appeared to age significantly after running for over 15 years. The aging phenomina include chalking, cracks and low hydrophobicity on the surface. In this paper, the macroscopic aging characteristics of the aging LSR sheds were investigated by using hydrophobicity and hydrophobicity transfer test, tensile and tear strength test. The microscopic characteristics of the aging sheds were obtained by using Scanning Electron Microsope (SEM), Fourier Transform Infrared Spectroscopy (FTIR), X-ray diffraction(XRD) and X-ray Photoelectron Spectroscopy (XPS).
Besides the aging LSR sheds, we also obtained the same kind LSR which is new-made, and by cutting off the internal layer of the aging sheds, this paper could be able to compare the macroscopic characteristics and microscopic structure of 3 kinds of specimens: (a)the external layers of LSR which appear severely aging; (b)the internal layers of LSR which nearly show no aging features; (c)the new-made LSR. Through contrastive study, this paper can get several conclusions as below:
(1) There is little difference between the internal layer of aging LSR sheds and the new-made LSR, indicating that the aging of the LSR shed developed from the outer surface to the inner bulk, and there is an obvious separation line between the chalking layer and the internal bulk, while the layer structure can be found in the chalking layer.
(2) In the view of macroscopic characteristics, the hydrophobicity, hydrophobicity transfer speed, tensile strength and tear strength of the aging sheds had different decrease in compare with the new-made ones. Furtherly, some specific macroscopic measurements are also useful for judgement of aging status, such as the color brightness, bending test, etc. The Shore hardness results show no significant difference between the aging and unaging specimens.
(3) At microcosmic angle of view, the aging sheds appeared irregular shaped patches with a reduced surface roughness and adhesion; FTIR and XPS results indicated that the side chain groups of PDMS in the chalking layer were oxidized, crosslinked with other polysiloxane chains, forming more Si-O-Si groups, accompanied by the precipitation of TiO2 and white carbon black fillers.
In a word, due to the change of silicone rubber material from organic to inorganic, the content of C and Si elements in the chalking layer decreased, and the content of O element increased. The paper also includes the try of repairing sging LSR sheds using special RTV coatings, and the result shows that, the hydrophobicity and hydrophobicity transfer of the aging LSR sheds can be improved by RTV repairment, but the appearance and hydrophobic changes still needs to be regularly monitored in the operation.