Aging Characteristics and Repairing Evaluation of Long-term Operating LSR Bushings

In southern power grid, four 500kV composite wall bushings used at a converter station was found significantly aging after 15 years' operation, such as chalking, cracks and low hydrophobicity. The housing material of these wall bushings is liquid silicone rubber (LSR). In order to extend the service life, the out door insulation was repaired by RTV. As the internal failure, one of the four bushings was forced to be out of service. In this paper, the outer shed of the wall bushing was used to evaluate the aging characteristics of the LSR sheds after long term operation. The macroscopic aging characteristics of 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) and X-ray Photoelectron Spectroscopy (XPS). It was found that the aging of the LSR shed developed from outer surface into the inside, and the layer structure including the chalking layer and the inner layer was obviously observed. The hydrophobicity, hydrophobicity transfer, tensile strength and tear strength of the aging sheds had different decrease. 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. 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 research also 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.