This paper constructs the aging model of SF gas-solid composite insulated transformer (GIT). The nondestructive evaluation of insulation state is realized by combining the analysis of frequency domain dielectric spectrum(FDS) and physicochemical characteristics.
Experimental/Modeling methods
PET film which is the winding insulation material in GIT was aged for 96h, 192h, 288h and 384h respectively at 150℃ in SF6 atmosphere. And then we measured FDS, DSC and NIRS characteristics of PET materials under different aging time respectively.
Results/discussion
Through the detection of PET materials with different aging degrees, it was found that the crystallinity of PET film rose in the early aging stage, and decreased with the aging process. At the same time, through FDS detection, it was found that the dielectric relaxation of PET material also increased first and then decreased with aging, as shown in Fig.1. It is speculated that the change of dielectric properties is related to crystallinity. In the early aging stage, the crystallinity increased, leading to the reduction of interface in PET film and weakening of dipole steering ability. Thus, dielectric loss of the material reduced. In the later stage of aging, molecular chains broke up to form smaller groups of molecules, and the polarization of the film interface increased, leading to the increase of loss. In contrast, with the aging process, the near-infrared spectra (NIRS) characteristics of PET films tend to increase. This is mainly caused by the change of material absorbance, as shown in Fig.2.
Conclusions
As a nondestructive testing method, the polarization loss peak of FDS can be used to characterize PET materials. At the same time, assisted by NIRS technology, it can more accurately realize the local identification of GIT.
Appendix (Figure, table, image…)
                     
Fig.1 The dielectric spectrum of 0.5mm             Fig.2 NIR spectra of 0.5mm PET
PET changes with aging time at 60℃                    at different aging times
 

GIT,PET,FDS,crystallinty,NIRS