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Polyimide film (PI) is widely used as dry-type smoothing reactor insulation for ultra-high voltage direct current (UHVDC) projects. In practice, the temperature of a central conductor can reach 80 °C to 100 °C at the rated power flow. Thus, the inner insulation composed of PI sustains these high temperatures. In this study, the space charge characteristics of PI at different temperatures (40 °C, 60 °C, and 80 °C) and thermal aging degrees (336, 500, 1000, and 1500 h) were examined with an improved pulsed electro-acoustic (PEA) measurement system to investigate the effects of temperature and thermal aging on dielectric performance. The apparent trap depths corresponding to various aging degrees were obtained according to the space charge decay dynamics at the interface. Results indicate that positive charges likely accumulate within the bulk of the material because of the thermal aging of homogeneous materials. The polarity of space charge at the interface also changes from positive to negative as aging degree increases. The space charge decay dynamics at the interface further shows that the trap energy increases as the thermal aging degree increases. The amount of residual space charge also increases remarkably as the electric strength and aging degree increase. The electric field distortion is closely related to temperature, thermal aging degree, and electric strength. [ABSTRACT FROM AUTHOR]

The polyethylene terephthalate (PET) is widely used in dry-type insulation application because of its excellent insulation properties. In practice, thermal aging of PET is often encountered, but the related mechanisms and effect of thermal aging on the PET dielectric properties still remain unclear. In the present paper, we presented a systematic and comparative study on the effect of thermal aging on surface microscopic morphological characteristics, DC breakdown strength, trap depths and the apparent mobility. The results indicate that when aged at 130?C, the surface topography becomes coarser than that aged at 80?C. The polarity reversal at the PET/PET interface during the polarization indicates that the injection abilities of Al and Sc electrodes are gradually enhanced when aged at 80?C and 130?C, respectively. Moreover, the measured isothermal discharge currents during depolarization show that the trap depth corresponding to the maximum trap density increases significantly in the beginning of aging, and then it decreases gradually when aged at 80?C, but it increases evidently when aged at 130?C with increasing the aging degree. It also has been found that the apparent trap-controlled charge mobility is greatly enhanced when aged at 80?C with the increase in thermal aging degree. This enhancement is proved to be associated with the decrease in trap energy level at the PET/PET interface. Also, it can be preliminary concluded that the enhancement of DC breakdown strength (at 1500 h) is closely related to the increase in electron type trap energy and its density when aged at 130?C. [ABSTRACT FROM PUBLISHER]

To understand the effect of thermal ageing mechanism on space charge formation and dissipation in double-layered polyester films (PET), the time dependence of space charge characteristics at physical interface and in the bulk of PET films have been investigated. The measurements of space charge characteristics in specimens of different thermal ageing conditions (80 and 130 °C) were conducted under different DC field strengths (polarization) and short circuit conditions (depolarization) using the Pulsed Electro Acoustic (PEA) measurement system. The influence of electric field distortion caused by the space charges on average AC breakdown strength of PET specimens was also investigated. Results indicate that the thermal ageing leads to polymer interface acts as positive charge traps at ageing temperature of 80 °C, but acts as electron traps at ageing temperature of 130 °C and provides deeper trapping level for positive charge carriers. Besides these, average breakdown strength of the composite decreases due to the electric field distortion caused by the space charges. A preliminary study of this phenomenon is given and further investigation is needed. [ABSTRACT FROM AUTHOR]

Estimation of the degree of aging and the remaining lifetime of composite insulators is an important issue. In this paper, one shed of a 500 kV AC composite insulator that has operated for three years on a transmission line was studied. The insulator shed was cut longitudinally into four layers from the upper surface to the bottom surface and tested by the thermally stimulated current (TSC) method, and each layer was also tested by Fourier transform infrared (FTIR) spectroscopy. The results show that the first layer has fewer trap charges than the other layers in the TSC test, but it also has the highest trap level, and the trap depth of the first layer is deeper than that of the other layers. The trap levels and the quantities of shallow traps decrease successively from the second to the fourth layer. In the FTIR spectra, the absorption peaks of the first layer are lower than those of the other layers, and the other three layers have similar absorption peaks. The aging state of the first layer is more serious than that of the other layers. The study indicates that long-term contamination and ultraviolet (UV) radiation are the main reason for the aging differences among the different insulator shed layers. [ABSTRACT FROM AUTHOR]