In order to prove the effectiveness of direct fluorination in improving dc flashover performance of epoxy insulators in SF6 gas and also to provide evidence for the importance of surface conductivity of solid insulators, sheet-shaped and truncated cone epoxy samples were prepared and were fluorinated in a laboratory stainless vessel using a F2/N2 mixture with 12.5% F2 by volume at 0.1 MPa and 85 °C for 30 min. Physicochemical characteristics of the fluorinated surface layer were evaluated by ATR-FTIR and SEM techniques, and the results showed substantial chemical modification of the surface layer, which has a thickness of 0.89 μm and a roughened surface. Further, as expected on the basis of previous studies, measurements of surface electrical properties of the surface fluorinated sample, compared to the unfluorinated one, revealed a four orders of magnitude higher surface conductivity and a much more rapid decay of surface potential after corona charging. Dc flashover tests were performed on the truncated cone samples in SF6 gas at 0.1 MPa with a stepwise increasing voltage before and after the fluorination. The flashover test results showed a definite improvement in dc flashover voltage. For example, the flashover voltage at 63.2% probability or the mean flashover voltage for 2 min duration of the voltage step increased by 13.8% or 13.6% after the fluorination. The performance improvement is mainly attributed to easy leakage and dispersion of the charge deposited on the surface fluorinated sample from the gas phase, due to high conductivity of the fluorinated layer. The flashover test results also showed the influences of the duration of the voltage step on the flashover voltage and on the increase rate of flashover voltage. This means that even the time constant of the fast gas phase charging should be larger than 2 min, and that there should be different influences of the inhomogeneous surface conduction between the virgin sample and the fluorinated sample.

Improved DC flashover performance of epoxy insulators in SF6 gas by direct fluorination

In this paper, three methods, including dielectric barrier discharge (DBD) etching and deposition, and direct fluorination, were used for the surface modification of Al2O3-filled epoxy resin (Al2O3-ER) insulators to improve surface charge dissipation properties. The surface charge dissipation properties of the Al2O3-ER samples after these three treatments were compared. Firstly, the experimental results showed that DBD deposition reduced the most accumulated surface charges among the three treatments. The surface charge decay rates after DBD deposition and direct fluorination exceeded 98%. Then, the surface morphology, chemical components, and electrical parameters before and after the treatments were analyzed. Among the above three treatments, the surface roughness of the DBD deposited sample was the smallest, which facilitated the reduction of surface charge accumulation. The increase of the surface conductivity of the DBD deposited sample accelerated surface charge dissipation. Furthermore, the ageing effects of these three treatments were investigated. After five days of storage, it showed no significant difference between the dissipation properties of the DBD etched and the untreated samples due to the reorientation of polar groups towards the interface. However, there was only subtle changes in the surface charge decay rates of those DBD deposited and fluorinated samples, indicating that no obvious ageing effects were observed after DBD deposition and direct fluorination. The introduction of SiOx and C-Fn functional groups improved the anti-ageing properties of the DBD deposited and fluorinated samples, respectively. These comparative results can stimulate the development of eco-friendly plasma deposition techniques for the surface modification of Al2O3-ER insulators.

Atmospheric pressure plasmas and direct fluorination treatment of Al2O3-filled epoxy resin: A comparison of surface charge dissipation

Superhydrophobic and high-flashover-strength coating for HVDC insulating system

Space charges tend to accumulate on the surface and at the interface of ethylene–propylene–diene terpolymer (EPDM), serving as high voltage direct current (HVDC) cable accessory insulation, which likely induces electrical field distortion and dielectric breakdown. Direct fluorination is an effective method to modify the surface characteristics of the EPDM without altering the bulk properties too much. In this paper, the surface morphology, hydrophobic properties, relative permittivity, and DC conductivity of the EPDM before and after fluorination treatment were tested. Furthermore, the surface and interface charge behaviors in the HVDC cable accessory were investigated by the pulsed electroacoustic (PEA) method, and explained from the point of view of trap distribution. The results show that fluorination helps the EPDM polymer obtain lower surface energy and relative permittivity, which is beneficial to the interface match in composite insulation systems. The lowest degree of space charge accumulation occurs in EPDM with 30 min of fluorination. After analyzing the results of the 3D potentials and the density of states (DOS) behaviors in EPDM before and after fluorination, it can be found that fluorination treatment introduces shallower electron traps, and the special electrostatic potential after fluorination can significantly suppress the space charge accumulation at the interface in the HVDC cable accessory.

https://www.mdpi.com/2073-4360/10/5/500/pdf

Surface Layer Fluorination-Modulated Space Charge Behaviors in HVDC Cable Accessory.

In the present literature survey, we focused on the performance of polymeric materials encompassing silicone rubber (SiR), ethylene propylene diene monomer (EPDM) and epoxy resins loaded with micro, nano, and micro/nano hybrid fillers. These insulators are termed as composite insulators. The scope of the added fillers/additives was limited to the synthetic inorganic family. Special attention was directed to understanding the effect of fillers on the improvement of the thermal conductivity, dielectric strength, mechanical strength, corona discharge resistance, and tracking and erosion resistance performance of polymeric materials for use as high-voltage transmission line insulators. The survey showed that synthetic inorganic fillers, which include silica (SiO2) and hexagonal boron nitride (h-BN), are potential fillers to improve insulation performance of high-voltage insulators. Furthermore, nano and micro/nano filled composites performed better due to the better interaction between the filler and polymer matrix as compared to their only micro- or nano filled counterparts. Finally, some aspects requiring future work to further exploit fillers are identified and discussed.

/pdf/review-of-the-performance-of-high-voltage-composite-ctindmn1.pdf

Review of the Performance of High-Voltage Composite Insulators

The photoinduced evolutions of surface physicochemical characteristics and electrical properties of the fluorinated epoxy resin have been investigated. Cured epoxy resin sheets were surface fluorinated in a laboratory vessel using a F2/N2 mixture with 12.5% F2 by volume at 0.1 MPa and 95 °C for 30 min. The fluorinated epoxy sample together with the unfluorinated (original) one for a comparison were exposed to ultraviolet (UV) radiation with wavelengths of 320 to 390 nm. During UV exposure, the evolution of surface physicochemical characteristics was investigated by attenuated total reflectance Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray energy dispersive spectroscopy, and the sessile drop technique. The corresponding evolution of surface electrical properties was evaluated by measurements of surface potential decay and surface conductivity. These results have shown that the crosslinking reaction rather than photodegradation occurred for the fluorinated sample, compared to a continuous degradation of the original sample during the UV exposure. These results also indicate that surface conduction and its sensitivity to humidity increased very significantly with UV exposure time for the original sample, while the opposite changes in surface conduction and its moisture sensitivity with the exposure time were found for the fluorinated sample. A relationship between the evolutions of surface electrical properties and surface physicochemical characteristics has been established.

Evolutions of surface characteristics and electrical properties of the fluorinated epoxy resin during ultraviolet irradiation

Commercialization of cyclic butylene terephthalate oligomers has opened the door to bulky poly(butylene terephthalate) (PBT) matrix composite parts. In view of the potential applications of PBT in high voltage insulation or other sectors, where surface charge can play an unwanted role, PBT sheets were surface fluorinated in a laboratory vessel using a F2/N2 mixture with 12.5% F2 by volume at the different temperatures of 25, 55, and 85 ° C for the same time of 30 min, to enhance surface electrical conduction and to investigate the effect of fluorination temperature. Conductivity measurements reveal that the fluorinated samples have a much higher surface conductivity than the unfluorinated sample, and surface conductivity significantly increases with fluorination temperature. In accordance with the conductivity measurements, surface potential measurements after corona charging show that the fluorinated samples have a lower or much lower initial surface potential or even no surface potential, strongly depending on fluorination temperature and ambient humidity, compared with a high surface potential of the unfluorinated sample. ATR-IR analyses indicate that fluorination led to substantial changes in surface composition and structure, depending on fluorination temperature. SEM cross section and surface images show an increase of the fluorinated layer thickness with fluorination temperature and a modification of the surface morphology. Contact angle measurements of water show that fluorination also led to a significant increase in surface hydrophilicity, especially at higher fluorination temperatures. The intrinsic increase in surface electrical conduction is considered to be a result of competition between surface compositional changes and surface structural changes or the chemical traps and the physical traps.

Danli Xie

Papers

Improved DC flashover performance of epoxy insulators in SF6 gas by direct fluorination

Evolutions of surface characteristics and electrical properties of the fluorinated epoxy resin during ultraviolet irradiation

Significantly enhanced surface electrical conduction of poly(butylene terephthalate) insulators by direct fluorination