Yong Ma

Bio: Yong Ma is an academic researcher from Tongji University. The author has contributed to research in topics: Surface layer & Corona discharge. The author has an hindex of 2, co-authored 4 publications receiving 47 citations.

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

Abstract: 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.

High resistance of surface fluorinated epoxy insulators to surface discharge in SF 6 gas

Abstract: Our recent studies have indicated that direct fluorination can suppress surface charge accumulation of epoxy resin insulators and thus enhance the DC flashover performance. It was also confirmed that the fluorinated epoxy surface layer has a significantly improved resistance to corona discharge. Further, in this work, the degradation or tracking resistance to surface discharge and the AC flashover performance of the surface fluorinated epoxy insulator were investigated. A sheet sample/finger electrode arrangement was used in the AC flashover test in SF6 gas. The sample/electrode arrangement can keep the discharge along the same surface path for successive flashover tests, and thus can cause a lasting damage of the sample surface. Test results show that the fluorinated epoxy surface layer has a high degradation or tracking resistance, although direct fluorination only leads to a marginal increase in AC flashover voltage for the sample/electrode arrangement. The resistance improvement is mainly attributed to the formation of C-F bonds and the disappearance of conjugated double bonds in the surface layer by substitution and addition of fluorine atoms, as indicated by ATR-FTIR analysis. The marginal effect of the fluorination on AC flashover voltage is due to limited surface charge accumulation for the symmetrical sample/electrode arrangement during the short period between the partial discharge initiation and the occurrence of flashover.

Dc flashover performance and surface discharge resistance of the fluorinated epoxy insulator in nitrogen gas

Abstract: The surface of the epoxy insulator was modified by direct fluorination in a laboratory vessel using a F 2 /N 2 mixture with 12.5% F 2 by volume at 55 °C and 0.1 MPa for 30 min. The modified surface was examined by ATR-FTIR and SEM techniques. ATR-FTIR results show substantial changes in chemical composition and structure of the surface layer by the fluorination, and SEM observations reveal the fluorinated surface layer having a thickness of 0.76 μm and a roughed surface. Dc flashover performance and surface discharge resistance of the fluorinated surface layer in nitrogen gas were simultaneously evaluated using a simple sample/finger electrode arrangement in a stainless steel chamber at room temperature. The test results indicate a definite improvement in dc flashover voltage and a great increase in tracking resistance to the flashover discharge. The former is mainly attributed to easy leakage and dispersion of the charge from the cathode triple junction on the fluorinated surface due to high conductivity of the fluorinated layer, and the latter is due to the formation of C-F bonds and the disappearance of conjugated double bonds in the surface layer by the substitution and addition of fluorine atoms.

Investigation on resistance of the fluorinated epoxy surface layer to corona discharge in SF 6

Abstract: Surface fluorinated epoxy samples together with the unfluorinated (virgin) ones were exposed to the SF 6 corona discharge generated by a multi-needle-to-plate electrode system, to investigate the resistance of the fluorinated layer to the corona discharge. SEM observations and ATR-IR analyses show that thickness of the fluorinated layer was not reduced and no clear evidence for the cleavage of C-F bonds by the corona discharge, although it caused surface morphology change due to attack on some weak points in the fluorinated layer, chain scission, and cross-linking reaction. In contrast with this, the virgin surface layer was degraded by the corona exposure in the form of ablation. Measurements of surface potential and water contact angle indicate that the corona discharge reduced surface conduction of the fluorinated sample and rendered the surface conduction insensitive to humidity. On the contrary, the corona exposure led to an increase in surface conduction of the virgin sample, which is sensitive to humidity.

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

Abstract: 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.

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

Abstract: 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.

Oriented Boron Nitride Nanosheet Films for Thermal Management and Electrical Insulation in Electrical and Electronic Equipment

Abstract: Insulation failure and insufficient thermal management in electrical and electronic equipment threaten their safe and stable operation and restrict development toward high power and high integratio...