[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

Surface charges have the effect of changing the electric field over spacers in compressed gases, which may affect the stable operation of gas insulated equipment and could possibly trigger a catastrophic surface flashover without warning. In this paper, we review the literature on surface charge accumulation and related surface flashover phenomenon on spacers in compressed gases. A summary of experimental research of surface charge accumulation is presented and, the surface charge accumulation mechanism is also summarized, with some newly identified factors in mind that may affect surface charge transport and measurements thereof. Subsequently, the correlation between surface charge accumulation and the surface flashover phenomenon is thoroughly discussed, and the effect of surface charges on surface breakdown under applied voltage pulses of various waveforms is analyzed. Finally, several important research topics in this area are proposed. In addition, some ideas on potential surface charge control methods are presented to aid the design of a better spacer free from surface charges in future studies.

Understanding surface charge accumulation and surface flashover on spacers in compressed gas insulation

The electric field distributions along gas-solid interfaces determine the reliability, lifetimes and sizes of gaseous insulated switchgears/pipelines (GIS/GIL), which also affect the reliability of power systems. In this study, the characteristics of steady and transient electric field distributions were first introduced, followed by the concept of functionally graded materials (FGMs). The development histories of FGM applied in electrical engineering and the related optimisation methods were described in detail. The field regulation effect and fabrication technology of different FGM insulators were also compared. To overcome the limitations of traditional FGM insulators, the design of surface FGM (SFGM) was proposed, together with their preparation methods and electrical performance. Furthermore, the future development prospects of FGM and SFGM insulators for compact GIS/GIL were summarised.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/hve.2019.0327

Promising functional graded materials for compact gaseous insulated switchgears/pipelines

Electrical field distortion induced flashover is a bottleneck for the stable operation of HVDC gas insulated system (GIS). Functional graded materials (FGM) have attempted to homogenize the electrical field distribution along the gas-solid interface; however, the complicated fabrication process and huge dielectric loss makes it difficult into industrial application. This paper proposed a novel concept as that a spacer with functional surface was fabricated by magnetron sputtering ZnO onto the epoxy resin forming a surface conductivity gradient, namely surface functional graded materials (SFGM). The feasibility was first demonstrated by simulating the electrical field distribution. Two kinds of disc-type spacers with SFGM were designed as linear and inverse proportional annular gradients. Results show that the more uniform electrical field distribution contributes to the improved surface flashover voltage of the SFGM spacer. Therefore, the SFGM epoxy spacer has great potential in HVDC GIS insulation.

Surface functional graded spacer for compact HVDC gaseous insulated system

Surface charge accumulation on the spacers is one of the key issues restraining the development of HVDC GIS/GIL. The precise measurement of surface charge properties provides the basis for further study of the surface charge transport mechanism as well as the charge-induced flashover mechanism under DC voltage. In this study, the authors discuss their perspective on the current status, development needs and potential developing orientation of surface charge characterisation techniques. Different surface potential measurement methods and charge inversion algorithms are reviewed regarding the previous studies and future research needs. Drawbacks and outlooks of surface charge measurement techniques are also discussed with the background of laboratory experiment results and on-site measurements. It is hopefully that this study can serve as a useful guide reference for researchers within the same research field. More importantly, it is authors’ hope that this study can inspire some novel ideas for readers into developing of more accurate and scientific interface charge characterisation techniques.

Gas–solid interface charge characterisation techniques for HVDC GIS/GIL insulators

GIS spacers made of epoxy resin always suffer from the flashover due to surface charge accumulation. Therefore, it is important to seek a new method to improve the performance of epoxy resin. The epoxy/SiC composites have been proven to have nonlinear conductivity and the potential of being used in the spacer manufacture because of their field grading function under DC voltage. A high temperature rise on the central conductor of GIS can cause a large temperature gradient along the spacers. Exploring the effects of temperature on the electrical properties of the epoxy/SiC composites is helpful in the design the space dependent SiC filler content in GIS spacers. In this paper, three temperatures 20, 50 and 70 °C were studed on the temperature dependent surface potential decay (SPD) and flashover characteristics of the epoxy/SiC composites. At 20 °C, the epoxy/SiC composites, especially the 14 vol % samples, have much better SPD performances and higher flashover voltages than the pure epoxy. As the temperature increases, the SPD performances of all samples are greatly improved. The SPD rate of the 0 vol % samples approaches that of the 6 vol % samples at 50 °C and the 14 vol % samples at 70 °C. When the temperature is over 50 °C, flashover of the epoxy/SiC composites occur at lower voltages compared with the pure epoxy resin. With a further increase of temperature from 50 to 70°C, the flashover voltages of all samples have a significant drop, indicating that the temperature effects on flashover fault must be included in the spacer design and manufacture.

Temperature dependent surface potential decay and flashover characteristics of epoxy/SiC composites

The flashover faults due to the accumulation of surface charge have restricted the further development of spacers in HVDC GIS/GIL transmission systems. Many researchers have proposed a variety of mathematical models to simulate the surface charge accumulation process; however, few of them have taken into consideration the gas collision ionization and charge trapping-detrapping processes. This paper combines plasma hydrodynamics and charge transport equations to build a modified model. For the basin-type spacer, the model shows that the surface charge has the same polarity as the applied voltage on the lower surface but the opposite polarity on the upper surface of the spacer. A decreased charge density and even a polarity reversal is seen near the conductor. For the disc-type spacer, the surface charge has the same polarity as the applied voltage near the shell but opposite polarity near the conductor under negative voltage. But, under positive voltage, negative charge exists almost on the whole surface. The most serious distortion of the electric field occurs at the triple junction of the epoxy spacer. Under load condition, joule heating of the conductor increases the temperature which has a great influence on the electric field distribution. As the conductor temperature increases, the field strength near the conductor is reduced owing to the temperature-dependent volume conductivity of the epoxy spacer. The application of shielding electrodes grades the electric field at the triple junction, and is referred to as the DC GIS/GIL design.

Electrical field distribution along SG 6 /N 2 filled DC-GIS/GIL epoxy spacer

Epoxy disc-type spacer, as the major insulator of gas insulated transmission line, plays a significant role in the reliability and safety of the entire power grid. While surface charge accumulation on the spacer could induce flashover and accelerate degradation of the insulator, which threats the operation of high-voltage DC transmission and grid. In this study, a corona discharging system was employed to charge the epoxy samples before the charge dissipation measured, and the surface flashover voltages of samples with different modification time under DC voltage were also measured, the results are obtained from the research. It is found that the carrier mobility and surface flashover voltage of samples are affected by modification time, and maximum value of both can be obtained when the sample is modified for 60 min. Under the combined voltages, the initial surface charge density and carrier mobility are affected by both pulse voltage and modification time. It is indicated that surface modification is an appropriate method which can significantly inhibit the surface charge accumulation, and improve the flashover characteristics of epoxy sample by increasing carrier mobility. The trap distribution characteristics suggested that the modification treatment and charging condition have a significant effect on the depth and density of trap.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/iet-gtd.2017.0984

Hucheng Liang

Papers

Promising functional graded materials for compact gaseous insulated switchgears/pipelines

Temperature dependent surface potential decay and flashover characteristics of epoxy/SiC composites

Surface functional graded spacer for compact HVDC gaseous insulated system

Electrical field distribution along SG 6 /N 2 filled DC-GIS/GIL epoxy spacer

Carrier mobility and trap distribution dependent flashover characteristics of epoxy resin