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.

This paper introduces an HVDC SF6/N2 gas insulated transmission line (GIL) prototype that operates under 100 kV DC application. In the first part, the development history of GIL products over the past 50 years is reviewed. After that, the design concept of the 100 kV HVDC SF6/N2 GIL is introduced. A GIL prototype was prepared and the DC withstanding voltage test and DC superimposed lightning impulse voltage test were performed. The results show that this GIL prototype passed all the required tests, including hydrostatic test, gas tightness test, thermal cycling test, DC withstanding test, and DC superimposed impulse test, etc. The successful preparation of this product promoted a major step in researches of DC GIL and the prototype laid a solid foundation for the development of subsequent industrial products.

With the rise of renewable energy resources, especially offshore wind power plant, the DC transmission concept has attracted more and more attention. The selection of eco‐friendly insulating gas, as most critical insulating part of gas‐insulated equipment is top priority for developing highly reliable system. Moreover, it is acknowledged that eco‐friendly C ₃ F ₇ CN/CO ₂ gas mixtures have become a potential alternative to SF ₆ gas due to its excellent performance. This study reviews the basic physical properties of C ₃ F ₇ CN gas/gas mixtures and insulation properties including the gas gap breakdown and surface flashover performance at DC, as well as lightning impulse voltage. Investigation about the gas stability is presented, including the compatibility of gas with solid materials in gas‐insulated transmission line (GIL), and the decomposition characteristics under long‐term electrical and thermal stresses. The important progress on the charge accumulation characteristics of gas–solid interface in the novel gas environment under DC voltage stress is analysed. Finally, the key issues that need to be paid attention to and further followed through regarding the application of this novel gas in GIL are summarised and put forward. This study hopefully can provide a complete reference for the development of eco‐friendly DC gas insulation equipment with C ₃ F ₇ CN/CO ₂ gas mixtures.

Charging of insulators modifies local electric field distribution and increases potential threat to the safety of the gas insulated equipment. In this paper, surface charge tailoring techniques are classified and reviewed by introducing a Dam-flood model. Technical solutions of different charge tailoring methods are compared and discussed. The outlook of potential solutions to suppress charge accumulation is recommended and discussed based on industrial consideration. This paper serves as a guide handbook for engineers and researchers into the study of charge tailoring methods. Meanwhile, we hope that the content of this paper could shed some lights upon charge-free insulators to promote the industrial application of HVDC GIL/GIS.

The hetero-polar charges on the spacer surface due to the gas ionization has been verified as one of the hazard charge sources threatening the safe operation of HVDC gas-insulated transmission line (GIL). This letter reports the differences in surface charging properties of spacers in SF ₆ and C ₃ F ₇ CN/CO ₂ gas mixtures, and a rigorous re-examination of environmental firendly gas assessment for DC GIL is introduced. The results show that when the maximum value of electrical field strength at the spacer surface is in 0.45 kV mm ⁻¹ , homo-polar surface potential in different insulating gases indicates a dominant charge originating from bulk injection. As the maximum value of electrical field strength at the spacer surface reaches 0.68 kV mm ⁻¹ , local hetero-polar surface potential areas are detected and the area ratio of these regions is higher in CO ₂ gas, followed by 4% C ₃ F ₇ CN/96% CO ₂ gas mixtures. While in the 10% C ₃ F ₇ CN/90% CO ₂ gas mixtures, homo-polar potential regions are still dominant with very low area ratio of hetero-polar potential regions, which is close to that measured in SF ₆ . An indicating factor is put forward to further clarify the key role of hetero-polar charges plays in potentially triggering surface flashover. This letter provides an important basis for determining contents and components of insulating gases, which serves as one of the key issues in developing HVDC GIL.

Interface engineering, by modulating defect distribution and impedance at interfaces and inducing interfacial polarization, has proven to be an effective strategy for optimizing dielectric properties. However, the inherent incompatibility between heterogeneous phases presents a significant challenge in constructing multi-heterointerfaces and understanding how their distribution influences dielectric performance. Herein, we constructed an EGaIn@Ni/NiO/Ga₂O₃ composite structure by employing a low-intensity ultrasound-assisted galvanic replacement reaction followed by high-temperature annealing. The controlled addition of Ni salts allowed for the fine-tuning of Ni, NiO, and In concentrations and their spatial distribution within the interfacial architecture. Annealing treatment induced a transition from amorphous to crystalline phases, triggering dual relaxation behaviors between EGaIn/Ni and NiO/Ga₂O₃. Additionally, significant charge accumulation was observed at the NiO/Ga₂O₃ interface, likely due to the substantial work function difference between Ni and NiO, coupled with the low barrier height between EGaIn and Ni, which facilitates electron migration. Consequently, the optimized samples exhibited a maximum absorption bandwidth of 7.92 GHz, which is the highest among the EGaIn-based absorbers reported in the literature. This work not only elucidates the mechanism by which multi-heterogeneous interfacial distributions regulate the dielectric properties but also provides an effective approach for modulating the electromagnetic wave performance of liquid metals.