1. Introduction

In the field of power transmission and distribution, sulfur hexafluoride (SF₆) has long been the dominant insulating gas in high - voltage electrical equipment, favored for its non - toxic, non - flammable nature and excellent dielectric and arc - quenching properties. However, SF₆ is also a potent greenhouse gas with an extremely high global warming potential (GWP), about 23,500 times that of CO₂, and a long atmospheric lifespan exceeding 3200 years. As global concerns about climate change intensify, the search for environmentally friendly alternatives to SF₆ has become an urgent priority. Among the various candidates, perfluoroisobutyronitrile (C₄F₇N) has emerged as a highly promising option.

C₄F₇N exhibits remarkable dielectric strength. Studies have shown that its dielectric strength can reach more than two times that of SF₆ under certain conditions. This high dielectric strength allows electrical equipment to maintain reliable insulation performance, ensuring the safe operation of power systems. For instance, in gas - insulated switchgear (GIS) and gas - insulated transmission lines (GIL), the ability of C₄F₇N to withstand high electrical stress without breakdown is crucial for preventing electrical failures.

One of the most significant advantages of C₄F₇N is its relatively low GWP, which is only 2210. Compared to SF₆, this represents a substantial reduction in the potential to contribute to global warming. Additionally, C₄F₇N has zero ozone - depletion potential (ODP), making it a much more environmentally sustainable choice. This environmental friendliness aligns with the global trend towards reducing greenhouse gas emissions and promoting a cleaner energy future.

The liquefaction temperature of C₄F₇N is - 4.7 °C at 0.1 MPa. While this means it cannot be used alone in many practical applications due to the risk of liquefaction under normal operating temperatures, it can be mixed with buffer gases such as N₂, CO₂, or dry air. These mixtures can be tailored to have appropriate liquefaction temperatures for different operating environments, expanding the scope of its application in electrical equipment.

Many prototype GIS units using C₄F₇N - based gas mixtures have been developed and tested. For example, some 245 kV and 145 kV GIS prototypes have been constructed. These prototypes have undergone a series of tests, including those specified by IEC 62271 - 203 standard, such as 温升 tests (temperature rise tests), insulation tests, and tests related to breaker performance like terminal fault (TF), short - line fault (SLF) at different levels (e.g., L75 and L90), and capacitor switching tests. The successful completion of these tests has demonstrated the potential of C₄F₇N - based mixtures as both insulating and arc - quenching media in GIS. General Electric has also explored the application of C₄F₇N mixtures in high - voltage current transformers (CT), introducing a 245 kV gas - insulated CT.

In the context of GIL, C₄F₇N - based gas mixtures have also shown promise. Some 252 - 800 kV GIL prototypes have been studied. The high - voltage and long - distance power transmission requirements of GIL demand excellent insulation and long - term stability. The properties of C₄F₇N, such as its high dielectric strength and relatively good chemical stability, make it a candidate for use in GIL, where it can help ensure reliable power transmission over long distances with reduced environmental impact.

Research has been carried out on the application feasibility of C₄F₇N/CO₂ gas mixtures in GIT. Aspects such as insulation, thermal conductivity, and material compatibility have been systematically investigated. It has been found that a 15% C₄F₇N/85% CO₂ gas mixture shows superior dielectric strength and self - recovery performance. However, the thermal conductivity of C₄F₇N/CO₂ is lower than that of SF₆, resulting in an average temperature rise of 63.7 - 86.9 K under 100% load. Despite this, the compatibility of the gas with most typical materials used in GIT indicates its potential for use in this type of equipment.

In August 2023, the first set of C₄F₇N - based environmentally friendly insulation gas ring - main units in Hubei Province, China, was successfully put into operation in the State Grid Xiangyang Power Supply Company. These 12 kV ring - main units were jointly developed by the Xiangyang Hubei University of Technology Industrial Research Institute, Hubei Chuyun Electric Co., Ltd., and the State Grid Hubei Electric Power Research Institute. Compared to traditional RMUs using SF₆ as the insulating medium, the new units using C₄F₇N reduce CO₂ equivalent emissions by over 99%, demonstrating significant environmental benefits. They also show better insulation and arc - extinguishing performance compared to units using dry air or N₂, while having a smaller footprint.

Although studies have shown that C₄F₇N - based gas mixtures are generally compatible with common metals such as copper, aluminum, silver, and zinc at 120 °C, there are still some concerns. For example, at higher temperatures or in the presence of certain impurities, reactions may occur. The interaction between C₄F₇N and metal surfaces can affect the long - term reliability of electrical equipment. For instance, some research has suggested that in the case of copper, under specific conditions in a C₄F₇N - N₂ mixture, surface corrosion may occur at elevated temperatures, which could potentially compromise the electrical conductivity and mechanical integrity of the equipment.

Among non - metal materials, issues exist with components like sealing rubbers and some adsorbents. Sealing rubbers may experience swelling, hardening, or degradation when in contact with C₄F₇N, which can lead to gas leakage and reduced equipment performance. Regarding adsorbents, γ - Al₂O₃ has a strong adsorption effect on C₄F₇N, making it unsuitable for use in C₄F₇N - based gas mixture equipment. Although 5A molecular sieves show more promising adsorption performance compared to 3A and 4A molecular sieves, further research is needed to optimize their performance and long - term stability in the context of C₄F₇N applications.

When C₄F₇N decomposes under electrical stress or during arc - quenching processes in electrical equipment, various by - products are generated. Some of these by - products, such as CF₃CN, C₂F₅CN, C₂N₂, CO, C₃F₆, COF₂, and HF, which contain CN groups, are highly toxic and corrosive. These substances pose potential threats to the health of maintenance personnel and the safety of equipment. Although the amount of by - products generated is generally small, and some have good insulation strength, the overall impact on safety needs to be carefully evaluated. In addition, due to the lack of standard gases for some decomposition products, it is currently difficult to accurately study the biological safety of C₄F₇N - based gas mixtures under operating or fault conditions, and establish a clear relationship between discharge parameters (such as partial discharge intensity, spark discharge frequency, and number of interruptions) and the acute inhalation toxicity of the gas.

The production and handling of C₄F₇N are currently more expensive compared to SF₆. The complex manufacturing process of C₄F₇N, as well as the need for special handling and storage due to its properties, contribute to the high cost. Additionally, since C₄F₇N often needs to be used in mixtures with buffer gases, the cost of formulating and ensuring the quality of these mixtures also adds to the overall expense. This high cost is a significant barrier to the widespread adoption of C₄F₇N - based technologies in the power industry, especially for large - scale projects where cost - effectiveness is a crucial consideration.

Despite the challenges, the future of C₄F₇N as an environmentally friendly insulating gas looks promising. With continuous research and development efforts, it is expected that solutions to the current problems will be found. For example, research into new materials that are more compatible with C₄F₇N is ongoing, which could lead to the development of more reliable and long - lasting electrical equipment. In terms of safety, further studies on the decomposition products of C₄F₇N will help in formulating more effective safety protocols and developing adsorbents or filters to remove toxic by - products. As the production scale of C₄F₇N increases, economies of scale are likely to reduce its cost, making it more competitive in the market. Overall, C₄F₇N has the potential to play a significant role in the transition towards a more sustainable and environmentally friendly power transmission and distribution system in the coming years.