Supercritical Fluids as Alternative Insulation and Arc-Quenching Medium

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High Voltage, Arc-Quenching, Insulation, Supercritical-Insulated Switchgear (scIS), Circuit Breakers, Electrostatic Generators.

Abstract

This paper reviews the historical progression of arc-quenching media and examines the unique properties of supercritical carbon dioxide (scCO₂), including its transport characteristics, electrical breakdown resilience, and structural behavior. Through analysis of ionization mechanisms, mean free path, and heat dissipation, scCO₂ emerges as a viable insulating and arc-quenching medium, offering competitive performance and reduced environmental impact. Projected performance metrics for arcing time and dielectric strength show scCO₂’s competitive edge. The limitations of alternative supercritical fluids and the potential benefits of scCO₂ mixtures are discussed. In addition, the paper highlights the development of the first 72 kV scCO₂ AC circuit breaker, marking a significant step toward sustainable high-voltage applications. This work positions scCO₂ as a viable, environmentally friendly alternative to SF₆, with promising implications for future power systems.

1. Introduction

1.1. Motivation

The growing need for sustainable and environmentally friendly solutions in high-voltage electrical equipment has intensified the search for alternatives to traditional insulating gases. Sulfur hexafluoride (SF₆), widely used in circuit breakers for its exceptional dielectric properties, boasts a high global warming potential (GWP) of 23,500, an atmospheric lifetime of approximately 3200 years, and toxic post-arcing byproducts [1,2]. International efforts such as the Kyoto Protocol and the European Union’s environmental regulation mandate a total phase-out of SF₆ by 2030 [3], while G7 committed to the phase-out of the use of sulfur hexafluoride (SF₆) in new switchgear applications by 2035 [4].

Several potential substitutes have been explored, including perfluorinated nitriles (PFN) and ketones (PFK), which offer lower GWP than SF₆ while maintaining some acceptable dielectric characteristics [5]. However, these substitutes have failed to meet the rigorous demands of the industry. PFN and PFK suffer from several critical shortcomings, including susceptibility to chemical degradation under electrical stress, the formation of toxic byproducts, and condensation at typical operating conditions, leading to unreliable insulating performance [6,7]. Moreover, PFN and PFK often require the addition of carrier gases, such as carbon dioxide or nitrogen, to balance their dielectric and arc-quenching properties, especially at low operating temperatures, which introduces additional operational complexity and increases costs. As a result, the widespread adoption of these alternatives has been slow, and the energy sector continues to seek more effective and sustainable replacements.

In light of these challenges, supercritical fluids (SCFs), particularly supercritical carbon dioxide, have gained attention as a viable alternative [8,9,10]. Supercritical fluids are unique in that they exist at temperatures and pressures above their critical point, exhibiting properties of both liquids and gases. This combination of properties results in high densities, low viscosities, and tunable thermodynamic properties that make them ideal for a wide range of applications, including as the electrical insulating medium in electrical systems.

Among the SCFs, supercritical carbon dioxide has emerged as a particularly attractive alternative as it offers high dielectric strength and excellent arc-quenching capabilities, and is both non-toxic and abundant, making it cost-effective and environmentally benign compared to SF₆. Despite these advantages, the application of supercritical fluids in high-voltage circuit breakers remains a relatively unexplored area of research. While SCFs have been extensively studied in other fields, their potential for use in electrical power systems is still being evaluated. This paper aims to bridge that gap by examining supercritical fluids’ dielectric and arc-quenching behavior, focusing on scCO₂. By exploring its thermophysical properties under various operational conditions, we seek to understand how scCO₂ can replace electronegative gases like SF₆ in high-voltage circuit breakers. By leveraging their unique properties, SCFs have the potential to overcome the limitations of existing SF₆ alternatives, providing a sustainable pathway for future electrical grid technologies.

In the following sections, we aim to illustrate how scCO₂ can replace traditional insulating arc-quenching media. This paper focuses on scCO₂’s electrical and transport properties, unique structural characteristics, and electrical breakdown behavior. By looking more closely at ionization processes, this work seeks to gauge and gain further understanding of scCO₂ arc-quenching capabilities while identifying key knowledge gaps. It also explores the potential use of other supercritical fluids to predict and compare their performance against that of scCO₂. The paper concludes with a brief discussion of life expectancy and a case study of the development of a first-of-its-kind scCO₂ 245 kV circuit breaker.

1.2. Early Arc Quenching—A Brief History of Pre-SF₆ High-Voltage Circuit Breakers

Today, it seems obvious that an electronegative gas, a gas composed of molecules that have a high affinity for electrons, meaning they tend to capture free electrons from their surroundings (e.g., SF₆, CF₃I, C₄F₇N, C₅F₁₀O), is required to avoid arc restrike and arc reignition in high-voltage circuit breakers. However, early circuit breakers in the 19th and early 20th centuries were based on water, air, and oil, none of which exhibited strong electronegativity [11]. The environmental concerns with fluorinated arc-quenching media beg the question: Why could we not simply return to those early arc-quenching principles? The reasons are twofold: performance and byproducts.

In the early 1900s, electrical engineers explored different ways to quench arcs in high-voltage systems. J. N. Kelman, a pioneering engineer, designed a circuit breaker in 1901 that combined water and oil as an innovative solution for arc suppression [12]. Kelman’s design layered water and oil in a confined chamber, using water’s high thermal capacity to dissipate the arc’s energy. When an arc forms, water vaporizes, creating steam that helps cool and extinguish the arc, minimizing restriking [13]. However, water’s low dielectric strength restricts the operational voltage range [14]. Furthermore, water is susceptible to contamination and degradation over time. Dissolved metal from the contact system and the tank reduces the water’s resistivity by ionic conduction, compromising its insulating properties and increasing the likelihood of premature breakdown. This contamination necessitates regular water quality maintenance and monitoring, increasing operational complexity and degrading the system’s durability and reliability. Despite these limitations, Kelman’s breaker was initially effective. It was installed at the Colgate Powerhouse in 1902, where it successfully interrupted circuits until March 1903. After repeated operations in a short time frame, the breaker failed, releasing flaming oil and igniting the power house’s wooden structure.

Air circuit breakers were among the earliest types of circuit breakers. Arc quenching in air relies on elongating the arc and cooling it through convection and thermal dissipation. As the circuit breaker contacts open, an arc forms between them. Elongating the arc increases resistance, while cooling it reduces conductivity, thereby facilitating the arc’s extinction [15]. In air-blast circuit breakers, compressed air is used to force cool and extinguish the arc more effectively. Compressed air circuit breaks are generally less power-dense than their sulfur hexafluoride (SF₆)-based counterparts, compared to the same fault-clearing capability. While they might have appeared compact back when they were first introduced, their fault current ratings were nowhere close to what today’s breakers are capable of.

Bulk oil and minimum oil breakers appeared to be a much-improved solution. Oil circuit breakers quench arcs by immersing them in oil. The arc vaporizes the surrounding oil, forming a gas bubble mainly composed of hydrogen, which effectively cools the arc and increases dielectric strength [15]. Despite their excellent dielectric and quenching properties, the oils decompose and dissociate in the arc, creating hydrogen gas, which is beneficial to cool the arc but leaves behind carbonized solids. Besides the maintenance issue of replacing the oil and removing the solids, they also posed a fire hazard.

Given these challenges, SF₆ circuit breakers emerged as a comprehensive solution, becoming the industry standard for high-voltage applications. Developed in the 1950s, SF₆ outperformed compressed air, bulk oil, and minimum oil circuit breakers by offering superior arc quenching, faster dielectric recovery, and a compact design [16]. As a result, SF₆ breakers replaced older technologies in high-voltage systems.

Yet, the question remains: Are electronegative substances truly necessary for effective arc quenching? While their large electron attachment cross-section efficiently removes free electrons, suppressing electron avalanches, restrikes, and reignition, a dense enough medium with a sufficiently short mean free path can achieve the same effect. In such a medium, avalanches, restrikes, and reignition are inherently suppressed. Therefore, the ideal arc-quenching medium would have a short mean free path, high thermal stability, the ability to recombine, and minimal risk of increased electrical conductivity, whether ionic or electronic. Additionally, properties such as efficient heat transfer, low viscosity, compressibility, non-toxicity, non-corrosiveness, low cost, and wide availability are highly desirable. This is where supercritical carbon dioxide excels, offering a compelling alternative.

1.3. Review Methodology

It is important to note that the intention behind this paper is not to serve as a comprehensive literature review; rather, it is a position paper to encourage discussion and further investigation. This paper highlights key findings from selected studies to introduce a new perspective. As such, the literature cited has been selected for relevance and illustrative value rather than completeness.

Nevertheless, IEEE Xplore, ScienceDirect, Google Scholar, and ChatGPT-4o were utilized to conduct a systematic literature search, followed by a full-text assessment to screen for relevance. The following steps were followed to search, screen, and survey existing knowledge on keywords, which included “supercritical carbon dioxide dielectric,” “supercritical fluids arc quenching,” “SF₆ alternatives,” and “dielectric transport properties.”

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Step 1: Database Search. We used Google Scholar as the primary academic search engine, supplemented with IEEE Xplore, ScienceDirect, and NIST technical reports.

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Step 2: Screening. Articles were filtered based on relevance.

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Step 3: Thematic Clustering. Relevant works were categorized into themes: (i) transport properties of SCFs, (ii) arc quenching in dense fluids, (iii) SF₆ environmental alternatives, and (iv) high-voltage applications of SCFs.

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Step 4: AI-Assisted Synthesis. ChatGPT was used not as a primary source of information, but as a tool for searching, structuring, and synthesis. Specifically, the model was employed to:

• Compare and summarize overlapping findings from multiple papers.
• Suggest gaps in the literature that align with our experimental work.
• Identify older sources that did not surface in standard search queries but proved relevant to the historical and theoretical context of SCFs.

ChatGPT was chosen because the research was conducted during a period when it was among the most accessible and widely adopted AI tools. While other specialized AI platforms exist, they were often cost-prohibitive, limited to subscription-based databases, or narrower in scope. ChatGPT provided a balance of accessibility, breadth of knowledge, and flexibility for interactive refinement. It was not used to generate new knowledge but to identify and organize existing peer-reviewed knowledge more efficiently. All outputs were validated against primary sources.

2. Properties of Supercritical Carbon Dioxide

2.1. Transport Properties

In its supercritical state, CO₂ exhibits a combination of gas-like and liquid-like properties that are advantageous for electrical insulation applications. Key transport properties, including low viscosity and high diffusivity, enable efficient movement through confined spaces, while enhanced thermal conductivity facilitates rapid heat dissipation. These characteristics make scCO₂ particularly suitable for applications that require rapid temperature regulation and minimal resistance to flow. Notably, thermal conductivity and heat capacity both peak near the critical point, providing increased capability for heat dissipation in high-energy environments. This is essential for applications like circuit breakers that require efficient temperature control under high stress.

Figure 1 presents and compares key transport properties of supercritical carbon dioxide with those of supercritical hydrogen (scH₂) and supercritical nitrogen (scN₂), using data available from the NIST Chemistry WebBook [17]. scCO₂ stands out due to its significantly higher density, specific heat, and viscosity compared to scH₂ and scN₂, particularly at lower temperatures. This is mainly because scCO₂ is being operated near its critical point, where small changes in temperature or pressure can result in dramatic shifts in its properties. As a result, scCO₂ demonstrates enhanced density and specific heat, which enable it to absorb more thermal energy and contribute to stable arc quenching and dielectric performance. In Section 4, we will further discuss how these differences influence the suitability of scN₂ and scH₂ compared to scCO₂ as potential electrical insulation media. We will examine their respective dielectric strengths and transport properties.

2.2. Unique Structural Characteristics

A unique feature of scCO₂ is the formation of transient molecular clusters near the critical point. These clusters result from weak van der Waals forces and exhibit short lifetimes, leading to dynamic fluctuations in local density [18]. These density fluctuations create microenvironments within the fluid that affect electron mean free paths and impact dielectric performance. Near the critical point, small variations in temperature and pressure can significantly alter the size and distribution of clusters, affecting properties like dielectric strength and ionization potential.

To effectively leverage supercritical CO₂ as a dielectric medium, it is crucial to manage density fluctuations near the critical point, as these fluctuations can hinder scCO₂’s performance as an insulation medium. Near the critical point, high-density molecular clustering increases the electron mean free path, which can inadvertently reduce the material’s overall dielectric strength. For this reason, it is advantageous to operate scCO₂ slightly above its critical point. In this range, the fluid’s density is high and more homogeneous, reducing clustering, which minimizes the electron mean free path. This stability enhances the dielectric strength and ensures better insulation performance.

2.3. Breakdown Voltage

The breakdown voltage of supercritical CO₂ depends on several factors, including pressure, temperature, electrode gap distance, and the specific configuration of the applied electric field. Due to these conditions, typical breakdown voltage values for scCO₂ can range widely but generally reach around 360 kV/mm in optimized settings (uniform field, short gaps) above the critical point [19]. For comparison, the SF₆ breakdown voltage at 200 kPa ranges between 9.5 and 12.6 kV/mm [20]. scCO₂’s substantially higher breakdown voltage enables reduced insulation clearances, thereby facilitating more compact and efficient high-voltage system designs.

2.4. Conductivity and Permittivity

The study of conductivity and the dielectric constant in supercritical CO₂ mixtures has shown some foundational trends, though gaps remain in our understanding of these properties under varying conditions. The dielectric constant of CO₂ in its supercritical state is generally low but can increase with pressure or the addition of polar cosolvents, like methanol, which enhance molecular interactions [21]. However, CO₂’s behavior as a non-polar molecule means that even under supercritical conditions, the dielectric response is modest, and shifts in the dielectric constant occur gradually with changes in pressure and composition.

Conductivity in supercritical CO₂ is influenced by pressure-sensitive electron attachment and detachment processes. As pressure increases, CO₂ molecules tend to cluster around ions, a phenomenon known as electrostriction, which enhances the dielectric constant and alters conductivity [22,23]. These effects make supercritical CO₂ unique but also challenging to characterize precisely. Due to these complexities, more experimental data are needed, especially to understand how variations in pressure and temperature impact both the dielectric constant and conductivity in CO₂.

3. Ionization in Supercritical Carbon Dioxide

3.1. Thermal Ionization

Thermal ionization is the process by which high temperatures in an arc provide sufficient energy for atoms or molecules to lose electrons, creating a plasma of free electrons and positive ions. In scCO₂, this process is impacted by the unique thermophysical properties of the medium, including high density, significant thermal capacity, and moderate ionization potential.

One of the defining characteristics of supercritical carbon dioxide is its high thermal capacity [24,25], which plays a significant role in managing thermal ionization within the arc. The ability of scCO₂ to absorb and distribute large amounts of heat without reaching ionization thresholds makes it particularly effective at stabilizing arcs under high-temperature conditions commonly encountered in circuit breakers. In the arc column, temperatures can rapidly escalate, increasing ionization as particles gain the energy needed to free electrons from atoms and molecules. However, due to scCO₂’s high thermal capacity, rapid ionization is delayed as energy is distributed more efficiently throughout the fluid. This delay in the onset of thermal ionization means that the arc is less likely to experience a sudden influx of free electrons, which could otherwise create a conductive path that destabilizes the arc. Instead, scCO₂’s thermal absorption moderates the rate at which ionization occurs, allowing for a more controlled arc profile that supports predictable current interruption. The high thermal capacity of scCO₂ also contributes to enhanced arc stability, helping mitigate rapid thermal spikes that can lead to runaway ionization.

Additionally, the ability of scCO₂ to absorb heat without quick volumetric expansion prevents drastic pressure changes that could impact the arc’s shape and position. This containment of thermal expansion further stabilizes the arc, as the fluid’s density remains relatively constant, preserving the arc’s conductive path and preventing sudden, uncontrollable arc movement. This stability is crucial for achieving consistent performance in high-voltage circuit breakers, especially during high-current interruptions where pressure surges could otherwise hinder arc-quenching efforts.

3.2. Ionization by Collision

Ionization by collision is a fundamental process in sustaining arc conductivity by generating free electrons. This process occurs when high-energy electrons collide with molecules, transferring enough energy to dislodge additional electrons and create positive ions.

The molecular density and molecular size of scCO₂ are the defining factors in how ionization by collision functions. In supercritical states, CO₂ exhibits a density approaching that of liquids while retaining gas-like diffusivity, creating an environment where electron movement is frequent but contained. This dense molecular environment shortens the average distance an electron can travel before colliding with a molecule. Unlike low-density gases, where electrons have longer paths and may accumulate energy before a collision, the dense structure of scCO₂ creates a highly controlled ionization environment. Electrons lose energy more frequently due to regular collisions, which prevents excessive energy buildup and limits the risk of rapid ionization increases. Moreover, the density-driven short mean free path (MFP) in scCO₂ enables efficient electron recombination after collisions, which helps to balance ionization with de-ionization processes. This balance further stabilizes the arc by ensuring that ion and electron densities remain within a manageable range, avoiding situations where excessive ionization could lower arc resistance, making the arc difficult to control or extinguish. As such, scCO₂’s dense environment makes it ideally suited for applications requiring precise ionization control and efficient arc quenching.

The mean free path plot, Figure 2, shows how the MFP for different gases, CO₂, N₂, and H₂, as a function of temperature at a pressure of 10 MPa. The MFP was calculated using the Clausius mean free path equation:

$$\lambda ={\displaystyle \frac{1}{\sqrt{2}n{d}^2}}$$

(1)

where λ is the MFP, n is the number density of molecules (molecules per unit volume), and d is the molecular diameter.

In this plot, scCO₂ exhibits the shortest mean free path across the temperature range. This effect is more pronounced given CO₂’s critical point proximity to our operating conditions, where its density remains high, maintaining a shorter MFP.

In high-voltage applications, maintaining a stable ionization level near critical points, like the current zero, is particularly important. At current zero, the electric field’s influence over ionization becomes prominent, as field strength alone may not be sufficient to sustain electron flow. Minimal yet consistent ionization near current zero is essential to prevent an abrupt arc interruption that could result in transient overvoltages [26], especially in circuits with inductive components, where this can result in a steep rate of current change (di/dt). The dense nature of scCO₂ and the increased rate of ionization collisions promote a manageable level of ionization, allowing for a stable, minimal flow of electrons and preventing the arc from extinguishing too abruptly.

3.3. Formation of Negative Ions

The formation of negative ions, also known as electron attachment, occurs when free electrons attach to neutral atoms or molecules, resulting in the creation of negative ions. This process is particularly effective in electronegative gases, such as sulfur hexafluoride, where the high electron affinity leads to strong electron attachment, quickly reducing free electron density and extinguishing arcs. This effect becomes especially critical during the dielectric phase of fault clearing, when the peak transient recovery voltage imposes a strong electric field across hot contact surfaces, releasing large quantities of thermal electrons into the gap. Electron attachment in such environments contributes significantly to arc quenching by reducing conductivity and supporting rapid dielectric recovery.

In supercritical carbon dioxide, however, the formation of negative ions is minimal due to CO₂’s relatively low electron affinity. Unlike SF₆, which readily captures free electrons, CO₂ molecules are less inclined to form stable negative ions, limiting electron attachment as a quenching mechanism. However, electrostriction, the phenomenon of localized density increases around ions, becomes more pronounced at critical conditions. When an electron attaches to a CO₂ molecule, it forms a CO₂⁻ ion, which induces surrounding CO₂ molecules to compress around it. This creates a high-density region that stabilizes the CO₂⁻ ion and lowers the energy needed for further electron attachment. The increased stability of attached electrons promotes a rapid increase in the attachment rate because electrons are more likely to stay attached to CO₂ molecules rather than detaching [22]. The sharp increase in electron attachment rate constants and equilibrium constants near the critical pressure of supercritical fluids [23] hints at a high capacity for electron attachment in scCO₂.

4. Supercritical CO₂ as the Optimal Dielectric and Arc-Quenching Medium

In the search for suitable supercritical fluids to serve as both an electrical insulation and an arc-quenching medium, several options were considered, including supercritical nitrogen, hydrogen, oxygen (scO₂), water (scH₂O), and ammonia (scNH₃). Each of these fluids offers unique characteristics; however, upon closer examination, supercritical CO₂ emerges as the most effective option due to its favorable physical and chemical properties, aligning closely with the requirements outlined in previous sections for efficient arc extinction and dielectric recovery.

4.1. Mean Free Path and Heat Dissipation

One of the critical advantages of scCO₂ is its shorter mean free path (MFP) compared to scN₂ and scH₂. As presented in Figure 2, scCO₂’s MFP is approximately 2× shorter than scN₂ and 4× shorter than scH₂, a characteristic that significantly enhances its performance in arc-quenching applications. As discussed earlier, a shorter MFP means that electrons have less distance to travel before colliding with molecules, reducing the probability of sustained ionization that would otherwise support the arc. This property enables scCO₂ to suppress arc formation more effectively, providing a stable insulating environment.

Additionally, from Figure 1, scCO₂ possesses a specific heat capacity 4× higher than that of scN₂ and scH₂, directly contributing to its efficiency in dissipating heat from the arc zone. As established in previous sections, efficient heat dissipation is crucial for arc quenching, as it facilitates rapid cooling, thereby extinguishing the arc and restoring the system’s dielectric strength. The higher specific heat capacity of scCO₂ allows it to absorb and remove thermal energy from the arc region more quickly than the other supercritical fluids, leading to faster recovery and preventing arc re-ignition.

4.2. Arcing Time Constant−Projected Performance for scCO₂

The arcing time constant τ is a crucial parameter in evaluating the effectiveness of gases in interrupting electric arcs. It describes how quickly a gas can cool and de-ionize the arc plasma after the current zero crossing, thus restoring its insulating capability. A shorter time constant means the gas can more effectively suppress the arc and prevent re-ignition. SF₆, with its highly electronegative properties, has long been the gold standard for arc quenching, boasting an arcing time constant as low as 0.8 µs. Under the same low-pressure conditions, the time constants for CO₂ and N₂ were measured to be 1.5 µs and 210 µs, respectively [27]. However, studies indicate that CO₂, although slightly higher than SF₆ at low pressures, demonstrates promising reductions in arcing time constant as the pressure increases [27,28]. This performance makes CO₂ a strong candidate for arc quenching, with the potential to narrow the gap with SF₆ under higher pressures.

In our application, CO₂ will operate at approximately ten times the pressures tested in the literature, upwards of 8 MPa. Based on the observed trends between 0.6 and 0.8 MPa, we expect that over 8 MPa or higher, the arcing time constant of CO₂ will be considerably shorter, possibly outperforming SF₆. This anticipated performance improvement aligns with CO₂’s physical behaviors described above, as the increased gas density allows for rapid thermal dissipation and significantly faster dielectric recovery after the arc is extinguished.

4.3. Thermophysical Properties Relevant to Arc-Quenching Performance

A broad set of thermophysical properties influences how gases perform in arc-quenching applications. These include electrical conductivity, ionization energy, mean free path, and dielectric strength, among others. However, in this section, we focus on a smaller group of properties for which accurate, temperature- and pressure-dependent data are available through NIST and related databases. Specifically, we examine the speed of sound, kinematic viscosity, thermal diffusivity, and dynamic viscosity. Each of these plays a meaningful role in how gases behave under the extreme thermal and mechanical conditions that arise during arc formation and interruption. Figure 3, compares the temperature-dependent behavior of speed of sound, viscosity, and diffusivity for CO₂ and SF₆ under relevant operating pressures.

4.3.1. Speed of Sound

The speed of sound specifies how quickly pressure waves move through a gas, which is particularly important during the rapid gas expansion that follows arc initiation. In systems that rely on blast quenching, faster pressure wave propagation helps disrupt the arc channel and improve dielectric recovery. This is especially relevant in high-voltage circuit breakers that rely on blast-quenching mechanisms.

In our evaluation, Figure 3, CO₂ at 10 MPa exhibits significantly higher sound speeds than SF₆ at 0.7 MPa across the tested temperature range. This implies that, even under subsonic flow conditions, CO₂ enables faster dynamic response, better energy transport, and more efficient pressure relief around the arc.

Several prior studies support this interpretation. Bini et al. demonstrated that arc-induced pressure waves and shock fronts propagate at or near the local speed of sound, influencing the evolution of the arc column and pressure buildup in high-voltage circuit breakers [29]. Popovtsev et al. further modeled nozzle flow conditions in SF₆ breakers. They highlight that modern interrupters in SF₆ circuit breakers are specifically designed for supersonic gas flow (Mach numbers > 1), utilizing convergent–divergent nozzle geometries to maximize the cooling and de-ionization of the arc column during current interruption [30]. Moreover, Babrauskas reviewed the mechanics of arc explosions. They highlighted that the rapid release of energy results in the generation of strong acoustic and shock waves, with the speed of sound acting as the primary constraint on how quickly these forces propagate through the surrounding medium [31]. Taken together, these insights underscore that even if Mach 1 is not reached in operational flow, the local speed of sound remains a critical performance metric in arc interruption physics.

4.3.2. Kinematic Viscosity

Kinematic viscosity (ν), calculated as the dynamic viscosity divided by the gas density, characterizes the diffusivity of momentum within a fluid and directly impacts gas mobility through nozzles and contact chambers during arc quenching. It describes the fluid’s resistance to flow when inertial forces dominate over viscous forces. While higher kinematic viscosity reflects greater momentum diffusion [32], it also imposes a drag on gas mobility, limiting the fluid’s effectiveness in high-speed arc interruption scenarios.

CO₂ at 10 MPa consistently exhibits the lowest kinematic viscosity, while SF₆ shows the highest, Figure 3. This suggests that CO₂ can be accelerated more easily, making it better suited for high-speed gas-blast designs where rapid gas movement is essential.

4.3.3. Dynamic Viscosity

Dynamic viscosity (μ) describes the internal resistance of a gas to shear or flow. SF₆ generally has a lower dynamic viscosity than CO₂, but this alone does not tell the whole story. Since SF₆ also has a much lower density (at the typical operational conditions), its effective resistance to flow, as captured by its kinematic viscosity, is higher. By contrast, scCO₂’s higher density offsets its higher μ, resulting in improved flow performance in practical systems. In the supercritical regime, both viscosity and density increase with pressure; however, density increases more rapidly, resulting in a decrease in overall kinematic viscosity.

4.3.4. Thermal Diffusivity

Thermal diffusivity (α) describes how quickly heat spreads through the gas. As shown in Figure 3, SF₆ has higher thermal diffusivity than CO₂ at all conditions tested, which suggests it may perform better in systems where passive heat removal plays a larger role. That said, the slower heat diffusion of CO₂ is balanced by its stronger dynamic transport properties, meaning it can still perform well in quenching systems that rely more on forced gas motion than on thermal conduction.

4.4. Limitations of Alternative Supercritical Fluids

Other potential supercritical fluids, such as scO₂, scH₂O, and scNH₃, were excluded from consideration due to various practical limitations. scO₂, for instance, poses significant oxidation challenges due to its high reactivity, which can result in material degradation and safety hazards within the circuit breaker environment. Meanwhile, scH₂O and scNH₃ require incredibly high temperatures to reach the supercritical phase, placing excessive thermal and material demands on the system. Additionally, the strong polarity of scH₂O and the reactivity of scNH₃ make them incompatible with typical materials used in electrical applications, further limiting their practicality.

5. Supercritical CO₂ Mixtures

Mixtures of supercritical CO₂ with other fluids can help to tailor dielectric, thermal, fluid dynamic, and chemical properties. Azeotropic supercritical mixtures of CO₂ with ethane, C₂H₆, have shown lower critical temperatures than pure CO₂ [33]. However, this came at the cost of lower dielectric strength and decomposition during breakdown. Experiments with mixtures of supercritical CO₂ with trifluoroiodomethane (CF₃I) have shown an increase in dielectric strength [34]. Yet, the addition of CF₃I is problematic from an environmental and economic point of view. It has been speculated that adding hydrogen (H₂) could increase the thermal conductivity, especially at the high temperatures during the arcing phase. The problematic aspect is the potential subsequent formation of water, which could lead to corrosion and tracking on insulator surfaces. For circuit breaker applications, the most promising mixture consists of carbon dioxide with oxygen (O₂). While its impact on the dielectric strength and thermal conductivity is low, it helps with the life expectancy of the medium by reacting with metal vapors from the arcing contact surfaces and reducing carbon deposits.

6. Supercritical CO₂ Dielectric Life Expectancy

As with any arc-quenching medium, supercritical CO₂ will be dissociated in the electric arc due to the high temperatures in and around the arc. With increasing temperature, the mole fraction of carbon monoxide, atomic oxygen, and atomic carbon will increase. Assuming that the medium only consists of oxygen and carbon, it would recombine to the different oxidation states of carbon after the arc has been quenched—mostly carbon dioxide and, to a lesser degree, carbon monoxide and solid carbon (soot). Unreacted atomic oxygen will turn into molecular oxygen, i.e., mostly dioxygen and traces of ozone. For comparison, the dissociation of SF₆ yields many more different species, including SF₅, SF₄, SF₃, SF₂, F₂, S₄, S₂, and others [2]. After arc quenching, they recombine, constituting a long list of problematic substances, some highly corrosive and toxic (especially after secondary reactions with water and moisture). Compared to SF₆, CO₂ appears to be less problematic. Additionally, it is important to note that alternative gases such as perfluorinated ketones and fluoronitriles also face significant challenges related to the generation of toxic and corrosive byproducts during arcing events [6].

However, this is not where it ends. In real circuit breaker applications, metal vapors released from the arcing contacts will react with the dissociated constituents of CO₂. These metal vapors—mostly copper and tungsten—are highly reactive with oxygen. As the arc is quenched and the temperature drops, most metal vapors will compete with carbon for the oxygen. As the metal steals the oxygen, solid carbon (soot) deposits are expected to appear inside the switching chamber. Therefore, it is common practice to increase the oxygen content by adding oxygen to the CO₂ medium. With every arc-quenching operation, some of the oxygen will react with metal vapors, reducing the oxygen concentration over time. Thus, the life expectancy of the medium is determined by the oxygen concentration. The initial oxygen concentration is typically limited to below 20% to avoid excessive erosion/oxidation of the nozzle and the fire hazard associated with any other polymeric materials in the switching chamber. Typical values are between 5% and 20% of oxygen concentration for CO₂ gas circuit breakers [35,36,37,38].

While solid byproducts typically accumulate on the chamber walls and the bottom of the vessel, the decomposed gases are typically sorbed by a molecular sieve such as Zeolite [38]. These significantly reduce the potential impact of such decomposition products but do not change the basic process of oxygen being consumed by reacting with metal vapors after every breaking operation. Systematic modeling and experimental validation of the life expectancy of the scCO₂ + O₂ medium under arc-quenching operation is needed to design reliable circuit breakers, and it is currently an outstanding task.

7. Tough and Ecological Supercritical Line Breaker for AC (TESLA)

7.1. Development of SF₆-Free High-Voltage Circuit Breaker

In the United States of America, the Advanced Research Projects Agency-Energy (ARPA-E) is currently supporting three research projects as part of their “Sulfur Hexafluoride-Free Grid (SF6-FREE)” program. Several projects focused on circuit breakers with fluorinated gas mixtures as well as gas handling and management, while one project studied the feasibility of a 245 kV-rated scCO₂ circuit breaker.

An extensively explored approach to achieving SF₆-free circuit breakers involves replacing the insulating technology with vacuum interrupters, often combined with hybrid insulation systems such as vacuum-dry air or vacuum-solid insulating configurations [39,40]. These solutions use a vacuum interrupter for arc quenching and a series-connected disconnect switch for isolation. Dry air or a solid dielectric material can serve as the insulating medium inside the circuit breaker. In early 2021, Siemens Energy debuted its new SF₆-free circuit breaker, called “145 kV Blue Dead Tank Circuit Breaker,” which uses dry air as the insulation medium in combination with vacuum interrupters [41]. However, scaling up these circuit breakers for higher power ratings will be challenging due to the limited arc-quenching capabilities of vacuum. Furthermore, the physical footprint of dry-air-insulated circuit breakers is often larger, which poses challenges when replacing SF₆ circuit breakers in existing substations.

A team from Georgia Institute of Technology, in collaboration with the University of Wisconsin, Milwaukee, has developed the “Tough and Ecological Supercritical Line Breaker for AC” (TESLA), a novel SF₆-free high-voltage circuit breaker.

Figure 4 shows the 72 kV, 2 kA nominal, 20 kA fault single-phase prototype featuring a slim high-pressure chamber, 72 kV high-pressure bushings, and a hydraulic cylinder integrated into the switching chamber. TESLA’s high-pressure vessel was manufactured by McAljon in Savannah, Georgia, and is certified by the American Society of Mechanical Engineers (ASME) and pressure rated to 12.0 MPa.

Like conventional SF₆ high-voltage AC puffer-type circuit breakers, TESLA has a fast-moving contact system actuated by the hydraulic cylinder. Unlike conventional breakers, the hydraulic power unit can be detached from the chamber, further reducing the physical footprint of the breaker. The novel scCO₂ mixture insulation is formulated to maximize the recombination of dissociated arcing byproducts, extending maintenance intervals. Furthermore, its thermal and dielectric properties allow for the reduction in contact travel distance while maintaining fault-clearing capabilities. The team is advancing with developing the TESLA breaker by scaling up the 72 kV prototype to a model rated for 245 kV, with a nominal current capacity of 4 kA and a fault current rating of 63 kA.

TESLA’s design utilizes standardized, off-the-shelf components that comply with ASME standards, which are widely used in the petroleum industry. This strategic choice ensures that TESLA meets stringent safety and performance requirements and simplifies the supply chain by allowing the use of readily available, proven components. By leveraging these ASME-compliant parts, TESLA benefits from a streamlined manufacturing process, reduced lead times, and lower costs, while maintaining high reliability and compatibility with industry standards.

No commercial high-voltage bushings are currently available for electrical apparatuses operating at such high internal pressures. To address this need, 72 kV-class epoxy-molded bushings have been specifically designed and fabricated for TESLA circuit breakers at Georgia Institute of Technology, as illustrated in Figure 4.

The internal mechanism of TESLA includes the arcing and main contact system, nozzle, a puffer cylinder, busbars, actuation rod, and hydraulic cylinder, as shown in Figure 5. The hydraulic system was designed and assembled at the Georgia Institute of Technology. The contact system was designed and manufactured at the Georgia Institute of Technology. The actuation rod drives the motion of the contacts within the interrupter chamber. During opening, the puffer cylinder compresses the insulating medium, forcing it through the nozzle to cool and extinguish the arc formed between the tulip and pin arc contacts. The busbars provide the current path into and out of the interrupter. The interrupter integrates Stäubli MULTILAM connectors at multiple interfaces: one for the bushings, ensuring robust current injection into the device; and another for the stationary main contact, ensuring low-resistance conduction under continuous current. The tulip and pin arc contacts, shown in the lower right, represent a common contact geometry used in high-voltage breakers, optimized for both mechanical alignment and the controlled formation/erosion of arcs during switching events. The assembly is more compact than conventional SF₆ circuit breakers because of the better dielectric and thermal properties of the scCO₂. The team is currently conducting tests on the 72 kV TESLA breaker prototype based on the standards developed for SF₆ breakers.

7.2. Economic Tradeoffs in Circuit Breaker Design

While a detailed cost analysis is outside the scope of this paper and not currently available to reference, key cost-relevant points can be drawn from a recent life cycle assessment by Chen et al. in [42], which directly compares an SF₆ circuit breaker with the scCO₂ prototype presented in the previous section.

7.2.1. Raw Material Cost

The supercritical state of CO₂ is achieved when we overcome the fluid’s critical point, which occurs at 31 °C and 7.38 MPa. SF₆ circuit breakers typically operate around 0.6 MPa. This significant increase in pressure implies the use of thicker-walled vessels capable of withstanding the pressure requirements and safety factors. This leads to a circuit breaker that uses significantly more material mass (4058 kg for scCO₂ vs. 1631 kg for SF₆) [42], which translates to higher manufacturing costs, especially in the early prototype phase. Yet, as the technology matures, it is expected that the material mass will be significantly reduced as new designs leverage the higher dielectric strength and heat capacity of scCO₂ to enable more compact circuit breakers.

7.2.2. Fluid and Inventory Management Cost

Given market volatility, regional pricing disparities, and rising costs tied to regulatory compliance, this paper refrains from providing a specific market price for SF₆. Yet, given that SF₆ is an engineered specialty gas, we can expect CO₂ to be orders of magnitude cheaper as it is naturally abundant and easier to procure. Additionally, SF₆ incurs administrative costs linked to mandated inventory tracking and leakage reporting, which are not currently required for the use of CO₂. On the other hand, scCO₂ requires thermal management to maintain the fluid above its critical temperature, which adds operational energy costs, especially for outdoor installations in cold environments.

8. Conclusions

This study highlights supercritical carbon dioxide as a promising and environmentally sustainable alternative to sulfur hexafluoride for high-voltage circuit breakers. As summarized in

Table 1. Summary of key findings.

Property (Section Ref.)	Key Insights	Implication
Transport Properties (Section 2.1)	Liquid-like density with gas-like mobility enhances thermal conductivity and convective cooling.	Supports stable arc cooling and dielectric recovery.
Structural Characteristics (Section 2.2)	Unique fluid microstructures near the critical point reduce the mean free path.	Improves dielectric strength beyond conventional gases.
Breakdown Behavior (Section 2.3)	Breakdown strength can be tuned by adjusting the pressure–temperature condition.	Ensure reliable insulation under high-voltage stress.
Thermal Ionization (Section 3.1)	High density and heat capacity improve energy transport and cooling, limiting excessive ionization growth.	Prevents thermal runaway and supports stable post-arc dielectric recovery.
Collision Ionization (Section 3.2)	High density boosts collisions, limiting electron avalanches.	Stabilizes insulation under fault stress.
Negative Ion Formation (Section 3.3)	Transient CO2− ions capture electrons and accelerate recombination.	Reduce electron availability for avalanche growth, accelerating arc extinction.
Arc-Quenching Dynamics (Section 4.1, Section 4.2 and Section 4.3)	Short arcing time constants, strong convective cooling, and short MFP.	Enables reliable current interruption and reduced contact erosion.
Limitations of Other SCFs (Section 4.4)	N2, O2, and H2 have impractical critical points; NH3 faces toxicity and material issues.	Positions CO2 as uniquely suitable among SCFs.
CO2 Mixtures (Section 5)	Additives can tailor dielectric, thermal, and quenching behavior.	Offers an optimization pathway beyond pure CO2.
Applications and Case Study (Section 6)	The 72 kV prototype breaker demonstrates feasibility, with cost–performance tradeoffs being manageable.	Confirms technical viability and pathway for scaling.

, scCO₂’s combination of strong dielectric strength, efficient heat dissipation, and stable ionization dynamics suggests that scCO₂ can deliver reliable arc-quenching performance and meet industry standards for interruption capability. The first 72 kV scCO₂ AC breaker now under development at Georgia Tech is a strong step toward proving this concept in practice, and it also highlights the economic trade-offs, such as increased raw material costs, that may be offset by reduced gas and compliance expenses. At the same time, critical knowledge gaps remain. Comprehensive studies are required to assess scCO₂’s long-term dielectric behavior, performance under varying pressure-temperature regimes, and potential fluid mixtures. Addressing these questions will be essential for translating the unique physical properties of scCO₂ into robust engineering solutions. Beyond circuit breakers, scCO₂ also shows promise in emerging applications such as ultra-fast switchgear, electrostatic rotating machines, and advanced generator technologies, where its ecological and dielectric benefits could be transformative. If pursued, this research direction could enable a new generation of sustainable and efficient high-voltage technologies, reducing reliance on legacy electronegative gases and advancing the transition toward greener power systems.