Analysis and Research of the Electric Field in the Vacuum Arc Interrupter of a 31.5 kV Vacuum Circuit Breaker

Abstract

The insulation performance of vacuum arc interrupters greatly affects the reliability of vacuum circuit breakers. Optimizing the electric field distribution of a vacuum arc interrupter can improve its insulation performance. Here, we establish a two-dimensional model of vacuum arc interrupter with a shield system and study the electric field distribution of the vacuum arc interrupter under different structures through finite element simulation. Research has shown that as the electrode gap, contact chamfer radius, and main shield radius increase, the maximum value of the electric field in the vacuum arc interrupter decreases. The conductive rod at the static end of this vacuum arc interrupter is thickened, and the structure of the moving and static ends is asymmetric, resulting in asymmetric distribution of electric fields at the moving and static ends, which is not conducive to uniform electric fields. Optimization can be achieved by adjusting the structure of the main shielding cover. The research results will provide a basis for optimizing the electric field distribution of vacuum arc interrupters.

1. Introduction

The development of national industries relies heavily on the support of electric power. With the advancement of industrialization, both the voltage levels and loads of power grids have been continuously increasing [1,2,3]. Circuit breakers, owing to their excellent interruption performance, can quickly and effectively clear faults in power systems. Their stability and reliability are therefore critical for the safe operation of power grids [4,5,6]. As the core component of circuit breakers, the vacuum interrupter plays a decisive role in the insulation performance of circuit breakers, which is closely related to the distribution of the internal electric field [7,8,9].

Sulfur hexafluoride (SF₆) has been widely used in conventional gas-insulated switchgear (GIS) due to its superior insulating and arc-quenching properties. However, because of its severe environmental impact, power equipment utilizing SF₆ is gradually being phased out. Vacuum circuit breakers have thus become an excellent alternative to SF₆ circuit breakers [10,11,12,13,14,15,16]. To meet the demands of modern industry, vacuum interrupters are evolving toward miniaturization and higher voltage levels, while also extending their application to lower voltage ranges [17].

The electric field distribution within vacuum interrupters is a key factor affecting the overall performance of circuit breakers. Numerous studies have attempted to reduce breakdown risk and improve performance by homogenizing the internal electric field. Most existing works rely on electrostatic field simulations and use these results to guide structural optimization of vacuum interrupters. Factors such as contacts, contact gap, and external insulation all influence the electric field distribution. In addition, the floating potential of the main shield—affected by both its geometry and surrounding environment—exerts a significant impact on the electric field distribution inside the interrupter [18]. As a result, researchers have pursued various approaches from different perspectives. Guo et al. identified electric field concentration regions in external insulation via simulation, subsequently optimized the structure, and validated it through prototype testing [19]. Wu built a model of 12 kV vacuum interrupter, using finite element analysis to study the effect of the contact gap on the electric field and proposed a comprehensive optimization design [20]. Regarding 40.5 kV equipment, Fang et al. developed a simplified 2D model to examine the influence of the main shield radius, providing optimization recommendations [4]. Similarly, Wang and Chen demonstrated that equalizing shields of different shapes can effectively homogenize the electric field within the interrupter [21]. Furthermore, Zhang and Zhuang compared models with and without a main shield, confirming its crucial role in electric field equalization [22]. In many reported models, the diameters of the moving and stationary electrodes are identical. In practice, however, the stationary electrode can be designed thicker than the moving electrode, as it does not require mechanical actuation. This asymmetry alters the field distribution inside the interrupter. The fundamental principle behind this asymmetric design lies in the distinct functions of the stationary and moving contacts. The stationary contact, being fixed, can be prioritized for thickening to enhance structural stability, current-carrying capacity, and thermal dissipation. Conversely, the moving contact must account for its kinematic function, requiring lightweight construction to enable rapid opening and closing by the operating mechanism.

For large-capacity generator circuit breaker interrupter chambers, the elevated voltage level makes the fixed contact edge particularly prone to electric field concentration, posing a risk of insufficient insulation reliability [23,24,25]. Regarding the modeling approach, since the core objective of this study is to analyze the electric field distribution in the contact region—which exhibits strict axisymmetric characteristics—a two-dimensional axisymmetric model was adopted for simulation [26]. Accordingly, this research establishes a 2D axisymmetric model of the vacuum interrupter chamber with a thickened fixed conductive rod, calculates its electric field distribution at the 31.5 kV voltage level, and systematically analyzes the influence of key parameters—including contact gap, contact edge radius, and main shield structure—on the electric field distribution. The relevant work can provide valuable references for the electric field optimization design of such vacuum circuit breaker interrupter chambers.

2. Vacuum Interrupter Model

2.1. Subsection

When a circuit breaker interrupts the current, the operating mechanism drives the separation of the moving and stationary contacts. During this process, a vacuum arc is formed between the contacts, which is sustained by the ionization of metal vapor generated from the electrode material. As the contacts continue to separate, the gap distance increases, the current decreases, and the metal vapor density is reduced. At the current zero crossing, the plasma diffuses outward and eventually extinguishes the arc [27]. This arcing and arc-extinguishing process takes place entirely within the vacuum interrupter.

A vacuum interrupter mainly consists of a stationary contact, a moving contact, a stationary conductive rod, a moving conductive rod, an insulating envelope, a bellows, metallic end plates, and a shielding system [28,29]. The shielding system functions to cool and condense the metal vapor produced during the arcing process, preventing its deposition on the inner wall of the insulating envelope, which would otherwise reduce its dielectric strength. Moreover, it facilitates the rapid decay of the residual plasma after arc extinction [30,31]. The shielding system typically includes a main shield, bellows shield, and end shield, with additional equalizing shields incorporated when necessary.

If the electric field inside the interrupter is unevenly distributed, local weak points in insulation may occur. An appropriately designed shielding system not only absorbs metal vapor but also adjusts the internal electric field distribution of the vacuum interrupter, thereby homogenizing the field and enhancing its insulation performance. It should be noted that this study primarily focuses on the dominant role of macroscopic geometric structures, and therefore reasonably simplifies microscopic factors such as surface roughness and triple-junction effects. The simulation model of the vacuum interrupter is shown in Figure 1, and its partial three-dimensional simulation model is presented in Figure 2. Some structural parameters of this interrupter chamber are listed in

Table 1. Structural Parameters of the 31.5 kV Vacuum Circuit Breaker Interrupter Chamber.

Parameter	Value	Parameter	Value
End Shield Length/mm	35	Main Shield Edge Curling Angle/°	160
Bellows Length/mm	110	End Shield Bending Arc Angle/°	220
Bellows Shield Length/mm	30	Interrupter Chamber Diameter/mm	160
Bellows Radius/mm	43	Total Chamber Length/mm	350

below.

2.2. Model Parameters

In the simulation model, a high voltage of 31.5 kV was applied to the stationary terminal, while the moving terminal was set to zero potential. The computational domain was extended with an infinite element region, with the potential at infinity set to zero. The main shield was assigned a floating potential. Material parameters for each component are listed in

Table 2. Material parameters of vacuum arc interrupter.

Components	Material	Relative Permittivity
Contact	CuCr50	1
Conductive Rod	Copper	1
Bellows	Stainless Steel	1
Shielding Cover	Stainless Steel	1
Insulating Envelope	Ceramic	9.1
Interior of Interrupter	Vacuum	1
Exterior of Interrupter	Air	1.0006

.

To investigate the electric field distribution within the vacuum interrupter, four paths were defined to record the variation in electric field intensity along each path. Path 1 was along the radial direction on the surface of the stationary contact, Path 2 along the radial direction on the surface of the moving contact, Path 3 along the radial direction through the center of the moving and stationary contacts, and Path 4 along the axial boundary between the moving and stationary contacts, as shown in Figure 3.

3. Effect of Contact Gap and Edge Chamfer Radius on the Electric Field in a Vacuum Interrupter

3.1. Effect of Contact Gap on the Electric Field in a Vacuum Interrupter

The contacts are the core components of the vacuum interrupter, which controls current interruption through the making and breaking of the moving and stationary contacts. When the circuit breaker operates, the region between the moving and stationary contacts is usually where the electric field is most concentrated within the interrupter. The contact gap significantly influences the electric field magnitude inside the interrupter. In this study, contact gaps of 20–25 mm were selected for simulation.

The electric field distributions along the path between the moving and stationary contacts (Path 3) and along the axial boundary between the contacts (Path 4) were recorded, as shown in Figure 4 and Figure 5. The relationship between the maximum electric field in the interrupter and the contact gap is presented in Figure 6. The electric field distribution patterns are similar under different contact gaps, with the maximum field occurring at the contact edges. As the contact gap increases, the maximum electric field within the interrupter decreases. Although increasing the contact gap can significantly reduce the maximum field, excessively large gaps increase the mechanical burden on the operating mechanism and may reduce the interrupting capability of the circuit breaker [32,33]. Therefore, in industrial applications, the contact gap should be selected by balancing multiple factors.

3.2. Effect of Contact Edge Chamfer Radius on the Electric Field Distribution

The simulation results indicate that the maximum electric field within the vacuum interrupter is generally concentrated at the edges of the contacts, primarily due to field enhancement at sharp edges. To reduce this field concentration, the contact edges are chamfered. In this study, chamfer radius ranging from 0.5 mm to 3 mm were selected for simulation to investigate the effect of contact edge chamfering on the electric field distribution.

Figure 7 shows the relationship between the maximum electric field in the interrupter and the contact chamfer radius, while Figure 8 presents the radial electric field distribution along the surface of the stationary contact (Path 1) for different chamfer radius.

It is evident that increasing the contact chamfer radius can mitigate the electric field concentration at the contact edges, with the maximum electric field decreasing as the chamfer radius increases. However, the reduction in field strength exhibits diminishing returns. The chamfer radius should not be excessively enlarged for three reasons: (1) the boundary attenuation effect reduces the benefit of further increasing the chamfer radius; (2) a larger chamfer reduces the contact area between the moving and stationary contacts, and excessive reduction may impair the performance of the interrupter; (3) the contact blade itself is relatively thin. To verify the reliability of the simulation results, a mesh independence study was conducted, as shown in Figure 9. The analysis demonstrates that further refinement of the baseline mesh results in a change of less than 0.8% in the maximum electric field intensity within the interrupter chamber, confirming that the current mesh configuration yields a mesh-independent solution.

Figure 10 illustrates the electric field distribution at the edge of the moving contact for chamfer radius of 0.5 mm and 2 mm. Severe field concentration occurs at the 0.5 mm chamfer, while at 2 mm the concentration is significantly alleviated. Considering the thinness of the contact blade, a chamfer radius of 2 mm was selected in this study.

4. Effect of Main Shield on the Electric Field Distribution in a Vacuum Interrupter

4.1. Effect of Main Shield Radius on the Electric Field Distribution

In addition to its shielding function, the main shield can also adjust the electric field distribution within the vacuum interrupter. The main shield radius is defined as the distance from the center of the contacts to the main shield [34]. Simulation models were established with main shield radius ranging from 70 mm to 75 mm to investigate their effect. The relationship between the maximum electric field in the interrupter and the main shield radius is shown in Figure 11.

As the main shield radius increases, the maximum electric field in the interrupter decreases. However, an excessively large radius can result in an oversized interrupter and may reduce the shielding effectiveness of the main shield. In industrial production, the selection of the main shield radius must consider multiple factors.

Figure 12 shows the distribution of the electric field intensity along the axial distance on the vacuum interrupter’s insulating envelope. The field strength is minimum at the central region of the contacts and gradually increases towards both ends. Furthermore, a comparison of the electric field curves for different main shield radii reveals that the overall distribution pattern remains consistent, with no significant differences observed despite the variation in radius.

4.2. Electric Field Distribution Optimization with an Asymmetric Main Shield in a Vacuum Interrupter

As discussed in the previous section, moderately increasing the main shield radius can reduce the maximum electric field within the vacuum interrupter, but the reduction is limited. The electric field distributions along the radial direction on the surfaces of the stationary and moving contacts (Paths 1 and 2) were recorded for main shield radii of 70 mm and 75 mm, as shown in Figure 13. The difference in the maximum electric field between the stationary and moving contact regions under different shield radii is presented in Figure 14.

It can be observed that, in this vacuum interrupter, as the main shield radius increases, the electric field at the edge of the stationary contact increases while that at the edge of the moving contact decreases, resulting in a higher degree of asymmetry. This asymmetry is unfavorable for achieving a uniform electric field and for further reducing the maximum field intensity.

Figure 15 shows the equipotential distribution for a main shield radius of 75 mm. The equipotential lines in the contact region are not symmetrically distributed, resulting in a significantly higher maximum electric field at the stationary contact compared to the moving contact. Generally, the maximum electric field within a vacuum interrupter occurs in the contact region, and asymmetry in this region leads to an increase in the overall maximum field. The key factors influencing the distribution of equipotential lines, and thus the electric field distribution, are: (1) the distance between the end of the main shield and the conductive rod (for the moving end, this is the distance between the main shield end and the bellows and bellows shield); (2) the distance between the end of the main shield and the end shield. For clarity, the former is referred to as Distance 1 l₁ and the latter as Distance 2 l₂, as illustrated in Figure 16. Due to the structural asymmetry between the stationary and moving ends of the interrupter in this study, these two distances differ, leading to an uneven electric field distribution. In such cases, an asymmetrically designed main shield can be used to improve the uniformity of the electric field distribution.

Taking the model with a main shield radius of 75 mm as an example, the spacing l₁ is 22.5 mm at the moving end and 12.5 mm at the fixed end. The fixed-end spacing l₁ is reduced to optimize the electric field distribution. The maximum electric field values in the contact regions of the moving and fixed ends under different fixed-end spacings l₁ are shown in Figure 17, and the maximum electric field values in the interrupter chamber under different fixed-end spacings l₁ are shown in Figure 18.

As the fixed-end spacing l₁ decreases, the maximum electric field in the fixed-end contact region decreases, while that in the moving-end contact region increases, thereby reducing the degree of asymmetry. Consequently, when the fixed-end spacing l₁ is reduced from 12 mm to 8 mm, the maximum electric field in the vacuum interrupter chamber decreases. However, when the fixed-end spacing l₁ is further reduced from 8 mm to 7 mm, the maximum electric field increases abruptly. This is because the distance between the fixed end of the main shield and the conductive rod becomes too small, causing the location of the maximum electric field in the interrupter chamber to shift from the contact edge to the fixed end of the main shield. Although further reduction in the fixed-end spacing l₁ can continue to decrease the maximum electric field in the contact region, it would lead to a sharp increase in the electric field at the fixed end of the main shield.

In the model with a main shield radius of 75 mm, the spacing l₂ is 33 mm at the moving end and 53 mm at the fixed end. The length of the main shield at the fixed end is increased to reduce the fixed-end spacing l₂, thereby optimizing the electric field distribution. The maximum electric field values in the contact regions of the moving and fixed ends under different fixed-end spacings l₂ are shown in Figure 19, and the maximum electric field values in the interrupter chamber under different fixed-end spacings l₂ are shown in Figure 20.

As the fixed-end spacing l₂ decreases, the maximum electric field in the fixed-end contact region decreases, while that in the moving-end contact region increases, leading to a reduction in the degree of asymmetry. When the maximum electric field values in the fixed-end and moving-end contact regions become equal, the asymmetry is minimized, and the maximum electric field in the interrupter chamber is also reduced to its lowest value. Further reduction in the fixed-end spacing l₂ beyond this point results in an increase in the maximum electric field within the interrupter chamber. The results in Figure 20 confirm this trend: as the fixed-end spacing l₂ decreases, the maximum electric field in the interrupter chamber first decreases and then increases, reaching a minimum at 35 mm.

During the adjustment of the fixed-end spacing l₂, the distance between the end of the main shield and the conductive rod remains unchanged. Therefore, this approach does not cause a sharp increase in the electric field at the fixed end of the main shield, making it more effective than adjusting the fixed-end spacing l₁.

5. Conclusions

This paper investigates the influence of contact gap, edge chamfer radius, and shielding on the internal electric field distribution of a vacuum interrupter chamber designed for 31.5 kV circuit breakers with a thickened fixed-end conductive rod. The main conclusions are summarized as follows:

• Increasing the contact gap reduces the electric field strength between the moving and fixed contacts and decreases the maximum electric field within the interrupter chamber. However, it does not alter the fundamental distribution pattern of the electric field inside the vacuum interrupter.
• A larger contact edge chamfer radius mitigates electric field concentration at the contact edges and reduces the maximum electric field within the interrupter. A chamfer radius of 2 mm is sufficient to significantly alleviate field concentration, thereby considerably reducing the risk of breakdown at this location.
• As the radius of the main shield increases, the maximum electric field in the interrupter chamber decreases.
• With an increase in the main shield radius, the asymmetry of the electric field between the moving and fixed contact regions in the studied interrupter becomes more pronounced, which is detrimental to achieving a uniform field distribution.
• The asymmetry of the electric field between the moving and fixed contact regions can be optimized by adjusting two distances: the spacing between the fixed-end main shield and the conductive rod, and the spacing between the fixed-end main shield and the end shield. Reducing these spacings decreases the maximum electric field in the fixed-end contact region while increasing it in the moving-end region. The electric field distribution across the contacts becomes most symmetric, and the maximum field value in the contact region is minimized, when the field maxima at the moving and fixed contacts are equal. However, when the spacing between the fixed-end main shield and the conductive rod is reduced to 7 mm, the location of the maximum electric field shifts from the contact region to the end of the fixed main shield, resulting in an overall increase in the maximum field within the interrupter. Further reduction in this spacing is therefore not advisable.

This study innovatively reveals the inherent principles of electric field distribution in asymmetric structures and proposes a concrete method for electric field optimization through asymmetric shield design, providing an effective solution to address electric field concentration issues caused by structural asymmetry in engineering practice. The quantitatively derived design parameters from simulations can be directly applied to the engineering design and optimization of vacuum interrupter chambers, offering significant practical value for enhancing the insulation performance and operational reliability of circuit breakers.

It should be noted that this work primarily focuses on static insulation characteristics, with reasonable simplifications applied to microscopic factors such as surface roughness and triple-junction effects. Furthermore, while dynamic processes including vacuum arc behavior, plasma characteristics, and post-arc dielectric recovery are recognized as critical physical mechanisms, these aspects are reserved as key directions for future research. Such in-depth investigations will further refine the theoretical framework for evaluating breaking capacity and contribute to more comprehensive guidance for designing high-performance interrupter chambers.