Insulation performance analysis of 10kV outdoor vacuum circuit breakers

Introduction

The vacuum interrupter is the most crucial component in a vacuum circuit breaker. It boasts numerous advantages, such as large breaking capacity, frequent operability, excellent arc - extinguishing performance, no pollution, and a compact size. As vacuum circuit breakers are evolving towards higher voltage levels, in - depth research on the internal and external insulation performance of outdoor vacuum interrupters is of greater necessity.

The electric field distribution within the interrupter significantly impacts the insulation performance of the vacuum circuit breaker. An uneven electric field distribution can lead to the breakdown of the contact gap, ultimately resulting in the failure of the circuit breaker to open. Installing a grading shield inside the vacuum interrupter can homogenize the internal electric field distribution, making the structure of the vacuum interrupter more rational and compact.

However, the addition of the shield also causes changes in the electric field distribution within the interrupter. To accurately verify the insulation performance of the interrupter and analyze the influence of the shield on the electric field distribution, conducting numerical analysis of the electric field of the outdoor vacuum circuit breaker is a key step in validating the reliability of the product.

Therefore, this paper analyzes and designs the insulation structure of a new type of 10kV outdoor high - voltage AC vacuum circuit breaker independently developed and manufactured by domestic switch production enterprises.

When conducting an electrostatic field analysis of the vacuum circuit breaker, a voltage is applied to the boundaries of the model, and tetrahedral meshing elements are employed according to the model's structure. The meshing of the grid is carried out using intelligent meshing. Since the vacuum circuit breaker has an axisymmetric structure, the vacuum interrupter is sectioned along the X-axis of the three-dimensional coordinate system. The advantage of using intelligent meshing lies in the fact that in areas where the curvature of the graph changes significantly, the grid division is very dense, while in areas with a more regular structure, the grid density is relatively low.

Based on the two working positions of the circuit breaker contacts, namely the breaking and closing positions, as well as the different open distances of the contacts during the breaking process, an electric field analysis is respectively performed on the vacuum interrupter. The characteristics of the electric field distribution and the points of field strength concentration are determined. The points of field strength concentration are the key areas of analysis in this paper. The electric field results obtained under various different conditions are compared.

Figure 1 Internal Enlarged Structure Diagram of the Vacuum Interrupter

Figure 1 - Stationary End Cover Plate; 2 - Main Shielding Cover; 3 - Contact; 4 - Bellows; 5 - Moving End Cover Plate; 6 - Stationary Conductive Rod; 7 - Insulating Housing; 8 - Moving Conductive Rod

Calculation Results and Analysis

This paper examines the insulation performance between the isolation break points under the rated lightning impulse withstand voltage. A high voltage of 125 kV is applied to the stationary contact of the circuit breaker, and a zero potential of 0 is applied to the moving contact. The potential distributions of the entire circuit breaker are obtained when the contact opening distances are 50%, 80%, and 100% respectively. The unit of potential is V, and the unit of electric field strength is V/m.

Due to the presence of the shielding cover in the vacuum interrupter, the electric field distortion is suppressed, resulting in a very uniform and symmetrical voltage distribution in the area near the contacts. The floating potential on the shielding cover is approximately 60 kV.

Potential distribution of the vacuum interrupter at 50% contact opening distance
Potential distribution of the vacuum interrupter at 80% contact opening distance
Potential distribution of the vacuum interrupter at 100% contact opening distance

In Figure 2, the figures (a) - (c) are the contour maps of the electric field strength distribution in the vacuum interrupter under the above three different contact opening distances respectively.

For the vacuum circuit breaker at 50% contact opening distance, the maximum electric field strength appears at the end of the shielding cover, with a value of 25.4 kV/mm. At this time, the electric field strength between the contacts is significantly higher than that at the previous two opening distances. The grading shielding cover makes the voltage near the contacts show a gradient distribution, and the electric field strength is evenly distributed, with a relatively large electric field strength between the contacts.

When the contact opening distances of the vacuum circuit breaker are 80% and 100%, the maximum electric field strengths are 21.2 kV/mm and 18.1 kV/mm respectively. The voltage near the contacts shows a gradient distribution, and the electric field strength is evenly distributed.

Electric field contour map of the vacuum interrupter at 50% contact opening distance
Electric field contour map of the vacuum interrupter at 80% contact opening distance
Electric field contour map of the vacuum interrupter at 100% contact opening distance

It can be seen from the figures that when the external insulating medium is constant and uniform, the areas with relatively large electric field distribution strength in the vacuum interrupter are mainly concentrated on the end surfaces of the moving and stationary contacts and the upper and lower ends of the shielding cover. These insulation - vulnerable areas are prone to insulation breakdown. Therefore, in the actual design of the product, the electric field distribution at the points of concentrated field strength can be improved through optimization design methods such as increasing the curvature of the end surfaces of the moving and stationary contacts and blunting the sharp corners at both ends of the shielding cover.

The electric field strength on the outer surface of the vacuum interrupter is relatively small. It can be seen from the figure that in the areas near the two ends of the ceramic housing of the vacuum interrupter and close to the end cover plates of the interrupter, the electric field strength values are larger than those at other positions along the surface.

When the contacts of the vacuum circuit breaker are closed, a high voltage of 125 kV is applied to the central conductor, and the potential at the infinite - far boundary is set to 0. After the loading, the calculation shows that the electric field strength is very small both inside and outside the circuit breaker, with the maximum electric field strength being 0.8 kV/mm. The electric field strength is evenly distributed, and the voltage around the contacts shows a gradient distribution trend centered on the contacts.

(a) Electric field contour map of the vacuum interrupter at 50% contact opening distance
(b) Electric field contour map of the vacuum interrupter at 80% contact opening distance
(c) Electric field contour map of the vacuum interrupter at 100% contact opening distance

Conclusion

Through the analysis and research on the electric field of the 10kV outdoor high - voltage AC vacuum circuit breaker, the variations in the electric field strength and potential of the circuit breaker under different boundary conditions have been obtained. From the above results, it is clear that by using ANSYS to accurately simulate the prototype of the object and applying the finite - element method for numerical calculations of the electric field and potential, precise calculations of the variations in the electric field and potential within the vacuum interrupter can be achieved.