Research and simulation analysis of crack failure mechanism of conductor spacer bar in ±800 kV converter station

The cracking of the conductor spacer bar in a ±800 kV converter station was detected and simulated to study the cause of failure. By means of penetration detection, fracture macroscopic inspection and metallographic inspection, it can be found that there are some casting defects such as segregation, pinhole and intergranular micro-crack in the chuck microstructure of the spacer bar, and the fatigue fracture occurs under the action of alternating stress. Through the structural simulation analysis, it can be concluded that the spacer rod body and the joint of the chuck have a structural mutation, which causes stress concentration and leads to the fracture of the spacer rod. Finally, suggestions are put forward to improve the design, manufacture and installation process of spacer rods to prevent similar failures from happening again and ensure the safe operation of the power grid.


Introduction
Spacer bars, as an indispensable tool in ultrahigh voltage transmission lines, are installed on multi-split high-voltage transmission lines to ensure that the split wires can maintain the same distance between the wires under the influence of wind, ice covering of wires, dance and short circuit current, etc., to meet the electrical performance, reduce the surface potential gradient, and avoid mutual whipping between sub-wires.It also has the effect of suppressing wind vibration and sub-range oscillation [1][2][3][4].
Because the spacer is in the field high altitude environment for a long time, it is affected by ice, snow, storms and other bad weather and the electromagnetic attraction between wires, and it is easy to be damaged, which seriously affects the safety of line operation [5][6][7][8].
Based on the macroscopic inspection, microstructure analysis and casting quality analysis of the conductor spacer bar in the converter station, preventive measures are put forward to prevent a large number of spacer bar fractures in similar projects in the future.

Penetration detection
The spacer rod was tested for penetration, and it was found that there were multiple welding pores in the weld position of the spacer rod, and there was a crack at the clamp head.The crack originated from

Macroscopic examination of fracture
The samples were separated along the fracture surface of the crack fracture, and the macro morphology of the fracture was observed.Black oxide existed on the surface of most of the fracture areas, which could not be completely removed after repeated ultrasonic cleaning, indicating that the crack took a long time to crack.There are obvious wear marks in the local convex parts of the section, indicating that there is relative movement and local friction on both sides of the fracture.In the lower left corner of the sample, as shown in the figure, there is a circular curved parallel texture (shell pattern), which has the macroscopic characteristics of fatigue fracture.The crack source is located at the inner tip of the lower left corner of the sample, as shown in Figure 2.

Metallographic examination
The macroscopic metallographic inspection was carried out on the sampling of the weld area of the spacer rod, and it was found that there were various welding defects such as porosity, under-penetration and under-fusion inside the weld, among which there was a large amount of porosity with a maximum diameter of 1.38 mm, and the total length of defects in the non-penetration and non-fusion parts could reach 2.50 mm, as shown in Figure 3.This proves that the welding process is not strictly controlled [9] and the welding quality is poor.The microstructure of the spacer bar base material and weld area is obviously different, and the weld seam and fusion area are relatively fine columnar crystals.Some of the base material grains are coarse, and the organization is not uniform and has a tendency of network distribution.Needle-shaped pores of different sizes can be seen in both the base material and the weld zone, as shown in Figure 4.The width of the main crack tip in the base metal region of the spacer rod chuck varies, and there is transgranular and intergranular propagation, as shown in Figure 5(a).There is a micro-crack next to the main crack, which is roughly parallel to the direction of the main crack, but exists independently of the branch crack of the main crack.Therefore, it can be judged that the micro-crack is the original casting crack of the base material, as shown in Figure 5

Simulation analysis
The spacer rod design drawing was referred to, and the selected material was aluminum alloy ZL102.
According to the mechanical properties of the material, the density was 2700 g/cm 3 , the elastic modulus was 72 GPa, the Poisson ratio was 0.33 and the strength limit was 271 MPa [10].The mesh cell type is a free tetrahedral mesh, with the largest cell being 2 mm and the smallest cell being 0.36 mm.In order to simulate the actual working condition of the spacer rod, any branch of the spacer rod and wire fastening part is chosen to apply fixed constraints, that is, the spacer rod fastening part has neither translation nor rotation in the x, y and z directions.Von Mises stress cloud images of the overall structure of spacer rods are shown in Figure 6.It can be seen from the figure that under the above conditions, the maximum Von Mises stress of the whole structure of the spacer rod appeared at the structural change at the joint site of the spacer rod body and the clamp.The maximum stress value was 332 MPa, higher than the yield strength of the material, and the material was cracked.The simulation results were consistent with the above analysis.It can be seen that structural mutation is one of the causes of spacer rod failure.

Conclusion
1.There are macroscopic welding defects such as porosity, non-penetration and non-fusion between the chuck and the main weld of the spacer bar.The maximum diameter of the porosity is 1.38 mm, and the total length of defects in the non-penetration and non-fusion parts is 2.50 mm, which indicates poor welding quality 2. There are some casting defects such as segregation, pinhole and intergranular micro-crack in the microstructure of the chuck of the spacer rod, which cause fatigue fracture under alternating stress.
3. There is a structural mutation in the connection part between the spacer bar body and the chuck, which causes stress concentration.Through simulation analysis, the maximum stress value is 332 Mpa, which is greater than the strength limit of the aluminum alloy, resulting in the fracture of the spacer bar.

Opinions and suggestions
1.The spacer rod structure design is optimized to increase the bearing cross-section of the sudden change part of the structure and reduce the stress concentration.
2. The spacer manufacturing process control and manufacturing quality supervision are strengthened to prevent macroscopic and microscopic casting defects and welding defects.
3. The optimization and specification of the installation process of the drainage wire and spacer rod are strengthened to minimize the pre-stress and bending moment of the spacer rod during installation.

Figure 3 .
Figure 3. Defects in weld area.The microstructure of the spacer bar base material and weld area is obviously different, and the weld seam and fusion area are relatively fine columnar crystals.Some of the base material grains are coarse, and the organization is not uniform and has a tendency of network distribution.Needle-shaped pores of different sizes can be seen in both the base material and the weld zone, as shown in Figure4.The width of the main crack tip in the base metal region of the spacer rod chuck varies, and there is transgranular and intergranular propagation, as shown in Figure5(a).There is a micro-crack next to the main crack, which is roughly parallel to the direction of the main crack, but exists independently of the branch crack of the main crack.Therefore, it can be judged that the micro-crack is the original casting crack of the base material, as shown in Figure5(b).
Figure 3. Defects in weld area.The microstructure of the spacer bar base material and weld area is obviously different, and the weld seam and fusion area are relatively fine columnar crystals.Some of the base material grains are coarse, and the organization is not uniform and has a tendency of network distribution.Needle-shaped pores of different sizes can be seen in both the base material and the weld zone, as shown in Figure4.The width of the main crack tip in the base metal region of the spacer rod chuck varies, and there is transgranular and intergranular propagation, as shown in Figure5(a).There is a micro-crack next to the main crack, which is roughly parallel to the direction of the main crack, but exists independently of the branch crack of the main crack.Therefore, it can be judged that the micro-crack is the original casting crack of the base material, as shown in Figure5(b).

Figure 6 .
Figure 6.Von Mises stress cloud image of spacer.