Failure analysis of strain clamps on a ± 800 kV transmission line

During X-ray testing of a ± 800 kV ultra-high voltage direct current transmission line tension clamp, it was found that the internal steel core had broken. To investigate the cause of the steel core fracture, the failed clamp was dissected and analyzed, including chemical material testing, mechanical performance testing, fracture morphology inspection, and analysis of the compression process of the tension clamp. A preliminary judgment was proposed on the cause of the internal steel core fracture of the tension clamp, And verification experiments and simulation analysis were conducted to preliminarily determine the cause of the steel core fracture, and it was ultimately concluded that the main reason for the steel core fracture was the incorrect compression sequence of the large cross-section wire tension clamp.


Introduction
To build a new type of power system, we need to vigorously promote large-scale transmission across provinces and regions, and transport clean electricity from power generation bases to load centers.Ultra high voltage transmission lines play a very important role in power transmission [1].As a hardware tool for fixing wires in power lines, tension clamps play a crucial role in power transmission systems, carrying all the tension of the wires [2].The tension clamps in ultra-high voltage transmission lines are usually fixed and connected to the wires and clamps through crimping.In the process of crimping construction, factors such as non-standard crimping process and lax control of crimping process often lead to poor crimping quality.In severe cases, it can lead to stress concentration in the steel core and even lead to steel core fracture [3][4].During the maintenance of a ± 800 kV ultra-high voltage direct current transmission line, it was found that there was a steel core fracture defect in the side crimping position of a tension clamp located on Tower 0383.The "three span" tension line with damaged steel core may be subjected to additional external forces such as wind load and icing during operation, resulting in the detachment of line conductors and clamps, leading to serious power safety accidents.In response to the quality issue of tension clamps, domestic and foreign researchers have conducted a series of simulation and experimental research work from the aspects of crimping construction, defect detection of clamp failure, and analysis of fracture causes.He Ximei et al.analyzed the causes of 750 kV wire breakage caused by improper compression process of tension wire clamps [5] .Zhu Dengjie et al. conducted gripping force experiments on NY400/35 strain clamp with normal crimping and crimping defects .The failure process and force transmission characteristics of the strain clamp were studied through displacement and strain experimental data [6] .Wang Yunhui et al.found that the excessive concentration of stress on the arc groove side of the wedge block of the wire clamp is an important reason for the failure of the NXJ type tension wire clamp, and proposed corresponding optimization plans [7].At present, the main methods for analyzing tension clamp accidents include macroscopic examination, scanning electron microscopy and energy spectrum analysis, microstructure observation, and material analysis [8].This paper analyzes the physical and chemical inspection, the crimping process inspection and the verification of the crimping experiment of the internal steel core of the NY-125070 tension clamp for a special High-voltage direct current#Back to back line, the fracture mechanism of steel core in large section tension clamp is studied.

Physical and chemical testing
Anatomy was conducted on the tension clamp of a certain ultra-high voltage direct current transmission line with internal steel core fracture.Material testing, mechanical performance testing, and fracture analysis were performed on the fractured steel core.The fracture situation is shown in Figure1.The model of the strain clamp is NY-1250/70, and the internal wire specification is JL1/G3A-1250/70.

Steel core material inspection
The main chemical composition of the fractured steel core was tested, as shown in Table 1, and the results showed that the steel core material was a manganese containing alloy steel with a carbon content of approximately 0.7%.

Mechanical performance testing of steel core
Two sections of steel core centerline were selected from the old wire far from the fracture of the line for mechanical performance testing.The results of single wire mechanical performance testing are shown in Table 2.The tensile strength and elongation after fracture of the steel wire, as well as the stress at 1% elongation, all meet the requirements of GB/T 1179-2017 "Round Wire Concentric Hinged Overhead Conductors" standard.2, the fracture surface of the center line steel wire is dark brown, which shows that the steel core has been broken for a long time and there is surface rust in the natural environment.There are obvious radial cracks in the fracture radiation region, and the crack direction has a certain bending, which indicates that there are partial torsional stress in addition to the axial tensile stress.

Scanning electron microscopy analysis of steel core fracture surface
SEM micro morphology analysis was conducted on the fracture surface of the fractured steel core.Due to oxidation on the surface of the section, no obvious micro morphology features such as dimples were observed.However, at a large magnification, larger cracks and a large number of small secondary cracks can be observed on the section, as shown in Figure 3.In the process of working, the micro-hole will form the internal crack, which will accelerate the tensile fracture and reduce the performance of the material [9] .

Inspection of crimping dimensions
Observing the crimping marks of the sample tension wire clamp, it was found that there were a total of 8 indentations on the wire side of the clamp, and 3 indentations on the steel anchor side.The indentation from the wire side to the steel anchor side is defined as indentation 1 to indentation 11.The maximum size of the indentation on the wire side of the tension clamp is 68.87mm, the minimum size is 68.25m, and the average size is 68.53mm.According to the regulations of Q/GDW1571-2014, the maximum allowable distance between hydraulic pipes after pressing is S=0.86D+0.2(mm).
Therefore, the allowable distance between the edges of the NY-1250/70 clamp after crimping should be ≤ 69.00 mm.The opposite edge distance size of the failed tension clamp after crimping meets the standard requirements.

Crimping sequence inspection
From the observation of the indentation depth of the tension clamp, it was found that the 8 segments of indentation on the wire side of the tension clamp were sequentially crimped from the wire side to the steel anchor side; Among the three indentations on the steel anchor side of the tension wire clamp, the middle indentations are deeper than the indentations on both sides, as shown in Figure 4.It is judged that the middle section of the aluminum pipe on the steel anchor side was first pressed during construction, and then the area near the steel anchor pull ring or near the non pressing area was pressed.The pressing sequence at this point is inconsistent with the requirements of the specifications.

Crimping simulation experiment
According to the relevant standard requirements of the "Strain Clamp" [10] and the construction process requirements of the "Hydraulic Crimping Process Specification for Overhead Conductors and Ground Wires in Power Transmission and Transformation Engineering", the crimping sequence of aluminum pipes for large-section strain clamps is "reverse pressing" [11]: that is, starting from the pipe mouth on the wire side to the steel anchor side one by one, pressing to the marked point of the same side non pressing zone, passing through the "non pressing zone", and then pressing from the marked point of the non pressing zone on the steel anchor side to the pipe mouth on the steel anchor side, as shown in Figure 5.  Comparing the dimensions of the tension clamp after crimping between the validation experiment and the original line failure tension clamp, it was measured that the length of the tension clamp after crimping in the validation experiment was 845mm, with an average edge distance of 68.92mm;The length of the failed strain clamp on the original line is 875mm, with an average edge distance of 68.53mm.The comparison found that the overall length of the tension wire after compression obtained from the validation experiment was smaller than the overall length of the tension wire after compression due to the failure of the original line.It was analyzed that the thickness of the aluminum tube used in the validation experiment may not be consistent with that of the original line.The original thickness of the aluminum tube used in the validation experiment was greater than the original thickness of the aluminum tube used in the validation experiment.
Digital radiographic testing is a very intuitive detection method [12][13][14][15].X-ray inspection was conducted on the tension clamp after the simulation experiment crimping, and it was found that there was a suspected steel core fracture inside the clamp, as shown in Figure 6.Anatomy was conducted on the suspected area of steel core fracture in the tension clamp after the verification experiment compression.After dissection, it was found that there was a fracture in the outer ring steel core, with a fracture direction at an angle of about 45°with the steel wire axis.There was a small amount of plastic deformation at the fracture, and the fracture was relatively flat.

Simulation analysis
To effectively verify the cause of failure, simulation analysis was conducted using incorrect process compression simulation, which involves first pressurizing from the steel anchor side, and then sequentially compressing from the wire side to the steel anchor side.The contact area between the steel anchor and the aluminum pipe is constrained by MPC, which is a rigid connection between the steel anchor and the aluminum pipe, to simulate the effect of pressing the aluminum pipe and fixing the steel anchor on the steel anchor side.Then, 8 sets of pressure blocks are sequentially loaded, and the displacement distance of the pressure blocks is consistent with the actual indentation.The simulation outputs the stress level at the failure position of the steel core when 8 sets of pressure blocks are pressed down on the wire side.According to the axial stress cloud diagram, during the pressing process, the stress of the steel core at the 8th group of compression blocks will suddenly increase, as shown in Figure 7. From the simulation analysis results, it can be seen that when the reverse process is used to press the tension clamp, the steel core is close to the non compression area, and the equivalent stress of the steel core material below the pressure head is concentrated.The equivalent plastic strain is relatively large, and the steel core undergoes significant plastic deformation, posing a risk of fracture failure.

Conclusion
By conducting material analysis, mechanical performance testing, and fracture analysis on the fractured steel core of the tension clamp, and conducting material analysis and tensile testing on the wire steel core, the experimental results all meet the requirements of relevant standards; The fracture characteristics indicate that the steel core fracture belongs to ductile fracture.Due to the fact that the ductility of the steel core in the steel core aluminum stranded wire is much greater than that of the aluminum wire, it was found in the tensile strength test room that when the external load is greater than the tensile strength of the steel core aluminum stranded wire, the outer aluminum wire first breaks, and the inner steel core finally breaks.The surface of the internal steel wire fracture of the tension clamp in this case is dark brown, with surface rust phenomenon, indicating that the steel core has been fractured for a long time.Therefore, it is judged that the steel core at this location has formed a very obvious stress concentration phenomenon before fracture.
Upon inspection of the compression process of the tension wire clamp, it was found that the compression marks in the steel anchor area were deeper than those on both sides.It was determined that the compression sequence was not carried out in accordance with the standard sequence.The main reason for the internal steel core fracture of the tension wire clamp was the incorrect compression process, which was inconsistent with the specification requirements.This became the main reason for the internal steel core fracture of the tension wire clamp, This leads to varying degrees of residual tensile stress in the internal steel core of the tension clamp after crimping, which overlaps with the tension of the wire during operation, causing the steel core to fracture during operation.

Figure 1 .
Figure 1.X-ray imaging of strain clamp for a ± 800 kV ultra-high voltage DC line.

Figure 2 .
Figure 2. Macro view of fracture surface of fractured steel core.

Figure 3 .
Figure 3. Microscopic diagram of fracture surface of fractured steel core.

Figure 5 .
Figure 5. Schematic diagram of crimping sequence for large cross-section wire tension clamps.

Figure 6 .
Figure 6.X-ray imaging image of tension wire clamp after compression in simulation experiment.

Figure 7 .
Figure 7. Axial stress nephogram in the process of reverse crimping.

Table 2 .
Room temperature tensile test.
3 2.3.Analysis of fracture morphology of steel core single wire As shown in Figure