Cross-linked polyethylene (XLPE) cable is an important part of the Chinese power system, and it plays a vital role in the transmission of power resources, therefore ensuring the normal operation of its lines is critical. According to some relevant cable fault statistics, the main cause of XLPE cable failure is a poor quality anti-stress cone in the cable joint, which is because all parameters of the anti-stress cone of cable joints are currently measured manually using contact measuring tools, such as tape measure. This measurement method has large errors and is prone to causing secondary damage to the measured object. The other cause is some nonstandard joints whose anti-stress cone size does not meet the design requirements are often connected to the power system. Under long-term high-voltage action, nonstandard joints can cause partial discharge due to insufficient resistance to axial stress, resulting in the insulation breakdown. Existing noncontact parameter measurement methods are difficult to apply to the parameter measurement of anti-stress cones. Therefore, we combined the structural characteristics of the cable joint with three-dimensional (3D) point cloud processing to propose a parameter measurement algorithm for the anti-stress cone of the XLPE cable joint, which can realize the effective measurement of all parameters of the cable joint anti-stress cone, which is critical to ensure safe and reliable operation of the power system.

First, this algorithm performs denoising and coordinate adjustment on a cable joint point cloud. Second, the target point cloud of the anti-stress cone and its adjacent area is obtained according to the XLPE cable joint characteristics. Then, the target point cloud is divided into strips and pieces using the point cloud space division method based on the angle and height information between each point and the coordinate axis in the target point cloud. Following that, the local point clouds of different regions on each strip point cloud are obtained using the included axis angle of the piece normal vector on the strip point cloud and the improved concave-convex criterion. On this basis, the random sample consensus (RANSAC) algorithm and Lagrange multiplier method are used to obtain the intersection line of the adjacent plane, and preliminary measurement results are obtained on the basis of the distance relationship between each point on the strip point cloud and the intersection line. Finally, residual estimation is used to correct the error of the preliminary measurement results to obtain the final measurement results.

The proposed XLPE cable joint anti-stress cone parameter measurement algorithm has high-measurement accuracy and robustness. When it measures the parameters of the cable joint anti-stress cones with standard size, the absolute error is less than 0.2 mm, and the relative error is less than 0.5%; when it measures the parameters of the anti-stress cones of the cable joints polished by different technicians, the absolute error is less than 1.0 mm, and the relative error is less than 1.5% (Table 4), which meets the industry measurement accuracy requirements. Compared with the radius change method, it has higher measurement accuracy (Table 2). To address the problem that the number of points between the pieces obtained using the existing point cloud space division method is relatively large, resulting in the instability of local features, a new point cloud space division method that can achieve a good division effect is proposed (Figs. 6 and 7, Table 3), the best angle range of strip division is 4°-6° (Table 6), and the best setting constant range of piece division is 0.4-0.6 mm (Table 5). To address the problem that the initial measurement value is shifted to the anti-stress cone region due to the structure of the cable joint, a residual estimation error correction method is proposed, which effectively improves the measurement accuracy of the algorithm (Fig. 12, Table 4). The optimal threshold range of D_th3 and D_th4 is 0.35-0.45 mm (Tables 7 and 8).

In this paper, we proposed a 3D point cloud-based algorithm for measuring the anti-stress cone parameters of XLPE cable joints. The target point cloud is obtained by the proposed obtaining method; on this basis, the measurement of the parameters of the anti-stress cone of the cable joint is realized, which reduces the interference of the nontarget area and improves the processing efficiency of the algorithm. The proposed cable joint with a quasi-cylindrical structure ensures the consistency of the piece element s description of the measured object s characteristics. The original concave-convex criterion is improved to eliminate the influence of the piece elements in the transition area based on the changes in the included angle of the piece elements on the strip point cloud. The robustness of the algorithm is ensured using a preliminary measurement method based on the structural characteristics of the anti-stress cone. To avoid the problem that the preliminary measurement results are shifted to the anti-stress cone region due to the arc-shaped transition structure at the junction point of the anti-stress cone, the residual estimation error correction method is used to improve the measurement accuracy of the algorithm. Two types of cable joint point clouds with standard dimensions and surface defects were used for measurement experiments, and the experimental results show that the proposed algorithm has high-measurement accuracy and robustness.