How to solve the common problems that occur in the production of power cables for rail transit

The pace of rail transit construction in our country has accelerated, and the increase in the mileage of railway operation has driven the expansion of the demand for rail transit cables, and the demand for wires and cables has also risen accordingly. Due to the particularity of rail transit construction, higher requirements have been put forward for the safety, environmental protection and other technical levels of wire and cable products. In addition to having excellent electrical performance and insulation performance, wire and cable must also have good flame retardancy, water resistance and rodent and termite resistance. Therefore, compared with traditional power cables, this product has undergone new updates in both material selection and process routes.

Power cables are cables used for transmitting and distributing electrical energy. They are commonly applied in urban underground power grids, outgoing lines from power stations, internal power supply in industrial and mining enterprises, and underwater power transmission lines across rivers and seas. In power lines, the proportion of cables is gradually increasing. Power cables are cable products used in the main lines of power systems to transmit and distribute high-power electrical energy, including various voltage levels from 1 to 500KV and above, as well as various types of insulation power cables.

The main problems that occur in the manufacturing process of power cables for rail transit include insulation eccentricity, unevenness of the comprehensive sheath and high breakdown rate of electric sparks. The surface of halogen-free, low smoke and flame-retardant polyolefin materials is not smooth and has pores, etc. After repeated analysis and experiments, it is believed that the main causes of these problems are as follows:

Insulation eccentricity occurs during three-layer co-extrusion.

Due to the relatively thick insulation thickness and the small contact area between the insulation and the conductor, under the high-temperature and high-pressure conditions inside the cross-linked pipe, the plastic is in a viscous flow state and will sag to a certain extent. This makes it easy for the eccentricity to be substandard.

The comprehensive protective layer is uneven and the electric spark breakdown rate is high.

The main causes of this problem are:

(1) The longitudinal wrapping aluminum-plastic tape is subjected to uneven force during the forming process, resulting in wrinkles on the longitudinal wrapping surface.

⑵ When the aluminum-plastic composite tape was longitudinally wrapped, the lap joint was not bonded, resulting in warping.

⑶ Flanging of aluminum-plastic tape

⑷ Foaming, perforation, etc. of halogen-free, low smoke and flame-retardant polyolefin materials;

The surface of halogen-free, low smoke and flame-retardant polyolefin materials is not smooth and has pores.

The main causes of this problem are:

(1) Improper storage of raw materials leads to moisture absorption;

⑵ Due to the relatively low processing temperature of this material, an excessively high extrusion temperature will cause the material to foam.

⑶ Due to its high flame retardancy, a large amount of fillers are added to the raw materials, which leads to an uneven appearance and cracks during extrusion.

The solutions to the insulation eccentricity problem during three-layer co-extrusion are:

Cross-linked polyethylene plastic is in a viscous flow state inside the cross-linked pipe. Dow Chemical's low-sag 35KV grade cross-linked insulating material from the United States can be used to increase the viscosity of the plastic itself, thereby reducing the sagging of the material in the high-temperature molten state. Additionally, an online eccentricity meter should be installed on the three-layer co-extrusion equipment to ensure real-time monitoring. Finally, after the insulating core cools down, it should be sliced in time to observe the roundness. In this way, The insulation performance and eccentricity of the product have been effectively controlled, fully meeting the requirements of Changsha Metro with an eccentricity within 5%.

The solutions to the problems of uneven comprehensive protective layer and high electrical spark breakdown rate are:

This phenomenon is mainly due to the relatively large outer diameter of the cable core. During the longitudinal wrapping of the aluminum strip, uneven force is applied due to deformation, resulting in unevenness after the longitudinal wrapping of the aluminum strip. Firstly, there are two sets of longitudinal wrapping molds for the aluminum-plastic composite strip, from large to small. Before production, the size of the molds must be strictly checked to prevent difficulties in forming the aluminum-plastic strip caused by overly large molds. If the aluminum-plastic tape is not on the same horizontal line from deformation to formation during production, wrinkles and unevenness will occur. At this time, the positions of the front and rear molds should be adjusted in time to ensure the flatness of the aluminum-plastic tape. The flanging of aluminum-plastic tape is often due to the misalignment of the wire laying frame and the forming mold of the aluminum-plastic tape. The wire laying frame needs to be strictly fixed to the floor. For the problem of non-adhesion at the lap joint of aluminum-plastic tape, timely adjustment of the position and temperature of the hot air gun can effectively solve this issue.

The solutions to the problems such as rough surface, foaming, porosity and perforation of halogen-free, low smoke and flame-retardant polyolefin materials are:

First, strictly prevent impurities from being mixed into the rubber compound. Upon entering the factory, check if the material packaging is damaged. If any damage is found, clean and handle it promptly. Second, it is necessary to prevent plastic from getting damp. If the weather is humid, the plastic should be dried for at least 4 hours before extrusion. Thirdly, we improved the extrusion die. During production, we found that the surface of the low smoke zero halogen flame retardant polyolefin material was not smooth when extruded with a common die. After analysis, it was believed that the working surface of the die sleeve was relatively long, and the plastic stretching led to a large internal stress. Therefore, we designed a special die to reduce the working surface of the die sleeve and adopted a water bath for segmented cooling from high temperature to low temperature to eliminate the internal stress. Fourth, a low compression ratio screw is adopted to reduce the shear heat of the screw. 5. Control the stretch ratio and keep it below 2.0 to reduce the risk of delamination due to overcrowding of the sheath. After trial production, the appearance problem was effectively solved. Finally, by taking the above measures, problems such as uneven surface, foaming, porosity and breakdown have been effectively solved.