At present, there are various on-site treatment methods for the armor layer of low-voltage direct-buried cables, including single-end grounding and two-end grounding. There are also two ends that are left floating and not grounded. According to the different treatment methods of steel tape armoring at both ends of the cable, after the cable fails, the appearance of the fault point will be different.
The steel strips at both ends of the cable are all suspended and not grounded. After a short-circuit fault occurs in the cable, the breakdown point may only be a breakdown and burning hole in the local position of the cable line, which will not cause long-distance large-area burning and carbonization. Because when the cable is partially damaged by accidental mechanical damage and the insulation of the sheath is damaged, the system may not trip and power off immediately. Due to the action of moisture and moisture in the soil at the damaged point, the live wire will produce intermittent flashover discharge on the ground, which eventually develops into Permanent grounding and phase-to-phase short circuit trip and power failure. Since the discharge current of the live wire to the ground is limited to the damaged point of the cable, the discharge current does not form a branch circuit to the ground through the steel strip, so after the cable fails, there is generally only one point in the whole cable. Fault. However, the surface of the armored layer will be charged at this time. For the sake of safe electricity use, the exposed armored layer at both ends of the cable must be insulated and sealed.
The steel strip of the cable line adopts single-end grounding or double-end grounding method. After a short-circuit fault occurs in the cable, the fault may be a section of the cable, and the long-distance surface burning and carbonization adhesion may occur in the local area of the cable. Because the steel strip adopts this connection method, when a single-phase grounding fault occurs locally in the cable, a relatively large grounding short-circuit current will flow in the steel strip of the cable; at the same time, the three-phase load current of the cable will also appear unbalanced, and the steel There may also be eddy currents in the belt. After the two currents flow through the steel belt together, the steel belt will heat the sheath and insulation of the cable like a high-power electric furnace. Poor heat dissipation, excessive thermal resistance, accumulation of partially reserved coils in the cable, poor heat dissipation and other unfavorable reasons may cause long-distance and large-area burning and carbonization adhesion of cable insulation and sheath. The burnt area is relatively random, it may be near the fault point, or it may be in another section. Often, the section with the most difficult heat dissipation and the largest thermal resistance burns the most. The system may not trip until the single-phase grounding develops into a two-phase short circuit, and it cannot be reclosed for power transmission.
For low-voltage cable armored cables, it is necessary to strengthen the real-time online detection and monitoring of the three-phase current of the cable. At the same time, after the armored layer is grounded, the armored layer current transformer should be installed to monitor the current of the steel strip from time to time. The single-phase-to-ground short-circuit fault of the cable should be detected and dealt with in advance, so as to avoid the long-distance burning of the cable and cause unnecessary economic losses of electricity, and ensure the economical, reliable, stable and safe operation of the power grid.
According to normal analysis, after a short-circuit fault occurs in a directly buried low-voltage cable, there should generally be only one fault point. However, during the excavation process of the actual field cable fault point, it was found that two or more fault points may occur in the low-voltage cable fault, and at the same time, the long-distance insulation sheath may be heated, burned, and carbonized. Adhesion phenomenon. The author believes that the difference in the failure phenomenon of low-voltage armored cables may be related to the grounding or ungrounding of the cable armor, and the views and opinions are not necessarily correct. It is hoped that professionals with sincere insights into such phenomena can provide more scientific and authoritative analysis and views. to uncover the underlying reasons for this phenomenon.