Analysis of Operation Failure of Armored Cable

At present, there are various on-site treatment methods for the armor layer of low-voltage direct-buried cables, including single-ended grounding and two-end grounding. There are also those that are not grounded at both ends. According to the different treatment methods of the steel tape armor at both ends of the field 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 burn-through hole in a local location of the cable line, which will not cause long-distance and large-area burning and carbonization. Because when the cable is partially damaged by accidental mechanical damage and the sheath insulation is damaged, the system may not immediately trip and power off. The damaged point will cause intermittent flashover discharge on the earth due to the action of moisture and moisture in the soil, which will eventually develop into Permanent grounding and short-circuit between phases cause a trip and power failure. Since the discharge current of the live wire to the ground is limited to the location of the damaged point of the cable, the discharge current does not form a branch circuit to the ground through the steel belt, so there is generally only one point in the entire cable after the cable fails. malfunction. However, the surface of the armor layer will be electrified at this time. In consideration of safe use of electricity, the armor layer exposed at both ends of the cable must be insulated and sealed.

The steel belt of the cable line adopts single-ended grounding or double-ended grounding. After a short-circuit fault occurs in the cable, the fault may be a section of the cable, and the local area of ​​the cable may have long-distance surface burnt and carbonized adhesion. Because after the steel belt adopts this connection method, when a single-phase ground fault occurs in the cable, a relatively large ground short-circuit current will flow in the steel belt of the cable; at the same time, the three-phase load current of the cable will also appear unbalanced. Eddy current may also occur in the belt. After the two currents flow through the steel belt, the steel belt will be like a high-power electric furnace, heating the cable sheath and insulation, and the customer’s switch selection is not appropriate, and the soil is localized. Poor heat dissipation, excessive thermal resistance, partial accumulation of cable coils, poor heat dissipation and other unfavorable reasons may cause long-distance and large-area burning and carbonization of the cable insulation and sheath. The burned area is relatively random. It may be near the fault point, or in another section. It is often the section with the most difficult heat dissipation and the section with the largest thermal resistance that burns the most. The system may not trip until the single-phase grounding develops into a two-phase short circuit and cannot be reclosed to transmit power.

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 armor layer is grounded, an armor layer current transformer should be installed to monitor the current of the steel strip from time to time. The single-phase grounding short-circuit fault of the cable should be detected and dealt with in advance to avoid the long-distance burning of the cable and cause unnecessary power economic loss, and to ensure the economy, reliability, stability and safety of the grid operation.
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 actual field excavation and processing of cable fault points, it is found that low-voltage cable faults may have two or more fault points, and may also be accompanied by long-distance insulation sheath heating and burning and carbonization adhesion. The author believes that the difference in the failure of low-voltage armored cables may be related to the grounding or non-grounding of the cable armor, and the opinions and opinions may not be correct. It is hoped that professionals who have sincere insights into such phenomena can provide more scientific and authoritative analysis and opinions. To uncover the underlying cause of this phenomenon.