Analysis of the Faults in the Operation of Low Voltage Directly Buried Armoured Cables

Analysis of the Faults in the Operation of Low Voltage Directly Buried Armoured Cables

At present, the on-site treatment methods for low-voltage buried cable sheaths are various, ranging from single-end grounding to grounding at both ends. There are also two ends are not open to ground. According to the different treatment methods of steel strips at both ends of the field cable, after the cable fails, the appearance of the fault point will be different.

Both ends of the cable are 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 part of the cable line, and it will not cause long-distance burning and charring in large areas. When the insulation of the jacket is damaged due to accidental mechanical damage to the cable, the system may not immediately trip or power off. Due to moisture and moisture in the soil, the fire line will produce intermittent flashover discharge on the earth and eventually develop into Permanent grounding and phase-to-phase short-circuit trip and power failure, because the firing current to the ground is limited to the location of the damage point of the cable, the discharge current through the steel belt does not form a branch circuit to the earth, so the cable is generally only one point after the failure of the cable malfunction. However, at this time, the surface of the armored layer will be electrified and it is considered to be safe for electricity. The armored layers exposed at both ends of the cable must be insulated and sealed.

The steel wire of the cable line adopts the single-end grounding or the double-end grounding. After a short-circuit fault occurs on the cable, the fault may be a section of the cable. The long-distance surface burns and carbonized and sticks may occur in the local area of ​​the cable. Because the steel strip adopts this connection method, when a single-phase earth fault occurs in the cable, a relatively large short-circuit current flows in the steel strip of the cable; at the same time, the three-phase load current of the cable may also appear unbalanced phenomenon in the steel. The vortex phenomenon may also occur in the belt. After the two electric currents flow through the steel belt together, the steel belt will be like a high-power electric furnace, heating the sheathing and insulation of the cable, plus the improper selection of the customer's switch. Heat is not good, thermal resistance is too large, the cable is partially reserved rims, heat dissipation is not good and other unfavorable reasons, it may cause cable insulation, sheath long-distance large area burned carbonized adhesion phenomenon. The burned area is relatively random, may be near the fault point, and may also be in another section, and is often the most difficult to dissipate heat, and the section with the largest thermal resistance is most severely burned. The system may not trip until the single-phase grounding develops into a two-phase short circuit, and it is not possible to reconnect the power supply.

For low-voltage cable armored cables, it is necessary to strengthen the real-time online monitoring and monitoring of the three-phase current of the cable. At the same time, after the braided layer is grounded, an armored layer current transformer should be installed to monitor the current of the strip. The single-phase ground short-circuit fault occurred on the cable is discovered and dealt with in advance to avoid long-distance burn-out of the cable, resulting in unnecessary power economic loss and ensuring the economical, reliability, stability, and safety of the power grid operation.

According to normal analysis, after a short-circuit fault occurs in a directly buried low-voltage cable, the fault point should generally be only one. However, during actual excavation processing of cable fault points, it was found that two or more faults may occur in the fault of the low-voltage cable, and at the same time, there may be a phenomenon that the long-distance insulation sheath burns and burns and carbonized and adhered. The author believes that the different phenomena of failure of the low-voltage armored cable may be related to the grounding or non-grounding of the cable armor, and the viewpoints and views are not necessarily correct. Those who wish to have a genuine insight into such phenomena can put forward more scientific and authoritative analysis and views. To uncover the underlying causes of this phenomenon.

Analysis of the Faults in the Operation of Low Voltage Directly Buried Armoured Cables

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