Common cable fault analysis

Common cable line faults include mechanical damage, insulation damage, insulation dampness, insulation aging and deterioration, overvoltage, and cable overheating. When any of these faults occur, the power supply to the faulty cable should be cut off, the fault point should be located, the fault should be inspected and analyzed, and then repairs and tests should be carried out. Any cables that need to be cut should be removed. Power supply can only be restored after the fault has been eliminated. The most direct cause of cable failure is insulation degradation leading to breakdown.

The main causes include:
a) Overload operation: Long-term overload operation will cause the cable temperature to rise, insulation to age, and eventually insulation breakdown, reducing construction quality.
b) Electrical factors: Inadequate cable termination construction, poor cable termination sealing, moisture intrusion into the cable, and decreased cable insulation performance; failure to take protective measures during cable laying, resulting in damage to the protective layer and reduced insulation.
c) Civil engineering factors: Poor drainage in manholes and trenches, long-term immersion of cables in water, damaging insulation strength; manholes that are too small, insufficient cable bending radius, and long-term damage from external pressure. This is mainly due to rough handling by machinery during municipal construction, damaging or breaking cables.
d) Corrosion: Long-term chemical corrosion or cable corrosion of the protective layer causes it to fail, reducing insulation. e. Poor quality of the cable itself or cable head accessories, poor sealing of the cable head, melting and cracking of the insulation glue, leading to the resonance phenomenon in the station. The line breakage fault causes the phase-to-phase capacitance and ground capacitance of the line to form a resonant circuit with the excitation inductance of the distribution transformer, thereby exciting ferroresonance. Hazards of resonance caused by line breakage faults In severe cases, the high-frequency resonance and the fundamental frequency resonance superimpose, which can make the overvoltage amplitude reach 2.5 times the phase voltage [P], which may lead to the displacement of the neutral point of the system, overvoltage of the windings and conductors, and in severe cases, insulation flashover, surge arrester explosion, and damage to electrical equipment. In some cases, the phase sequence of the load transformer may be reversed, and the overvoltage may also be transmitted to the low-voltage side of the transformer, causing damage. The main measures to prevent overvoltage caused by line breakage resonance are: (1) Do not use fuses to avoid non-full-phase operation. (2) Strengthen the inspection and maintenance of the line to prevent the occurrence of line breakage. (3) Do not leave unloaded transformers on the line for a long time. (4) Use a ring network or dual power supply. (5) Add interphase capacitors on the distribution transformer side. The principle is: Use capacitors as energy-absorbing elements to absorb energy in the transient process, thereby reducing the impact disturbance intensity and suppressing the occurrence of resonance. Add interphase capacitor ΔC on the distribution transformer side to increase 8-[Co+ 3(C U+ A0)/Ca, thereby increasing the equivalent capacitance C and the equivalent electromotive force Eo. The required capacitance value can be calculated according to the method in reference [6]. (6) Using a transformer with better excitation characteristics helps to reduce the probability of open circuit overvoltage.