Common cable failure analysis
Common faults in cable lines are mechanical damage, insulation damage, insulation moisture, insulation aging deterioration, overvoltage, cable overheating faults, etc. When the above-mentioned fault occurs on the line, the power supply of the faulty cable should be cut off, the fault point should be found, the fault should be inspected and analyzed, and then repaired and tested. The cut-off can be resumed after the fault is eliminated.
The most direct cause of cable failure is the breakdown of insulation and breakdown.
a, overload operation: long-term overload operation, will increase the cable temperature, insulation aging, resulting in breakdown of insulation, reducing construction quality.
b. Electrical aspects: the cable head construction process can not meet the requirements, the cable head sealing performance is poor, moisture intrusion into the cable, the cable insulation performance is degraded; when the cable is laid, the protective measures are not taken, the protective layer is damaged, and the insulation is reduced.
c. In terms of civil works, the drainage of the wells and pipes is not smooth, the cables are soaked by water for a long time, and the insulation strength is damaged; the work well is too small, the bending radius of the cable is not enough, and the external force is damaged by long-term extrusion, mainly the mechanical barbaric construction in municipal construction. Digging and cutting cables.
d. Corrosion, the protective layer is subject to chemical corrosion or cable corrosion for a long time, resulting in failure of the protective layer and reduction of insulation.
e, the cable itself or the cable head accessory is of poor quality, the cable head is poorly sealed, the insulating glue is dissolved, and the cracking occurs, resulting in the resonance phenomenon of the station. The line disconnection fault causes the line phase capacitance and the ground capacitance to resonate with the distribution transformer excitation inductance. The loop, thereby exciting the ferromagnetic resonance.
Breakage failure causes resonance hazard
In the severe case, the high frequency and the fundamental frequency resonance are superimposed, which can make the overvoltage amplitude reach 2.5 times of the phase voltage [P], which may cause the neutral point of the system to be displaced, and the winding and the wire may be overvoltage. The insulation flashes, the arrester explodes, and the electrical equipment is damaged. In some cases, the load transformer phase sequence may reverse and may also pass an overvoltage to the low side of the transformer, causing a hazard.
The main measures to prevent disconnection resonance overvoltage 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 disconnection.
(3) Do not hang the no-load transformer on the line for a long time.
(4) Powered by ring or dual power supply.
(5) Add an interphase capacitor on the distribution side.
The principle is:
Capacitance is used as the energy absorbing element to absorb the energy in the transient process, thereby reducing the impact disturbance intensity to suppress the occurrence of resonance. s one (o + 3C,,) 1C., add the phase-to-phase capacitance △C on the distribution side, so that 8 [Co+ 3(C U+ A0)/Ca increases, so that the capacitance required to increase the equivalent capacitance C and the equivalent electromotive force Eo can be obtained according to the method in . (6) The use of a transformer with better excitation characteristics helps to reduce the probability of occurrence of disconnection and overvoltage.