Cause of aging
1. External force damage. From the operational analysis in recent years, a considerable number of cable failures are caused by mechanical damage.
2. Insulation is damp. This situation is also common and generally occurs at cable joints in direct buried or drained pipes. For example, if the cable joint is unqualified and the joint is made under humid climatic conditions, the joint will be filled with water or mixed with water vapor. After a long period of time, water branches will form under the action of the electric field, which will gradually damage the insulation strength of the cable and cause malfunction.
3. Chemical corrosion. The cable is directly buried in the acid-base area, which often causes the cable armor, lead or outer sheath to be corroded. The protective layer is chemically corroded or electrolytically corroded for a long time, causing the protective layer to fail and the insulation to be lowered. Cable failure. Chemical: The corrosion of the cable in the unit is quite serious.
4. Long-term overload operation. Overload operation, due to the thermal effect of the current, the load current will inevitably lead to the conductor heating when passing through the cable. At the same time, the skin effect of the charge and the eddy current loss of the steel crucible and the loss of the insulating medium will also generate additional heat, thereby increasing the cable temperature.
5. The cable connector is faulty. Cable joints are the weakest link in cable lines, and cable joint failures caused by direct personnel failure (poor construction) often occur. During the process of making cable joints, if the joints are not tightly crimped or insufficiently heated, the insulation of the cable head will be reduced, causing an accident.
6, environment and temperature. The external environment and heat source where the cable is located can also cause excessive cable temperature, insulation breakdown, and even an explosion.
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, the moisture invades the cable inside, the cable insulation performance is degraded; when the cable is laid, the protective measures are not taken, the protective layer is destroyed, 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 destroyed by long-term extrusion. Mainly the mechanical barbaric construction in municipal construction. Digging and cutting cables.
d. corrosion. The protective layer has been subjected to chemical corrosion or cable corrosion for a long time, resulting in failure of the protective layer and lowering 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. Loop, thereby exciting 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. Insulation flashover, lightning arrester explosion, electrical equipment damage. In some cases, the phase sequence of the load transformer may reverse, and the overvoltage may be transmitted to the low voltage side of the transformer, causing harm.
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 network or dual power supply.
(5) Adding an interphase capacitor on the distribution side,
The principle is: the capacitor 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 phase-to-phase capacitance △C on the distribution side , so that 8[Co+ 3(C U+ A0)/Ca increases, thereby increasing the equivalent capacitance C and the equivalent electromotive force Eo.
(6) Excitation characteristics are adopted. Better transformers help reduce the chance of disconnection and overvoltage.
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