I. Causes of Cable Aging
1. External damage. Based on operational analysis in recent years, especially in Pudong, Shanghai, where the economy is developing rapidly, a significant number of cable faults are caused by mechanical damage.
2. Insulation dampness. This is also a common problem, usually occurring at cable joints in direct burial or conduit installations. For example, substandard cable joint construction or jointing in humid conditions can cause water or water vapor to enter the joint. Over time, under the influence of the electric field, water trees will form, gradually damaging the cable's insulation strength and causing a fault.
3. Chemical Corrosion. When cables are directly buried in areas with acidic or alkaline conditions, the cable armor, lead sheath, or outer protective layer often corrodes. Long-term exposure to chemical or electrolytic corrosion can cause the protective layer to fail, reducing insulation and leading to cable faults. For example, the cable corrosion situation at one unit was quite severe.
4. Long-term overload operation. Under overload operation, due to the thermal effect of current, the conductor will inevitably heat up when the load current passes through the cable. At the same time, the skin effect of the charge, as well as the eddy current loss of the steel armor and the dielectric loss of the insulation, will also generate additional heat, thereby increasing the cable temperature.
5. Cable Joint Failure. Cable joints are the weakest link in a cable line, and failures caused by direct human error (poor construction) are frequent. If construction workers fail to tighten the joint properly or heat it insufficiently during the cable joint fabrication process, the cable head insulation will decrease, leading to an accident.
6. Environment and temperature. The external environment and heat sources around the cable can also cause the cable temperature to become too high, insulation to break down, or even explode and catch fire.
II. Common Cable Faults
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.
Mainly:
a. Overload operation. Long-term overload operation will cause the cable temperature to rise, the insulation to age, and eventually the insulation to break down, reducing the quality of construction.
b. Electrical aspects include: inadequate cable head construction techniques, poor cable head sealing, allowing moisture to penetrate the cable and reducing its insulation performance; failure to take protective measures during cable laying, resulting in damage to the protective layer and reduced insulation.
c. Civil engineering issues include: poor drainage in manholes and trenches, causing cables to be submerged in water for extended periods, damaging insulation strength; manholes that are too small, resulting in insufficient bending radius for cables, leading to long-term damage from external pressure. The main problem is the rough handling of machinery during municipal construction, which damages or breaks cables during excavation.
d. Corrosion. Long-term exposure to chemical corrosion or cable corrosion can cause the protective layer to fail and reduce insulation.
e. Poor quality of the cable itself or cable head accessories, poor sealing of the cable head, melting or cracking of the insulation glue, leading to the resonance phenomenon in the station. The line breakage fault causes the phase-to-phase capacitance and the capacitance to ground of the line to form a resonant circuit with the excitation inductance of the distribution transformer, thereby exciting ferroresonance.
III. The Hazards of Resonance Caused by Open Circuit Faults
In severe cases, open-circuit resonance, where high-frequency and fundamental frequency resonances overlap, can cause overvoltage amplitudes to reach 2.5 times the phase voltage [P]. This can lead to neutral point displacement, overvoltage in 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 be transmitted to the low-voltage side of the transformer, causing harm.
The main measures to prevent open-circuit 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 line breakage.
(3) Do not leave unloaded transformers on the line for extended periods.
(4) Use a ring network or dual power supply.
(5) Add interphase capacitance on the distribution transformer side.
The principle is: using a capacitor as an energy-absorbing element to absorb energy in the transient process, thereby reducing the impact disturbance intensity and suppressing the occurrence of resonance. s一(o+ 3C，，) 1C., add interphase capacitance △C on the distribution transformer side, so that 8一[Co+ 3(C U+ A0)/Ca increases, 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.
