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Causes of CABLE aging

by:AAA     2021-03-04

Causes of aging
1. External force damage. Judging from the operational analysis in recent years, especially in Pudong, Shanghai, where the economy is developing rapidly, a considerable number of cable failures are caused by mechanical damage.

2. The insulation is damp. This situation is also very common, and generally occurs at the cable joints in direct burial or piping. For example: unqualified cable joints and joints made in humid weather conditions will cause the joints to enter water or water vapor. For a long time, water branches will be formed under the action of the electric field, which will gradually damage the insulation strength of the cable and cause failure.

3. Chemical corrosion. The cable is directly buried in the area with acid and alkali, which will often cause the cable armor, lead skin or outer sheath to be corroded. The protective layer suffers from chemical corrosion or electrolytic corrosion for a long time, resulting in the failure of the protective layer and the reduction of insulation. The cable is faulty. Chemical: The unit's cable corrosion situation is quite serious

4. Long-term overload operation. Overload operation, due to the thermal effect of the current, the conductor heating will inevitably be caused when the load current passes through the cable. At the same time, the skin effect of the charge, the eddy current loss of the steel armor, and the insulation loss will also generate additional heat, which will increase the temperature of the cable.

5. The cable connector is faulty. The cable joint is the weakest link in the cable line, and the cable joint failures caused by the direct fault of the personnel (poor construction) often occur. In the process of making cable joints, if there are reasons such as insufficient joint crimping, insufficient heating, etc., the insulation of the cable head will be reduced, which will cause an accident.

6. Environment and temperature. The external environment and heat source of the cable can also cause the cable to overheat, insulation breakdown, and even explosion and fire.

Common failures of folding
Common faults of cable lines include mechanical damage, insulation damage, insulation damp, insulation aging deterioration, overvoltage, cable overheating faults, etc. When the above-mentioned fault occurs in the line, the power supply of the faulty cable should be cut off, the fault point should be found, the fault should be checked and analyzed, and then repair and test should be carried out. The power supply can be restored after the removal of the fault.

The most direct cause of cable failure is breakdown due to insulation degradation.

There are:

a. Overload operation. Long-term overload operation will increase the temperature of the cable and age the insulation, which will lead to breakdown of the insulation and reduce the quality of construction.

b. Electrical aspects include: the cable head construction process fails to meet the requirements, the cable head has poor sealing performance, moisture penetrates into the cable, and the cable insulation performance is reduced; when the cable is laid, protective measures are not taken, the protective layer is damaged, and the insulation is reduced.

c. The civil works include: poor drainage of pipe trenches in industrial wells, long-term water soaking of the cables, which damages the insulation strength; too small wells, insufficient bending radius of the cables, and long-term damage by external squeezing forces. Mainly due to brutal mechanical construction in municipal construction. Cut and cut the cable.

d. Corrosion. The protective layer suffers from chemical corrosion or cable corrosion for a long time, causing the protective layer to fail and the insulation to decrease.

e. The quality of the cable itself or the cable head accessories is poor, the cable head is poorly sealed, the insulating glue is dissolved, and cracked. The resonance phenomenon in the station is the line disconnection. The line phase capacitance and the ground capacitance and the distribution transformer excitation inductance form resonance Loop to excite the ferromagnetic resonance.

Harm of resonance caused by disconnection fault

Disconnection resonance In severe cases, the superposition of high frequency and fundamental frequency resonance can make the overvoltage amplitude reach 2.5 times the phase voltage [P], which may cause the neutral point of the system to shift, and the windings and wires may experience overvoltage. Insulation flashover, lightning arrester explosion, electrical equipment damage. In some cases, the phase sequence of the load transformer may be reversed, and overvoltage may be transmitted to the low-voltage side of the transformer, causing harm.

The main measures to prevent disconnected resonant overvoltage are:

(1) No fuse is used 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) Adopt ring network or dual power supply.

(5) Add interphase capacitance on the distribution transformer side,

The principle is: the use of capacitors as energy-absorbing components to absorb the energy in the transient process, thereby reducing the intensity of impact disturbance to suppress the occurrence of resonance. s一(o+ 3C,,) 1C., additional phase-to-phase capacitance △C on the distribution transformer side , Increase 8一[Co+ 3(C U+ A0)/Ca, thereby increasing the equivalent capacitance C and the equivalent electromotive force Eo. The required capacitance value can be obtained according to the method in [6]. (6) Using excitation characteristics A better transformer helps to reduce the chance of breakage and overvoltage.


General principle

The rated voltage of the cable is equal to or greater than the rated voltage of the network where it is located, and the maximum working voltage of the cable must not exceed 15% of its rated voltage. In addition to the use of copper core cables in places that require movement or severe vibrations, aluminum core cables are generally used. Cables laid in cable structures should be bare-armored cables or aluminum-clad bare plastic sheathed cables. Directly buried cables use armored cables with sheath or aluminum-clad bare plastic sheathed cables. Heavy-duty rubber-sheathed cables are used for mobile machinery. Corrosive soils generally do not use direct burial, otherwise special anti-corrosion layer cables should be used. In places with corrosive media, the corresponding cable sheath should be adopted. For laying cables vertically or at places with large height differences, non-drip cables should be used. Rubber insulated cables should not be used when the ambient temperature exceeds 40℃.

Section verification

(1) Choose cables according to voltage: Choose according to the first of the above-mentioned general principles.

(2) Choose the cable section according to the economic current density: the calculation method is the same as that of the wire section.

(3) Check the cable cross section Iux≥Izmax according to the maximum long-term load current of the line

Where: Iux-allowable load current of the cable (A);

Izmax-the long-term maximum load current (A) in the cable.

We use this selection method the longest in our daily work. Usually, we first find the working current of the line, and then according to the maximum working current of the line, it should not be greater than the allowable current carrying capacity of the cable. The allowable long-term working current of the cable is shown in Table 1.

We often encounter this situation in actual work. Due to the increase in load and the increase in load current, the original cable has insufficient current carrying capacity and runs over current. In order to increase the capacity, considering the normal operation of the original cable, it is necessary to re-lay the cable. The construction is difficult and uneconomical, and we often adopt double or even triple merging.

In the choice of combined cables, many people think that the smaller the cable cross-section, the more economical and reasonable, as long as the current-carrying capacity requirements are met. Is this actually the case?

On January 3, 2006, the main cable from the 1# transformer to the power distribution room exploded. Two of the original 185mm four-core aluminum core cables exploded. In order to restore the power supply in time, the work area kept the other good cable and merged the two cables. A 120mm four-core aluminum core cable is used for power supply. After 10 months of operation, the main cable burst again on November 15, 2006. After inspection, it was found that the 185mm cable burst caused the accident.

Why did this accident happen? According to Table 1, we can find that the safe current carrying capacity of the three cables used together is 668A, and the maximum load current measured by the clamp-type ammeter is only 500A in the living area. According to the principle of Iux≥Izmax, this operation It should be safe and reliable. However, we ignore that the cable has resistance, because when the multi-parallel cable is connected, the contact resistance is different at the connection, and this contact resistance is often comparable to the resistance of the cable itself. As a result, the current distribution of the multi-parallel cable will be inconsistent. The current distribution of balanced, multi-parallel cables is related to the impedance of the cable.

Rough calculation of copper wire interface: S=IL/54.4U (S wire cross-sectional area in millimeters)

Rough calculation of aluminum wire interface: S=IL/34U

Resistance calculation

The DC standard resistance of the cable can be calculated according to the following formula:

R20=ρ20(1+K1)(1+K2)/∏/4×dn×10

In the formula: R20-the standard resistance of the branch current of the cable at 20°C (Ω/km)

ρ20--Resistivity of wire (at 20℃) (Ω*mm/km)

d--The diameter of each core wire (mm)

n--number of cores;

K1-core wire twist rate, about 0.02-0.03;

K2-the twisting rate of multi-core cables, about 0.01-0.02.

The actual AC resistance per kilometer of cable at any temperature is:

R1=R20(1+a1)(1+K3)

In the formula: a1-the temperature coefficient of resistance at t ℃;

K3-the coefficient that takes into account the skin effect and proximity effect, 0.01 when the cross-sectional area is less than 250 mm; 0.23-0.26 when the cross-sectional area is 1000 mm.

Capacitance calculation

C=0.056Nεs/G

In the formula: C-cable capacitance (uF/km)

εs-relative permittivity (standard is 3.5-3.7)

N--the number of hearts of the multi-core cable;

G--form factor.

Inductance calculation

For underground cables for power distribution, when the conductor cross-section is round, and the loss of armor and lead-cladding is neglected, the inductance calculation method of each cable is the same as that of the wire.

L=0.4605㏒Dj/r+0.05u

LN=0.4605㏒DN/rN

Where: L--inductance of each phase wire (mH/km)

LN-the inductance of the neutral wire (mH/km);

DN--the geometric distance between the phase line and the neutral line (cm);

rN-the radius of the neutral line (cm);

DAN, DBN, DCN-the center distance (cm) between the neutral line of each phase line.

illustration

The measured load current of work area 2# living variable load is 330A, the existing cable is a 120mm four-core copper core cable, and the safe current carrying capacity is 260A after checking the table. The cable is overloaded and there are hidden dangers of unsafe operation. In order to ensure the normal power supply, our work area It is planned to split the current with another cable to ensure normal power supply. (The cables mentioned below refer to 1KV, VV type armored polyethylene four-core copper-core cables).

If we look at 330A-260A=70A according to the safe current-carrying capacity, we only need to connect a cable with a current-carrying capacity of 70A to theoretically guarantee safe operation (ideally).
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