Cause analysis and preventive measures of cable insulation breakdown

Date：March 19,2024

Based on the experience of our company (Shanghai Shenghua Cable Technology Group Co., LTD.) in cable operation for many years, we analyzed the cause of insulation breakdown of power cable by applying the theory of solid bow edge. The purpose is to find specific measures to prevent cable insulation breakdown with electrical counterparts to improve the level of safe operation of cables and extend the service life of cables. Due to the limited level, the description is not necessarily comprehensive, there may be mistakes, hope the majority of electrical colleagues correct.

First, breakdown of solid insulation

1. Solid insulation breakdown in the form of insulating material also known as dielectric. When the voltage applied to the dielectric exceeds a certain critical value, so that the electric field strength in the dielectric exceeds the level it can withstand, the current through the dielectric surges, the dielectric breaks or decomposes, completely loses the insulation ability, this phenomenon is called dielectric breakdown. Different dielectrics, the form and mechanism of breakdown is not the same, the breakdown of field dielectric can be generally divided into electrical breakdown, thermal breakdown and electrochemical breakdown of three forms.
Electrical breakdown is when the voltage is high and the electric field is strong enough, a small number of free electrons in the solid dielectric will move under the action of the strong electric field and accumulate a large kinetic energy. If these kinetic energies are enough to free the medium atoms into electrons and ions, there will be many free charged particles in the medium, which can continue to produce new charged particles, and this intense development will eventually lead to breakdown.
Thermal breakdown occurs when the voltage is applied for a long time. Under the action of the electric field, the dielectric loss will make the insulating medium heat inside, the temperature rises, and at the same time, it also dissipates heat to the surrounding area. If the heat has been greater than the heat dissipation, the temperature of the medium will continue to rise, and finally the insulation characteristics will be completely lost due to the high temperature. Electrochemical breakdown generally occurs after the equipment has been running for a long time. In operation, insulation will be subjected to thermal, chemical and mechanical forces, so that the insulation performance gradually deteriorates and ages. Under the action of high voltage, due to the air gap between the electrode and the insulation or the existence of bubbles in the insulation, corona or partial discharge often occurs, and even local burning phenomenon. All of the above conditions will lead to a decrease in the electrical strength of the insulation, resulting in thermal breakdown under the action of a long period of voltage, or electrical breakdown under the action of a short period of overvoltage.

2. There are many factors affecting the breakdown of solid insulation, here only a few main factors are briefly introduced.
(1) As long as the electric field strength is selected enough to cause a small number of free electrons in the medium to produce a chain reaction of collision ionization, electrical breakdown can occur. The same applied voltage, the thicker the insulation layer, the lower the electric field strength (in a uniform electric field, V: Ed, type V is the voltage, E is the electric field strength, d is the distance between the two poles of the voltage, that is, the thickness of the insulation layer), so the breakdown voltage increases with the increase of the insulation thickness.

(2) When there are impurities in the insulation material, especially when there are bubbles, the insulation performance of the entire material can be reduced. On the one hand, bubbles reduce the effective thickness of the dielectric medium, more importantly, because in the dielectric medium with sandwich, the electric field strength is inversely distributed according to the dielectric coefficient of each layer of materials, the electric field is stronger in the smaller bubble, and the pressure strength of the bubble itself is low, so these bubbles are always free and partial discharge occurs, and then lead to the breakdown of the entire medium.

(3) The lack of contact between the charged body and the insulating material will also reduce the compressive strength of the medium. This is because the gap between the charged body and the medium will first create ionization like the bubble
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(4) The breakdown voltage will drop rapidly after the insulation is damp. This is because when the water content increases, the conductivity and dielectric loss of the medium will increase rapidly, and it is easy to appear thermal breakdown.

(5) For a certain insulating material, the breakdown voltage is higher in a uniform electric field, and the breakdown voltage will be greatly reduced in an uneven electric field. The 35 kV and above cable terminal uses the stress cone and the grounding shield ring to improve the distribution of the electric field near the edge of the lead sleeve, thereby improving the compressive strength.

Second, the reason analysis of cable insulation breakdown

1. Mechanical damage In the factory due to mechanical damage caused by cable insulation breakdown accident proportion is large, generally common reasons are the following examples:

(1) Directly damaged by external forces, such as heavy objects falling from a height to smash the cable, digging the cable accidentally.

(2) Excessive bending of blood during cable laying causes insulation injury, or severe extrusion during loading causes insulation and protective layer damage.

(3) The direct buried cable is damaged by excessive tension due to the formation subsidence, and the phenomenon of pulling and breaking the cable is serious.

2. Terminal head and intermediate connector failure Terminal head and intermediate connector are the weak links of cable lines, and a large proportion of cable failures occur in terminal head and intermediate head. The main reasons are mostly caused by poor construction and poor quality of materials used.

3. Insulation damp

(1) Poor construction of the terminal head and the intermediate joint causes water to invade the inside of the cable.

(2) Water invasion due to the damage of the inner protective layer of the cable. There are many reasons for the damage of the inner protective layer, such as direct mechanical damage, lead-covered cable laying near the source, fatigue cracking due to long-term vibration of the lead package due to chemical corrosion caused by holes or cracks on the lead package due to poor shooting.

4. Overvoltage Breakdown of insulation caused by atmospheric overvoltage or internal overvoltage. Overvoltage breakdown, especially the cable breakdown caused by overvoltage in the power system, is often multiple cables at the same time.

5. Insulation aging is due to the long-term effect of various factors in the operation process, resulting in the deterioration of the electrical and mechanical properties of the material, the main factor is heat. Overheating will make the insulation brittle and easy to break, the reason for overheating is often due to poor heat dissipation or overload.

Third, measures to prevent cable insulation breakdown

1. To prevent mechanical damage, the overhead cables, especially the cables laid along the wall, should be covered, and the moving soil in the plant should be signed by the moving soil department.

2. Before improving the construction quality of the terminal head and the intermediate joint, it has been introduced that bubbles and moisture have a great impact on the compressive strength of the insulation. Therefore, in the construction of the wrapping insulation must be tightened, can not produce gaps, epoxy resin and quartz powder must be strictly dried before the use of moisture. Because the electric field distribution at the end head is very uneven, the electric field intensity near the edge of the lead sleeve is very large, so the insulation at the edge of the lead sleeve should be treated well. For cables of 35 kV and above, the stress cone should be wrapped and the grounding shielding ring should be installed to make the electric field evenly distributed and improve the insulation level.

3. Due to the corrosion of the lead package, the accident of insulation moisture breakdown occurs, so the maintenance of the cable outer protective layer should be strengthened, and the asphalt paint can effectively prevent the corrosion of the cable steel strip every 2 to 3 years.