We combine our experience in cable operation over the years with the application of solid Bauer edge theory to analyze the causes of insulation breakdown of power cables.
The purpose is to cast a brick to attract jade, and electrical counterparts to find specific measures to prevent cable insulation breakdown, improve the level of safe operation of the cable and extend the service life of the cable.
I. Breakdown of Solid Insulation
1. Solid insulation breakdown of the form of insulation material is also known as dielectric.
When the voltage applied to the dielectric exceeds a certain critical value, the electric field strength in the dielectric exceeds the level it can withstand. Through the dielectric current increases sharply, the dielectric rupture or decomposition, the complete loss of insulation capacity. This phenomenon is called dielectric breakdown.
Different dielectric, the form, and mechanisms of breakdown are different, field dielectric breakdown can generally be divided into three forms electrical breakdown, thermal breakdown, and electrochemical breakdown.
Electrical breakdown is when the voltage is very high, the electric field is strong enough, and a small number of free electrons in the solid dielectric will be in the strong electric field under the action of movement, the accumulation of large kinetic energy.
If this kinetic energy is sufficient to make the medium atoms free into electrons and ions, there will be many free-charged particles in the medium. They can continue to generate new charged particles, so that the intense development continues, and finally lead to a breakdown.
Thermal breakdown occurs when the voltage is applied for a longer period. Under the action of the electric field, the dielectric loss will make the insulating medium internal heating, temperature rise, and at the same time also to the surrounding heat dissipation.
If the heat generation has been greater than the heat dissipation, the dielectric temperature will continue to rise, and finally, the insulation characteristics are completely lost due to the high temperature and breakdown. Electrochemical breakdown generally occurs after a long period of equipment operation.
In operation, the insulation will be subject to thermal, chemical, and mechanical forces, so that the insulation properties gradually deteriorate and age.
Under the action of high voltage, corona or partial discharge often occurs due to the air gap at the contact between electrode and insulation or the existence of bubbles inside the insulation, and even causes the local scorching phenomenon.
All of the above will lead to the insulation of the electric strength of the decline so that the long period of voltage under the action of thermal breakdown, or in a short period of overvoltage caused by electric breakdown.
2. The impact of solid insulation breakdown of the garden Tong affects the solid insulation breakdown of more factors, here is only a brief introduction to several major factors.
(1) As long as the electric field strength is selected to be sufficient to make a small number of free electrons in the medium to produce a chain reaction of collision ionization, an electrical breakdown can occur.
The same externally applied voltage, the thicker the insulation layer, the lower the electric field strength (in a uniform electric field. V: Ed, the formula scarf V is the voltage, E is the electric field strength, d is the distance between the voltage poles, that is, the thickness of the insulation layer). Therefore, the breakdown voltage increases correspondingly with the increase in insulation thickness.
(2) When there are impurities in the insulation material, especially when there are air bubbles, it can make the whole material’s insulation performance is reduced.
On the one hand, the air bubbles reduce the effective thickness of the dielectric. More importantly, because in the dielectric with interlayer, the electric field strength is inversely distributed according to the dielectric coefficient £ of each layer of material, £ smaller bubble in the electric field is stronger, and the bubble itself is lower pressure strength. So these bubbles are always first free and partial discharge, which leads to the breakdown of the whole medium.
(3) Charged body and insulating material contact is not closed will also make the medium voltage strength drop. This is due to the gap between the charged body and the medium asked and the bubble will be the first to produce a free reason.
(4) Insulation after the moisture breakdown voltage will rapidly drop. This is because when the water content increases, the conductivity of the medium and dielectric loss will rapidly increase, and it is easy to appear thermal breakdown.
(5) For a certain insulation material, the breakdown voltage is higher in a uniform electric field, and the breakdown voltage will be greatly reduced in an uneven electric field. 35 kV and above cable termination is the use of a stress cone and grounding shield ring to improve the distribution of the electric field near the edge of the lead jacket, thereby improving the voltage strength.
Ⅱ. Analysis of The Causes of Cable Insulation Breakdown
1. Mechanical damage in the factory due to mechanical damage caused by the cable insulation breakdown of a large proportion of accidents, generally common causes are the following examples:
(1) directly by external forces and damage, such as heavy objects from a high landing smashing cable, digging accidentally hurting the cable.
(2) laying the cable bend blood excessive insulation injury, or severe extrusion during shipment so that the insulation and protective layer damage.
(3) Directly buried cable due to ground subsidence and excessive tension so that the cable is damaged, in serious cases, there is a pull off the cable.
2. Terminal and intermediate joints fault terminal and intermediate joints are the weak link of the cable line, a large proportion of cable faults occur in the terminal and intermediate joints. The main reason is mostly caused by poor construction and the use of poor-quality materials.
3. Insulation Moisture
(1) Poor construction of the terminal and intermediate joints so that moisture intrusion cable is internal.
(2) Due to the damage of the cable inner sheath and moisture intrusion ^. There are many reasons for the breakage of the inner sheath. Such as direct mechanical damage, and lead pack cable laid in the vicinity of the earthquake source. Fatigue cracking of the lead package due to long-term vibration. Chemical corrosion produces holes due to poor shot-making lead packages with small holes or cracks, etc.
4. Overvoltage due to atmospheric overvoltage or internal overvoltage caused by the insulation breakdown. Overvoltage breakdown, especially the cable breakdown caused by internal overvoltage of the power system, often multiple cables to occur simultaneously.
5. Insulation aging is due to the long-term role of various factors in the operation process, resulting in the deterioration of the electrical and mechanical properties of the material. One of the main factors is heat. Overheating will make the insulation embrittlement and easy to fracture, overheating is often due to poor heat dissipation or overload.
Ⅲ. Measures to Prevent The Breakdown of Cable Insulation
1. Prevent mechanical damage to the overhead cable. In particular, the cable laid along the wall should be covered, and the plant earth moving should be signed by the earth moving department for the earth moving certificate.
2. Improve the construction quality of the terminal and intermediate joints that have been introduced before, bubbles and moisture on the insulation of the compressive strength. Therefore, in the construction of the wrap insulation must be tightened, so as not to produce gaps. Epoxy resin and quartz powder must be strictly dried to catch moisture before use.
As the electric field distribution at the terminal head is very uneven, the electric field strength at the edge of the lead jacket is large, so the insulation at the edge of the lead jacket should be dealt with.
For 35 kV and above, the cable should be wrapped with a good stress cone and grounding shield ring to make the electric field distribution even and improve the insulation level.
3. Prevent the insulation from moisture chemical plants due to corrosion of the lead package and insulation from moisture breakdown accidents occur from time to time. So you should strengthen the maintenance of the cable sheath, every 2 to 3 years to paint a bitumen paint can effectively stop the cable steel corrosion.