Direct buried power cable insulation resistance reduces the cause of the failure

April 27, 2023

The phenomenon that the insulation resistance of the direct buried power cable is reduced causes the cable line to fail frequently, and it is also the project with the most quality accidents between the cable user and the cable manufacturer. From the perspective of cable materials, cable manufacturing and cable construction, a comprehensive explanation of the causes of cable insulation resistance degradation is required, including cable materials, cable manufacturing, use environment, natural strips, and installation facilities. The main points are as follows:
First, the cable insulation is damp
1, cable raw materials damp
The raw materials used for cable insulation and sheathing are mainly plastic and rubber-based materials, and thus modified to produce a variety of materials with special functions. In the manufacture of materials, materials are mixed, kneaded, granulated, cooled and dried, and during the transportation and storage of materials, moisture levels vary from one to another, resulting in varying degrees of material content. Moisture. Therefore, the cable manufacturer must dry the material before it is squeezed onto the cable conductor. The extrusion unit is equipped with a material drying device so that the extruded insulation layer and the sheath do not occur. Bubbles and blisters, no blistering on the surface. This is the hard process specification of the cable manufacturer, otherwise the finished cable will not pass the factory withstand voltage test.
2, the cable manufacturing process is damp
During the insulation extrusion process, the insulation layer is scratched, causing the insulation layer to be broken or degummed, and the insulated core is filled with water in the cooling water tank, resulting in a decrease in insulation resistance. Or when the sheath is squeezed, the sheath is damaged and the water is poured, so that the insulation layer is damp and the insulation resistance is lowered. When manufacturing a multi-core cable, even if the insulation layer is squeezed intact, it may be damaged when the insulated core is twisted and twisted, and when the sheath is squeezed, the water entering the water is damp, so the finished cable can not pass the factory withstand voltage test. .
3, cable construction process damp
In the process of direct buried cable construction, if cable trench excavation, cable burying operation, cable intermediate joints and terminal joints are not standardized, it is likely to damage the cable sheath and insulation layer. If the soil is wet or water is accumulated in the cable ditch, the cable must enter the water. After the insulation is wet, the insulation surface resistance of the cable is lowered and the surface leakage current is increased, the insulation resistance is lowered, and the electric field distortion between the conductor and the insulating layer is also caused. The uneven distribution of the electric field inside the insulation causes the internal discharge of the insulation and even the cable breakdown. After-sales service practice has proved that more than 95% of the direct buried cable insulation resistance drop accident is caused by improper construction.
Second, the cable environment
1, the ambient temperature
According to media physics theory and engineering practice, the electrical resistance of insulating materials decreases exponentially with increasing temperature, while the conductance increases exponentially with decreasing temperature. An increase in temperature causes a drop in insulation resistance. This is because when the insulation temperature rises, the thermal motion of the molecules in the material increases, and the number of generated and migrated conductive ions increases. After the cable is energized, under the action of the voltage, the conduction current formed by the movement of the conductive ions increases, and the temperature of the insulation layer rises, which inevitably causes the insulation resistance to decrease.
Experiments have shown that the insulation resistance of cable insulation at 70 Â° C is only 10% at 20 Â° C. That is to say, the insulation resistance of the cable at the operating temperature of the conductor of 70 Â° C is only 10% of the measured value of the insulation resistance at a conductor operating temperature of 20 Â° C. If the power supply line is overloaded and the cable conductor temperature exceeds 70 Â° C, the insulation resistance will be more severe.
The ambient temperature of the cable is also very important for the insulation resistance. The insulation resistance of direct buried power cables measured in different climatic zones (tropical, subtropical, temperate and cold) is different. In China, although the cable product standard specifies the long-term operating temperature allowed by the conductor to ensure the insulation level of the cable, in the southern subtropical and tropical regions, the insulation resistance of the directly buried power cable is lower than that in the northern temperate zone and cold zone. The regional decline is much larger. This is an important difference in the performance requirements of electrical products for different regional climatic conditions.
2, the ambient humidity
It is well known that the ingress of water into the cable during manufacturing and laying operations is a major factor that jeopardizes the electrical performance and service life of the cable. Regardless of the cable manufacturer or the user, this is very important.
Practical experience has proven that the main causes of cable flooding are as follows.
1) Material purity
If the cable insulation is mixed with impurities, especially metal impurities, even the pigments of different colors used, it will directly affect the electrical properties of the insulation and reduce the insulation resistance. The reason is that the non-metallic impurities in the insulating layer absorb moisture when the cable is wet, forming a large number of conductive points; second, the metal impurities in the insulating layer are directly conductive points. Under the combined action of the conductor operating temperature and the external ambient temperature, these conductive points form conductive paths in the insulating layer, resulting in a decrease in insulation resistance and an increase in leakage current, which in turn causes insulation to be broken down.
2) Material moisture
If the cable insulation material is wet, it will not dry before being extruded on the conductor, and there will be a large number of pores in the insulation layer, the extrusion surface is not smooth, and the mechanical strength is reduced or even cracked. Therefore, cable manufacturers must dry the material when extruding the cable insulation. When extruding low-smoke halogen-free materials, pay more attention to drying. These are the basic process knowledge of cable manufacturers.
3, line overload
The experiment proves that when the power supply line is not overloaded and the dielectric insulating medium is at a low working electric field strength, the mobility of the conductive ions in the dielectric material is proportional to the electric field strength, that is, the mobility of the conductive ions in the medium varies with the electric field strength. Increased and increased. When the electric field strength is relatively high, the mobility of the conductive ions in the medium increases with the increase of the electric field strength, and gradually changes from a linear relationship to an exponential relationship. When the mobility of the conductive ions in the medium increases to a certain extent, the insulation resistance suddenly decreases drastically, and then the heat is generated. "Ion avalanche" causes an instant breakdown of the insulating layer. This type of failure usually occurs when the cable is overloaded for a long period of time. Before the product is shipped from the cable factory, the finished product withstand voltage test shall be carried out according to the product standard. The cable user should correctly select the cable model according to the rated voltage of the line, and try to avoid long-term overload operation of the cable line.
Third, the natural conditions
1, termite damage
Termites are the enemy of underground power cables, especially in the hot and humid regions of Southeast Asia and southern China. It is a frequent occurrence of termites eroding cable plastic sheaths. When termites encounter cables, in addition to biting, it also secretes formic acid, which seriously corrodes cable insulation and sheathing, resulting in cable insulation performance degradation or even short circuit. Therefore, in the laying procedures established by the cable use department, there are clear provisions on the anti-ant measures for cable lines.
There are three test methods for anti-ant performance of cables, namely the knockdown method, the group hair and the nesting method anti-ant test specified in the national standard GB2951.38 and the mechanical industry standard JB/T10696.9-2011. In the past many years, the most widely used group method. However, after years of experience in cable ant control, knockdown and group method tests do not truly reflect the anti-ant performance of cables in different environments. Therefore, Guangdong Power Grid Corporation has proposed that the anti-ant cable must pass the ant nest test in the power cable bidding from 2009, and the corrosion condition of the cable sample must reach the level I.
2, rodent damage
The damage to the underground cable by the rodent is mainly the mechanical damage caused by the bite. When the hardness of the cable sheath material is lower than the hardness of the rat incisor, the cable is likely to be bitten by the mouse. There is no uniform cable rodent test standard in the world, but each country has its own test methods. China's JB/T10696.10-2011 specifies the rat bite test method. In addition, the national standard â€œGeneral Rules for Anti-rat and Anti-ant Wire and Cableâ€, which was led by Shandong Huaneng Cable Co., Ltd., was held on March 19, 2016, and the official start-up meeting will be held soon. Ant cable product standard.
3, mold damage
As early as the end of the 1950s, some countries have already stipulated that electrical products used in wet tropics should have anti-mildew properties. In China, we have established relevant standards for mold and wire resistance for wires and cables used in these areas. In some parts of southern China, the degree of mildew of direct buried cables is not equal due to the degree of humidity, duration, duration, and environment of wire and cable used in various years.
According to the research report on microbial mold reproduction, the main conditions for mold growth are temperature and humidity. The general temperature suitable for mold growth is 15 Â° C to 35 Â° C, and the most suitable temperature is 25 Â° C to 30 Â° C. When the temperature is lower than 0 Â° C or higher than 40 Â° C, the mold actually stops growing. The relative humidity suitable for mold growth is 80% to 90%, and when the relative humidity exceeds 95%, it is the condition that mold growth is most vigorous. Therefore, when the ambient temperature is 30 Â° C Â± 2 Â° C and the relative humidity is greater than 95%, it is most suitable for the proliferation of mold. The hot and humid climate of Hainan Island is just right for the growth of molds.
If a large amount of mold grows on the surface of the cable, it will have a great influence on the performance of the cable, which may cause: discoloration, pitting, and decay of the cable surface; insulation resistance, volume resistivity, dielectric strength decrease, causing leakage, and even dielectric breakdown; insulation Chemical degradation of the material of the sheath material, the mechanical properties of the material are significantly reduced, and its protective effect is lost; moisture moisture enters the interior of the cable, causing serious electrical performance failures.
4, lightning impact
In the event of a thunderstorm, if the lightning arrester used on the line is of poor quality or the grounding protection is not proper, the lightning strike will hit the arrester, causing the line load to suddenly increase and generate an overvoltage, resulting in an overvoltage surge in the cable, resulting in cable insulation. wear. In areas where there are frequent thunderstorms in southern China, including Hainan Island, lightning strikes on cable lines are not uncommon.
Fourth, chemical corrosion
1. Laying environmental chemical corrosion
If the water in the cable trench or the buried soil contains corrosive components such as sulfuric acid or nitric acid, the surface of the cable may be in contact with these corrosive substances for a long time, and severe chemical corrosion may occur. If the cable sheath is damaged, moisture will spread longitudinally to the left and right after entering the cable. In the case where the groundwater quality and soil in some areas are seriously contaminated by chemical, if the cable path is improperly selected, the cable trench is poorly constructed, and the backfill is too corrosive, which will cause chemical degradation of the cable insulation and the molecules of the sheath organic material. The cable is corroded, the insulation resistance of the cable is reduced, and even the insulation resistance is lost.
2. Acid rain chemical corrosion
In addition to the water quality and soil conditions of the laying environment, there are also serious effects of modern acid rain.
The so-called acid rain is due to the massive burning of fossil fuels (coal, oil, natural gas) or biomass fuels. Acidic compounds (such as sulfur dioxide, carbon dioxide and nitrogen dioxide, mainly sulfur dioxide) are discharged into the air, causing sulfuric acid in the rain. The phenomenon of acidic substances such as nitric acid. The main component of acid rain is sulfur dioxide. It is generally believed that if the pH of the rain is less than 5.6, it can be considered as acid rain. The main cause of acid rain is the excessive emissions of sulfur dioxide from the plant. Now, the world is implementing â€œenergy saving and carbon reductionâ€ and â€œenergy saving and emission reductionâ€, the main purpose of which is to reduce the emission of sulfides and carbides to protect the clean atmosphere.
Acid rain disasters have occurred in more than 20 provinces and cities in China, mainly in the south of the Yangtze River. Acid rain not only causes very serious extinction damage to crops, forests, grasslands, fish, etc., but also corrodes metal objects. It is also very serious for wire and cable, railway tracks, ship vehicles, transmission lines, bridges, houses, mechanical and electrical equipment, etc. Both can cause serious damage.
The Journal of Sichuan University has published a research report "Corrosion Behavior of Copper in Acidic Acidification Process under Acid Rain". Experiments have shown that acid rain increases the corrosion rate of copper. The corroded surface of copper is mainly cuprous oxide (Cu2O) and copper oxide (CuO).
The harmful way of acid rain to direct buried cable is: sulfur dioxide in the air reacts with rainwater to form sulfurous acid, and sulfurous acid is oxidized to sulfuric acid:
SO2+H2O=H2SO3
2H2SO3+O2=2H2SO4
Rainwater containing sulfuric acid, in high temperature environment, enters the cable from the cable sheath damage point or cable joint, causing corrosion to the insulation layer, the sheath and the copper conductor. Sulfuric acid corrodes the cable sheath and the insulating layer, causing the molecular structure to be degraded and damaged, so that the insulation resistance is seriously degraded, and even the insulation and protection are lost. Sulfuric acid reacts with copper to form blue copper sulphate (CUSO4) crystals, which form a blue copper sulphate solution when exposed to water.
CU+2H2SO4=CUSO4+SO2â†‘+2H2O
In the past few years, a power supply department in Chongqing, where the soil was corrosive and acid rain was severely hit, the blue liquid and insulation layers were damaged at the end of the cable. If the cable sheath is damaged seriously, especially in the high temperature, high humidity, strong sunlight season, if acid rain occurs, or the sulfuric acid content in the soil is large, the cable enters a lot of water, and the blue copper sulfate solution will quickly follow the cable length. Spread up until it overflows from the cable break and cable ends. The copper sulfate solution can conduct electricity, and after infiltrating into the insulating layer, the conductivity of the insulating layer is strengthened, and the resistance of the insulating layer is drastically lowered, the insulating effect is lost, and a cable short-circuit accident occurs.
Five, mechanical damage
Over the years, the experience of after-sales service of cable products proves that more than 95% of the cases of cable mechanical accidents complained by users are caused by improper installation of cables or poor maintenance of lines. A power supply department has summarized the following aspects.
1) Installation damage: violation of operating procedures during installation; unskilled construction personnel; non-compliance with construction techniques when making cable intermediate joints and terminal joints; cable trenches do not meet requirements; any barbaric pulling; cable bending radius is too small. These can cause mechanical damage to the cable.
2) External force damage: There are other construction work on or near the cable laying route, which causes cable damage. This phenomenon is not uncommon.
3) Vehicle damage: If the cable is buried deep enough, the cable trench cover protection is poor after laying. In the case of frequent vehicle vibration, the cable frequently suffers from great pressure and vibration, resulting in cable structure deformation and damage.
4) Natural damage: Cable damage caused by natural phenomena such as excessive humidity, excessive temperature, excessive humidity, typhoons, earthquakes, etc., so-called irresistible damage.
Mechanical damage can have a significant impact on cable life, especially in tropical and subtropical regions. In the "high temperature, high humidity, strong light" season in these areas, direct buried cables work in very harsh environments, and are in a hot and humid environment all the time, just like the "wet heat aging test". If the buried cable sheath is damaged and moisture moisture enters the cable, the insulation resistance will drop sharply. Even if the damage is not serious, the power-on test after the laying is normal, but after a long time, moisture moisture will enter the cable, causing the insulation resistance to drop. This process, depending on the environment, natural conditions and degree of damage, is generally 2 to 12 months, and operational failure is likely to occur.
In the above five aspects, from the aspects of cable insulation moisture, cable use environment, natural conditions, chemical corrosion and mechanical damage, the reasons for the drop of the insulation resistance of the direct buried power cable, the cause of the line operation failure, and some corresponding corrective measures are proposed. . Only the cable material manufacturer, cable manufacturer and cable construction are closely coordinated with the user units, and each department is responsible for continuously improving the cable quality and ensuring the safety of the power supply line.
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