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Analysis: Causes of Failure of Insulation Resistance Reduction in Power Cables
First, cable insulation damp
1, cable raw materials damp
The raw materials used for cable insulation and sheathing are mainly plastics and rubbers, and a variety of materials with special functions are derived from this modification. Material manufacturers in the manufacture of materials, through the mixing agent mixing, mixing, granulation, cooling and drying processes, as well as in the material transport, storage, often occur in varying degrees of moisture, so that the material contains varying degrees of Moisture. Therefore, before the cable manufacturer squeezes the material on the cable conductor, the cable manufacturer must dry the material. 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, the surface will not blisters and other defects. This is a rigid process requirement of the cable manufacturer, otherwise the cable product fails the factory withstand voltage test.
2, the cable manufacturing process damp
In the insulation extrusion process, the insulation layer is scratched, causing the insulation layer to break or debond, and the insulated wire core enters the water in the cooling water tank, resulting in a decrease in the insulation resistance. Or when the jacket is squeezed, the protective layer is damaged and enters the water, so that the insulation layer gets wet and the insulation resistance decreases. When a multi-core cable is manufactured, even if the insulating layer is unsmoothed, it may be damaged and the moisture may enter moisture when the cable is stranded and when the sheath is extruded. Thus, the finished cable does not pass the factory withstand voltage test. .
3, cable construction process damp
In the process of directly burying cable construction, if the cable trench excavation, cable laying operation, cable intermediate joints and terminal joints are not standardized, etc., it is very likely to damage the cable sheath and insulation layer. If the soil is wet or the cable ditch water, cable water inflow must occur. When the insulation is wet, the surface resistance of the cable insulation decreases, the surface leakage current increases, the insulation resistance decreases, and the electric field distortion between the conductor and the insulation layer also occurs. Inhomogeneous distribution of electric field within the insulation will cause free discharge within the insulation, and even cause cable breakdown. After-sales service has proved that more than 95% of the direct buried cable insulation resistance drop accident is caused by improper construction.
Second, the use of cable environment
1, the ambient temperature
According to the theory of dielectric physics and engineering practice, the resistance of the insulating material decreases exponentially with increasing temperature, while the conductance increases exponentially with decreasing temperature. The increase in temperature causes a decrease in the insulation resistance. This is due to the fact that as the insulation temperature increases, the thermal motion of the molecules in the material increases, resulting in an increase in the generation and migration of conductive ions. After the cable is energized and operated, the conduction current formed by the movement of the conductive ions increases under the action of the voltage, and the temperature of the insulating layer increases, which will inevitably result in a decrease in the insulation resistance.
Experiments show that the insulation resistance value of cable insulation material at 70°C is only 10% at 20°C. In other words, the insulation resistance of the cable at the conductor operating temperature of 70°C is only 10% of the insulation resistance measured at a conductor operating temperature of 20°C. If the power supply line is overloaded and the conductor temperature of the cable exceeds 70°C, the insulation resistance will decrease more.
The temperature of the laying environment of the cable also has a great influence on the insulation resistance. The insulation resistance of buried power cables measured in different climate zones (tropical, subtropical, temperate and frigid) is different. In China, although the cable product standard specifies the permissible long-term operating temperature of the conductor to ensure the insulation level of the cable, in the southern subtropical and tropical regions of the country, the insulation resistance value of the power cable laid directly buried is lower than that in the northern temperate zone and the cold zone. The regional decline is much larger. This is an important difference in the performance requirements of electrical products in different regional climate conditions.
2, the environmental humidity
As we all know, the cable is affected by moisture in the process of manufacturing and laying operation, which is the main factor that jeopardizes the electrical performance and service life of the cable. Regardless of whether the cable manufacturer or the user, it is very important.
Third, natural conditions
1. Termite damage
Termites are the enemies of underground power cables, especially in the hot and humid regions of Southeast Asia and southern China. Termites often afflict cable plastic sheathing accidents. When the termite encounters the cable, in addition to the bite, formic acid is also secreted, which severely corrodes the cable insulation and sheath, resulting in degradation of the insulation performance of the cable and even a short circuit. Therefore, in the laying regulations established by the cable use department, there are express provisions on the ant-proof measures for cable lines.
There are three kinds of cable ant-proof performance test methods, namely the national standard GB2951.38 and the mechanical industry standard JB/T10696.9-2011 provisions of the knock down method, group hair and ant nest method ant ant test. In the past, the most adopted method was the group law. However, after many years of experience in cable ant pest control, knockdown and colony tests, it does not truly reflect the ant-resistance performance of cables in different environments. As a result, Guangdong Power Grid Corp. has stipulated that the ant-proof cable must pass the ant nest test in the bidding of power cables from 2009 onwards, and the etched condition of cable test specimens must reach Level I.
2. Rat damage
Rat damage to underground cables is mainly due to mechanical damage caused by biting. When the hardness of the cable sheath material is lower than the hardness of rat incisors, the cable is likely to be bitten by rats. There is no uniform cable ratproof test standard in the world, but each country has its own test method. China's JB/T10696.10-2011 regulations rat bite test method. In addition, the national standard “General rule for rodent prevention and anti-mosquito wire and cable†led by Shandong Huaneng Cable Co., Ltd. has already been held on March 19, 2016. The preparation work initiation meeting will be held soon. China will soon have a formal rat-proofing system. Ant cable product standard.
3, mold damage
As early as the end of the 1950s in the last century, some countries had already stipulated that the electrical products used in the humid tropics should have anti-mildew properties. China has established standards for the mildew resistance of wire and cable used in wetlands and tropical areas for wires and cables exported to these areas. In parts of southern China, due to differences in the degree of humidity and heat of the climate, different durations, and geographical and wire and cable use environments in various years, the degree of mold damage in direct buried cables varies.
According to a research report on microbial growth in molds, the main conditions for mold growth are temperature and humidity. The usual temperature for mold growth is 15°C to 35°C, and the optimum temperature is 25°C to 30°C. When the temperature is lower than 0°C or higher than 40°C, the mold actually stops growing. The relative humidity for mold growth is 80% to 90%, and when the relative humidity exceeds 95%, it is the most prosperous condition for mold growth. Therefore, when the ambient temperature is 30 °C ± 2 °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 proliferation of molds.
If a large amount of mold grows on the surface of the cable, it has a great influence on the performance of the cable and can cause: discoloration of the cable surface, pitting, decay; insulation resistance, volume resistivity, dielectric strength, causing leakage, or even insulation breakdown; insulation The chemical material of the sheathing material is chemically degraded, the mechanical properties of the material are significantly reduced, and the protective function is lost; the moisture of the moisture enters the inside of the cable and causes serious electrical performance failure.