Analysis of cable faults

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As power cables that connect various electrical devices and transmit and distribute electrical energy, they are being used more and more widely due to their advantages such as safety, low maintenance requirements, high stability, and the ability to improve the quality of electrical energy. Currently, faults caused by power cables account for a significant proportion of all power supply failures. How to quickly and accurately determine the location of the fault point and identify the type of fault has become one of the crucial technologies in the use and operation of power cables. 1. Causes of cable failures The most direct cause of cable failures is the breakdown due to reduced insulation. There are many factors that can lead to a decrease in insulation. Based on practical operational experience, these factors can be summarized as follows: 1.1 Mechanical damage – Damage during installation: The cable may be accidentally damaged during installation, pulled apart due to excessive mechanical force, or damaged as a result of excessive bending ； Direct damage from external forces: Construction activities carried out along or near the cable path after installation can cause direct physical damage to the cables. Indirect damage from external forces: Vibration or impact forces from passing vehicles can lead to cracks in the lead (aluminum) shielding of underground cables ； Damage caused by natural phenomena: such as the expansion of insulating glue inside intermediate connectors or terminal fittings, which leads to the cracking of the housing or cable sheath ； The cable insulation installed at the pipe end or support is scratched due to the natural movement of the cable ； Excessive tensile force caused by land subsidence breaks the intermediate joint or conductor. 1.2 Insulation moisture causes failures when the insulation becomes moist. The main reasons for cable moisture absorption are water ingress caused by an unsealed structure or improper installation of the splice box or terminal box ； The cable was poorly manufactured; the metal sheath has holes or cracks ； The metal sheath is punctured or corroded due to external objects ； 1.3 Insulation aging and deterioration: The air gaps within the cable insulation medium generate free charges under the influence of an electric field, leading to a decline in insulation quality. When the insulating medium is ionized, chemical compounds such as ozone and nitric acid are generated in the gas gap, which corrode the insulation ； Moisture in the insulation causes hydrolysis of the insulating fibers, leading to a decline in insulation performance. Overheating can cause insulation to age and deteriorate. Electrical ionization in the air gaps inside the cable causes localized overheating, leading to carbonization of the insulation. Cable overload is a significant factor contributing to cable overheating. Cables installed in areas with a high density of cables, in cable trenches and tunnels where ventilation is poor, cables housed in dry tubes, as well as those located near thermal pipelines, can all experience accelerated insulation damage due to overheating. 1.4 Overvoltage Overvoltage mainly refers to atmospheric overvoltage (lightning strikes) and internal overvoltage in cables. Analysis of actual failures shows that many failures of outdoor terminal blocks are caused by atmospheric overvoltage. Overvoltage causes the cable insulation to break down, resulting in a fault; the breakdown point is usually located at a material defect. 1.5 Poor design and manufacturing processes for intermediate connectors and terminal fittings, inadequate design of waterproofing and electric field distribution, improper selection of materials, as well as substandard manufacturing practices that fail to comply with regulatory requirements can all lead to failures in cable terminals. 1.6 Material defects: Material defects are mainly manifested in three aspects. The first is the issue with cable manufacturing, namely the defects left by the lead (aluminum) shielding layer ； During the insulation wrapping process, defects such as wrinkles, cracks, breaks, and overlapping gaps appear on the paper insulation ； The second is defects in the manufacturing of cable accessories, such as sand holes in cast iron parts, insufficient mechanical strength in ceramic parts, other components not meeting specifications, or poor sealing during assembly ； Third, poor maintenance and management of insulating materials result in the cable insulation becoming damp, dirty, and aged. 1.7 Sheath corrosion: Due to underground acid-base corrosion and the effect of stray currents, the outer lead coating of the cable is corroded, resulting in pitting, cracking, or perforations that cause failures. 1.8 Loss of insulation oil in cables: When oil-impregnated paper-insulated cables are installed in trenches that are uneven, or at outdoor terminals mounted on utility poles, the significant differences in elevation cause the insulation oil to flow from higher areas to lower ones, thereby reducing the insulation performance of the cables in those higher areas and leading to failures. 2. Fault Classification 2.1 Classification by fault resistance and core wire condition • Open circuit (broken wire) fault: An open circuit fault, also known as a broken wire fault, occurs when the insulation resistance between cable phases or between a phase and ground meets the required specification values, but the operating voltage cannot be transmitted to the terminals ； Or although there is voltage at the terminal, the load capacity is poor. When the insulation resistance = ∞, it indicates a broken wire fault. • Low resistance (short-circuit) faults: Low resistance faults, also known as short-circuit faults, occur when the insulation between cable phases or between a phase and ground is damaged, resulting in such low insulation resistance that it can be measured using the low-voltage pulse method. It is a type of fault for which the insulation resistance is 100 kΩ and cannot be measured using the low-voltage pulse method; it is considered in contrast to low-resistance faults. It includes two types: high-resistance leakage faults and high-resistance flashover faults. The above fault classifications are also intended to facilitate the selection of testing methods. According to the currently popular fault location techniques, open-circuit and low-resistance faults can be detected using the low-voltage pulse reflection method; high-resistance faults require the impulse flashover method, while flashover-type faults can be tested using the direct current flashover method. 2.2 Classification by surface phenomena • Open faults • Closed faults 2.3 Classification by grounding phenomena • Single-phase grounding fault • Inter-phase fault • Multi-phase grounding mixed fault 2.4 Classification by fault location • Joint faults • Cable body faults