With the growth of my country's national economy, power grid construction is moving toward large-scale, high-quality development. High-voltage cable construction is a crucial component of my country's power grid construction, and research on its construction process is of great significance. However, high-voltage cable failures are common. What causes these failures? Below, we share a brief analysis of the causes of high-voltage wire and cable problems.
What are the causes of high-voltage cable problems?
1. Manufacturer-related causes: Manufacturer-related causes can be categorized as cable-related, cable-related, and cable-related grounding system-related, depending on the location of the problem.
1. Wire and cable-related causes: Common problems that may occur during the cable production process include insulation eccentricity, uneven insulation shield thickness, impurities within the insulation, protrusions on the inner and outer shields, uneven cross-linking, moisture exposure, and poor sealing of the cable metal sheath. In some serious cases, failures may occur during final testing or shortly after commissioning. Most failures manifest as defects within the wire and cable system, posing a serious threat to the long-term safe operation of the wire and cable.
2. Wire and Cable Joint Manufacturing Issues: High-voltage wire and cable joints were previously made using wrap-around, die-cast, and molded types. These required significant on-site fabrication effort. Due to site constraints and manufacturing processes, air gaps and impurities inevitably formed between the insulating tape layers, making them prone to problems. The most commonly used types in China are assembled and prefabricated.
Wire and cable joints are categorized as either terminal joints or intermediate joints. Regardless of the joint type, joint failures typically occur at the cable insulation shield break, where electrical stress is concentrated. Manufacturing-related causes of cable joint failure include defects in the stress cone, problems with the insulation filler, and oil leakage from the sealing ring.
3. Wire and Cable Grounding System: The wire and cable grounding system includes the cable grounding box, wire and cable grounding protection box (with sheath protector), wire and cable cross-connector box, and sheath protector. Common problems arise primarily from poor sealing of the box, which can lead to water ingress and multiple grounding points, causing excessive induced currents in the metal sheath. In addition, improperly selected sheath protector parameters or poor quality can lead to unstable zinc oxide crystals, which can easily damage the sheath protector.
Second, Construction Quality: There are numerous cases of high-voltage wire and cable system failures caused by poor construction quality. The main causes are as follows:
1. Poor site conditions. Cables and connectors are manufactured in factories under stringent environmental and process requirements, and the temperature, humidity, and dust levels at the construction site are difficult to control.
2. During the construction of wires and cables, small scratches are inevitably left on the insulation surface. Semi-conductive particles and sand from emery cloth can also become embedded in the insulation. Furthermore, during joint construction, the insulation is exposed to air, absorbing moisture, all of which poses risks to long-term safe operation.
3. Failure to strictly adhere to the construction process or to consider potential problems during installation.
4. The DC withstand voltage test used during final acceptance testing can cause a reverse electric field to form within the connector, leading to insulation damage.
5. Poor sealing. Intermediate joints must utilize a sealed structure with a metal copper outer shell and a PE or PVC insulation layer. During on-site construction, ensure the lead seal is tight, effectively ensuring the joint's sealing and waterproof properties.
Third, design reasons: Cable compression and breakdown due to thermal expansion of wires and cables. When cross-linked wires and cables are under high load, the core temperature rises, causing the cables to expand due to heat. At bends in tunnels, the cables press against the support structure. Long-term high-load operation creates significant creep forces, which can cause the support structure to crush the outer sheath and metal jacket, pushing into the cable insulation and causing breakdown.
What issues should be considered during high-voltage cable construction?
1. High-voltage cable line selection: The selection of a cable line should primarily consider safety, economy, and ease of construction. Under normal operating conditions, its lifespan is generally over 30 years, and the investment cost is far greater than that of overhead lines. Therefore, the selection of the cable line route is extremely important, and the selection of the cable laying route is a key step in the entire construction process.
2. High-voltage Cable Type Selection: Common power cables include oil-immersed cables, polyvinyl chloride insulated cables, and cross-linked polyethylene cables. These cables are further divided into various specialized types depending on the application. Currently, with the continuous advancement of production technology and processes, cross-linked polyethylene cables have become the most widely used cable product. Cable selection should be based on the specific environment and conditions of use.
3. High-voltage Cable Laying Method: Cable laying projects should be carefully considered based on the actual project conditions, environmental factors, cable type, and cable quantity. Safety, maintenance, and economic factors should be considered during the laying process to facilitate routine maintenance. Each laying method has its advantages and disadvantages, and the appropriate method is determined by the specific circumstances. Generally, considerations include urban planning, existing building density, cable line length, number of cables to be laid, and environmental influences.
Based on the above analysis, high-voltage wire and cable failures can be broadly categorized into four main categories: manufacturer failures, construction quality failures, design failures, and external damage. In short, during the cable construction process, appropriate protective measures must be taken for the cables, and correct construction techniques must be used to lay the cables, so as to ensure the safe and stable operation of the power supply lines.
