High-voltage cable is a type of power cable used to transmit power between 10kV and 35kV (1kV = 1000V). It is primarily used in power transmission trunk lines. Below, Cable Baolai introduces the types and structures of high-voltage cables, as well as the causes of high-voltage cable failures.
1. Types of High-Voltage Cables
The main types of high-voltage cables include YJV cables, VV cables, YJLV cables, and VLV cables.
YJV cables stand for cross-linked polyethylene insulated and PVC sheathed power cables (copper core).
VV cables stand for polyvinyl chloride insulated and PVC sheathed power cables (copper core).
YJLV cables stand for cross-linked polyethylene insulated and PVC sheathed aluminum core power cables.
VLV cables stand for polyvinyl chloride insulated and PVC sheathed aluminum core power cables.
Due to the excellent electrical conductivity of copper conductors, copper-core power cables are increasingly being used in power supply trunk lines, while aluminum-core power cables are less commonly used. Copper-core cables are particularly popular in higher-voltage power systems. II. High-Voltage Cable Structure
The components of a high-voltage cable, from the inside out, include: conductor, insulation, inner sheath, filler (armor), and outer insulation.
Of course, armored high-voltage cables are primarily intended for underground use, where they can withstand high-intensity ground pressure and prevent damage from other external forces.
IV. High-Voltage Cable Usage Characteristics
This product is suitable for use in fixed-line power transmission and distribution lines with an AC rated voltage of 35 kV and below. The cable conductor has a maximum long-term operating temperature of 90°C. During a short circuit (maximum duration no more than 5 seconds), the maximum temperature of the cable conductor does not exceed 250°C.
Ultra-High-Voltage Cable
Low-voltage cables of 1 kV and below are considered low-voltage cables; medium-voltage cables of 1 kV to 10 kV; high-voltage cables of 10 kV to 35 kV; and ultra-high-voltage cables of 35 to 220 kV.
Ultra-high-voltage cables are a type of power cable that emerged with the continuous advancement of cable technology. They are generally used as hubs in large-scale power transmission systems. They are a high-tech, high-voltage cable primarily used for long-distance power transmission. V. Causes of High-Voltage Cable Failures
1. Manufacturer-Related Causes
Manufacturer-related causes can be categorized as cable-related, cable-connector-related, and cable-grounding-system-related, depending on the location of the failure.
2. Cable-Related Causes
Common problems that may occur during cable production 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 persist as defects within the cable system, posing a serious threat to the long-term safe operation of the cable.
3. Cable-Connector-Related Causes
High-voltage cable connectors were previously manufactured using wrap-around, die-cast, and molded types. These required extensive on-site fabrication. Furthermore, due to site conditions and manufacturing processes, air gaps and impurities between the insulation tape layers were unavoidable, making them prone to failure. Assembled and prefabricated types are commonly used in China. Cable joints are classified 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 seal ring.
4. Cable Grounding System
The cable grounding system includes the cable grounding box, cable grounding protection box (with sheath protector), cable cross-connect box, and sheath protector. Common problems arise from poor sealing of the box, which can lead to water ingress and multiple grounding points, causing excessive induced current in the metal sheath. Inappropriate sheath protector parameters or poor quality, such as unstable zinc oxide crystals, can also easily damage the sheath protector.
5. Construction Quality
There are numerous cases of high-voltage cable system failures caused by construction quality. The main reasons are as follows: First, poor site conditions. Cables and joints are manufactured in factories under stringent environmental and process requirements, and temperature, humidity, and dust levels are difficult to control at the construction site. Second, during cable construction, small scratches inevitably remain 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 a risk to long-term safe operation. Third, during installation, the construction process is not strictly followed, or potential problems are not considered in the process specifications. Fourth, the DC withstand voltage test used during final acceptance testing can create a reverse electric field within the joint, leading to insulation damage. Fifth, poor sealing can cause damage. Intermediate joints must utilize a sealed structure consisting of a metal copper outer shell with a PE or PVC insulation layer. Ensure a tight seal during on-site construction to effectively ensure the joint's waterproof performance.
6. Design Issues
Cable compression and breakdown due to thermal expansion. When cross-linked cables are loaded heavily, the core temperature rises, causing the cable to expand. At bends in tunnels, the cable presses against the support structure. Long-term high-load operation creates significant creep forces, causing the support structure to break through the cable's outer jacket and metal sheath, forcing it into the cable insulation and causing breakdown.
