Three-core cables typically employ grounding at both ends, as the total current flowing through the three cores equals zero during cable operation.
Consequently, there is virtually no induced voltage at both ends of the cable’s metal shield (typically for cables with a voltage rating of 35kV or less).
However, direct grounding at both ends is generally not applicable to single-core cables (usually for cables with a voltage rating of 35kV or higher).
The reason being: when current passes through the core of a single-core cable, induced current is generated in the metal shield, resulting in induced voltage at both ends of the cable.
The level of induced voltage is proportional to the length of the cable line and the current flowing through the conductor. When the cable line encounters a short circuit, suffers a lightning strike, or is subjected to overvoltage, a high induced voltage will form on the shield. This can pose a risk to personal safety and may even penetrate the cable’s outer protective cover.
When a single-core cable is grounded at both ends, the metal shielding layer of the cable may generate circulating currents.
According to relevant reports, the circulating current generated by grounding the single-core cable at both ends can reach 30%–80% of the normal current carrying of the cable core. This not only reduces the current carrying capacity of the cable but also wastes energy, resulting in losses, and accelerates the aging of the cable insulation.
Therefore, single-core cables should not be grounded at both ends.
When installing high-voltage single-core cable lines, special grounding methods should be adopted in accordance with the requirements of GB50217-1994 “Design Specifications for Power Engineering Cables”.
Different grounding methods should generally be selected according to specific line routes, and common methods include:
1. One end of the metal shield layer is directly grounded, while the other end is grounded via a protective layer protector.
2. The midpoint of the metal shield layer is directly grounded, and both ends are grounded through the protective layer protector.
3. One end of the metal shield layer is directly grounded, the protective layers in the middle of the cable are interconnected and grounded, and the other end is grounded through the protective layer protector.
4. One end of the metal shield layer is directly grounded, several protective layers are interconnected and grounded, the midpoint of the metal shield layer is directly grounded, several more protective layers are interconnected and grounded, and the other end of the metal shield layer is directly grounded.
5. Both ends of the metal shield layer are directly grounded (applicable only to short cables and small load cables).
The cable is primarily composed of the following four sections:
1. Conductive Core:
Constructed from high conductivity materials such as copper or aluminum.
Depending on the flexibility required by the installation conditions, each core wire may be composed of a single conductor or multiple conductors twisted together.
2. Insulation Layer:
The insulation material used in the cable should have high insulation resistance, high dielectric strength, low dielectric loss, and low dielectric constant.
Commonly used insulation materials for cables include oil-impregnated paper, polyvinyl chloride, polyethylene, cross-linked polyethylene, and rubber.
Cables are often classified by insulation materials, such as oil-impregnated paper insulated cables, polyvinyl chloride cables, and cross-linked polyethylene cables.
3. Sealing Sheath:
Protects the insulated core from damage from mechanical forces, moisture, humidity, chemicals, and light.
For insulation that is susceptible to moisture, a lead or aluminum extruded sealing sheath is generally adopted.
4. Protective Covering:
Designed to shield the sealing sheath from mechanical damage.
Typically, galvanized steel strips, steel wires, copper strips, or copper wires are used as the armoring wrapped around the sheath (known as armored cable).
The armor also serves as an electric field shield and protects against external electromagnetic interference.
To prevent the steel strips and wires from being corroded by surrounding media, they are generally coated with asphalt or wrapped with soaked hemp layers or extruded polyethylene or polyvinyl chloride sleeves.
Cables can be categorized according to their use as power cables, communication cables, and control cables.
Compared to overhead lines, the advantages of cables are that they require less insulation distance between lines, occupy less space, and can be laid underground, freeing up aboveground space.
They are not affected by environmental pollution, have high power transmission reliability, and cause minimal disturbance to human safety and the surrounding environment.
However, due to the high construction cost, complex manufacturing, and challenging maintenance, cable deployment is often inconvenient.
Therefore, cables are primarily used in densely populated areas with intricate electrical grids and congested traffic. They are particularly suitable for installation across rivers and under the sea, where they can prevent the use of large-span overhead lines.
Cables are also advantageous in scenarios where overhead lines may disrupt communication, where aesthetics are a concern, or where it is important to avoid exposing targets.
As the load current and environmental temperature fluctuate, power cables undergo thermal expansion and contraction.
The thermal mechanical stress generated by the expansion and contraction of the cable core is significant, and the larger the cable core cross-section, the greater this stress becomes.
The cable core and metal sheath can also experience creep due to repeated cycles of thermal expansion and contraction.
Given that thermal expansion and contraction pose a substantial threat to the operation of power cables, leading to cable displacement, slippage, and even damage to the cable and its accessories, it is crucial to pay attention to the thermal expansion and contraction issues of large cross-section cables.
The threats posed by the thermal expansion of cables under various laying methods on safe operation include:
(1) When directly buried, cables are constrained by the surrounding soil, preventing displacement along the entire length.
Consequently, under the influence of thermal mechanical forces, the cores generate substantial thrust at both ends of the line, causing end displacement. This presents a significant threat to the safety of cable accessories.
(2) During conduit installation, cables are not laterally constrained, resulting in bending deformation under the influence of thermal mechanical forces.
As the cable temperature continuously changes, repetitive bending deformation occurs, causing fatigue strain in the cable’s metal sheath.
(3) During tunnel installation, cables are generally placed on brackets without rigid fixation, hence they undergo significant thermal expansion.
This leads to a risk of slipping when laid on inclined surfaces; severe displacement can occur at bends. With continuous changes in cable temperature, repeated bending deformation can induce fatigue strain in the cable’s metal sheath.
(4) During vertical shaft placement, the weight of the cable and thermal mechanical forces can potentially cause excessive strain on the metal sheath, thereby shortening the cable’s lifespan.
(5) When laying cables in bridges, if placed within the bridge’s conduit, it presents the same issues as conduit installation. If the cable is installed within the bridge’s box girders, it encounters similar problems to tunnel installation. Additionally, cables laid on bridges are affected by bridge expansion and vibration, accelerating the damage to the metal sheath.
Appropriate countermeasures should be adopted in several areas including the design and production of cables and accessories, cable line design, and construction:
(1) Cables and accessories. To reduce the thermal expansion of large cross-section cables, split conductors should be used for cable cores. This not only reduces core loss but also results in smaller thermo-mechanical forces per unit area than other conductor types. The design of cable accessories must consider their ability to withstand the cable’s thermal mechanical forces without damage.
(2) There are currently two types of cable metal sheaths: aluminum and aluminum alloy, each having significantly different properties. Comparatively, aluminum sheaths can enhance the operating performance of cables. Hence, unless corrosion resistance requirements are exceptionally high, aluminum sheaths are generally the preferred choice for cable metal sheaths.
(3) For directly buried cables near terminal points, such as within the cable layer of substations, a serpentine layout can be adopted to absorb deformation and reduce terminal thrust. Rigid fixation should be applied at the bracket to prevent terminal damage due to cable displacement.
(4) When laying large cross-section cables in conduit, bentonite can be filled into the conduit to prevent the cables from bending and deforming. Disruptive fixation can be applied at the conduit outlet in the manhole, and rigid fixation is required on both sides of the cable joint to ensure its safety.
(5) In tunnels, cables can be laid in a serpentine manner to absorb deformations caused by thermal mechanical forces. The cables need to be fixed when laid on inclines, and rigid fixation is also required on both sides of the joints to ensure the safety of the cable joints.
(6) Large cross-section cables in vertical shafts can be laid in a serpentine manner using a clamp, and suspended fixation can be made at the top of the shaft to absorb deformations caused by thermal mechanical forces.
(7) Cables laid on bridges must be encased in aluminum to reduce the fatigue strain caused by bridge vibrations on the cable metal sheath. The laying method can reference conduit or tunnel. It is important to consider not only the thermal expansion of the cable, but also the expansion of the bridge. At the bridge expansion joint and the bridge entry and exit points, flexible fixation must be adopted, or cable racks that allow the cable to expand freely should be used.