What Cable Type is Used for Underground Power Distribution?

What is an Underground Power Cable?

A cable that has been buried underground is known as an underground cable. They distribute telecommunications or electrical power. Overhead cables, which are located many metres above the ground, can be substituted with these cables. 

Underground cables are frequently used in place of above-ground wires. The majority of the time, there are very few or no cables above ground in downtown districts with many tall buildings. This is done primarily for aesthetic reasons because underground cables are not visible. Because they are out of the way, they are also less dangerous for people. Though more expensive to install, they last longer, specially 33kv Underground Cable is the best.

The Design and working of Underground Power Cables

Electric power is transmitted and distributed using underground cables in situations when using overhead construction would be impractical. These areas may be congested areas where the cost of the right of way would be prohibitive or where local ordinances prohibit overhead lines for safety reasons, or they may be near plants and substations, or they may be at crossings of large bodies of water where the overhead crossing would be prohibited for a variety of reasons. The voltage and service requirements will determine the type of cables to be utilized. New developments in design and production have produced cables that can be used at high voltages. Because of this, it is now possible to transmit electric power across short or medium distances via underground wires.

A central core or many cores (two, three, or four) of tinned stranded copper conductors (an aluminium conductor is also occasionally used) make up underground cables. The conductors are separated from one another by paper, varnished Cambric, vulcanized bitumen, or impregnated paper. To guard against moisture intrusion, an aluminium, alloy, or lead metallic coating is supplied around the insulation. The only thing that prevented underground cables from being used for the transmission and distribution of electric power was their original high cost.

Mechanical factors and the voltage at which it must function determine the type of cable to be used at a certain place. Low-voltage cables are those that operate at voltages up to 1 kV, high-voltage cables are those that operate at voltages up to 11 kV; super-tension cables are those that operate at voltages up to 33kv Cable. Extra-high tension cables are those that operate at voltages up to 66 kV, and extra-super voltage power cables are those that operate at voltages above 132 kV.

In addition to the aforementioned considerations, the current rating of the cables is also greatly influenced by numerous other variables, including the manner of cable laying used, the distance between the cables, the number of cores, and the thermal conductivity of the soil. To handle each one, cable manufacturers use multiplying factors. Based on a maximum conductor temperature of 80 °C for cables with a voltage of 11 KV and higher, this calculation provides an approximation of the current carrying capability for Indian conditions. Throughout much of India, the ground temperature is 30 °C.

Advantages of Underground Power Cables

• Less vulnerable to damage from extreme weather (mainly lightning, wind, and freezing).
• In order to build underground cables up to 30 metres for 400 KV, a surrounding strip only needs to be 1 to 10 metres wide, whereas an overhead line needs a strip 20 to 200 metres wide to be kept continually clear for safety, maintenance, and repair.
• Neither animals nor low-flying aircraft are at risk from underground cables.

Classification Based on Cable Construction

• Belted Cable: These cables have their conductors (often three) bundled together before being enclosed by an insulating paper "belt." Each conductor in these cables is isolated using paper that has been treated with the appropriate dielectric. A fibrous dielectric substance, such as jute or hessian, is used to fill the spaces between the conductors and the insulating paper belt. This offers both flexibility and a round shape. The copper sheath and armouring are next placed over the jute layer, as we previously mentioned. This cable's unique feature is that it might not be exactly round in shape. To make better use of the available space, it is kept non-circular. 

• Screened Cable: 

1. H-type Cable: M. Hochstadter created the original version. The three cores are each individually wrapped in paper for insulation, and then a metallic screen or cover is placed on top. These metal covers have perforations in them. Because of this design, the three metallic screens are able to touch one another. Then, a metallic tape, often composed of copper, is used to group these three metallic covers together. This structure is surrounded by a lead sheath. Both the sheath and the metallic coverings are grounded. 
2. S.L. Type Cables: These cables are comparable to H Type Cables, except each of the three cores has a separate lead sheath. This clause does away with the requirement for the preceding overall sheath. The benefit of such a design is that the likelihood of a core-to-core breakdown is much reduced. The cable's elasticity has also been increased. The restrictions are rather strict. Only voltages up to 66kV are permitted for this type of construction. Since the individual sheaths are thinner, moisture may enter the cable through construction flaws and lower its dielectric strength.
3. Cables that combine H and S.L. types are known as H.S.L.-type cables: These cables have separate lead sheaths and impregnated paper insulation around each core.

• Pressure cable:  Solid cables become unreliable at voltages higher than 66 kV because electrostatic stresses in the cables are too high. This happens mostly because voltages greater than 66 kV induce voids to form. Therefore, we employ pressure cables rather than solid cables. These wires are often loaded with either gas or oil.
• Oil-filled cables: Oil is pumped through ducts that have been installed for this purpose at an appropriate pressure. Through reservoirs held at the appropriate distances, this oil supply and pressure are maintained. The oil used is the same one that is applied to paper insulators before impregnation.
• Gas-Filled Wires: In an airtight steel pipe, cables are encircled by a pressurised gas (often dry nitrogen). These cables have larger load-carrying capacities and higher operating voltages. However, the total cost is higher.

Based on How The Cable was Installed and laid, a Classification

Direct Buried: As the name implies, the conductors are buried bare in a trench below the Earth. If necessary, cooling pipes can occasionally be added. Once the cables are set up, nothing above the ground is noticeable.

Trough: Cables are put in concrete troughs that have been dug. They can be seen from the outside. The upkeep is simpler.

Tunnels: In some cases, tunnels are excavated for this reason. Such construction is typically used when a river needs to be crossed or when a significant city is the target of the intended electricity distribution. Although expansion and maintenance are simpler, the initial cost is higher.

Gas-insulated lines: This is a very recent technological advancement. Such a gas-insulated line structure is safer for cables handling greater power and higher voltages and currents. Modern projects use it for complex tasks.

22kV Cable Manufacturer in Australia

Znergy Cable is one of the best 22kV Cable manufacturers in Australia. With a team of experienced engineers and workers, we serve the best-quality 22kv cable, 33kv underground cable, etc. 

Also Read: Select The Best Quality Cable for Your Needs