Important Notes About Cable--Voltage Class
Classification of underground cables on the basis of voltage level is given belowType of Cable VS Voltage level (kV)
- Low tension (L.T.) Cable ----0 – 1 kV
- High tension (H.T.) Cable ----1 – 11 kV
- Super tension (S.T.) Cable ---11 – 33 kV
- Extra high-tension (E.H.T.) Cable ---- 33 – 66 kV
- Extra super-tension(E.S.T.) Cable ----66 kV and above
Parts of Cable - Lead Sheath -- Bedding
Lead sheath:
- In order to protect the cable from moisture, gases or other damaging liquids (acids or alkalis) in the soil and atmosphere, a metallic sheath of lead or aluminum is provided over the insulation. It has minimum dielectric stress in a cable.
- The extruded Lead sheath also serves as Metallic Screen. Copper Tape/Copper sheath can also be used but Copper is costly and would rather be used as overhead conductors with less mechanical stresses.
Bedding:
- Over the metallic sheath is applied a layer of bedding which consists of fibrous material like jute or hessian tape.
- It is to protect the metallic sheath against corrosion and from mechanical injury due to armoring.
Armoring:
- Over the bedding, armoring is provided which consists of one or two layers of galvanized steel wire or steel tape.
- Its purpose is to protect the cable from mechanical injuries while laying it or handling it.
Sheaths are used in underground cables to Protect the cable form moisture
Armoring protects a cable against mechanical injury
3 Phase Cables- Screened Cables
- In practice, underground cables are generally required to deliver 3-phase power. For the purpose, either three-core cable or three single-core cables may be used.
- For voltages upto 66 kV, 3-core cable (i.e., multi-core construction) is preferred due to economic reasons.
- However, for voltages beyond 66 kV, 3-core-cables become too large and unwieldy and, therefore, single-core cables are used.
The following types of cables are generally used for 3-phase service:
- Belted cables — up to 11 kV
- Screened cables — from 22 kV to 66 kV
- Pressure cables — beyond 66 kV.
Screened cables:
These cables are meant for use up to 33 kV, but in particular cases, their use may be extended to operating voltages up to 66 kV. Two principal types of screened cables are H type cables and S.L. type cables.
Insulation Property Of Cable
The satisfactory operation of a cable depends to a great extent upon the characteristics of insulation used. In general, the insulating materials used in cables should have the following properties.
- High insulation resistance to avoid leakage current
- High dielectric strength to avoid electrical breakdown of the cable
- High mechanical strength to withstand the mechanical handling of cables
- Non-hygroscopic i.e., it should not absorb moisture from air or soil; The moisture tends to decrease the insulation resistance and hastens the breakdown of the cable. In case the insulating material is hygroscopic, it must be enclosed in a waterproof covering like a lead sheath
- Non-Inflammable
- Low relative permittivity
- Low cost so as to make the underground system a viable proposition
- Unaffected by acids and alkalies to avoid any chemical action
The various insulating materials used in the manufacturing of cables are rubber, VIR, paper, Polyvinyl Chloride (PVC), Varnished Cambric, Polyethylene, Gutta-Percha, Silk, Cotton, enamel etc.
Cable Insulations:
Generally, insulation of cables is required to have high dielectric strength, to provide the necessary withstand against normal operating and impulse voltages.
There are mainly two types of cable insulating materials.
- Impregnated Paper
- Polymer.
- Impregnated paper is the most popular insulating material, especially for the land installations in the H.V and EHV lines.
- Polymer insulation is preferably used in new installations like DC and the submarine system.
- Around 83% of HV cables are paper insulated.
- Solid paper insulated cables are suitable for application up to the MV range. At higher voltage, there is some operation problem with them due to high dielectric losses and pressure of voids.
- In oil-filled cables at high voltage, the formation of voids is prevented by maintaining a flow of oil under pressure.
- The impregnation of paper improves the dielectric properties of paper insulation.
- Polymer insulation is immune to thermal degradation and must not be prone to defects such as voids, protrusions, or contaminants.
- For extra-high voltage (400 kV and higher) new types of types mode up of PPLP (Poly propylite laminated paper) are used.
- PPLP has the advantage of combining the technical traditional technology used for impregnated paper cables with the excellent electrical and dielectric properties of synthetic insulators.
Conductor Material Used For Cables
Annealed Copper:
- Due of superior conductivity, annealed copper became the international standard to which all other electrical conductors are compared.
- In 1913, the International Electrotechnical Commission defined the conductivity of commercially pure copper in its International Annealed Copper Standard.
- Copper is used as the electrical conductor in many categories of electrical wiring.
- Copper wire is used in power generation, power transmission, power distribution, telecommunications, electronics circuitry, and countless types of electrical equipment.
- Copper and its alloys are also used to make electrical contacts. Electrical wiring in buildings is the most important market for the copper industry.
- Roughly half of all copper mined is used to manufacture electrical wire and cable conductors.
- annealed copper conductors are most suitable for indoor and outdoor wires and cables.
Aluminium:-
- An aluminium wire have a 1.5 times larger cross-section to pass the same current as a copper wire, but two times lighter.
- Weight is one of the most important parameters for high-voltage power lines that transmit power over long distances.
- Therefore, only aluminium wires are used in main overhead power lines.
Hard drawn copper:-
- Hard-drawn copper has high electrical conductivity and great tensile strength.
- Use in transmission lines.
Silver:-
- Because of best thermal and electrical conductivity among all metals, it is best for electrical applications.
- Its cost is high and less abundant on Earth so has limited applications.
Some Most Asked MCQs
1. HT cables are generally used up to
A. 11 KV
B. 22 KV
C. 66 KV
D. 33 KV
Answer: A. 11KV
2. LT cables are generally used up to
A. 33 KV
B. 1 KV
C. 66 KV
D. 2 KV
Answer: B. 1 KV
3. Super tension Cables are Generally used up to
A. 66 KV
B. 33 KV
C. 132 KV
D. 220KV
Answer: B. 33 KV
4. Extra High tension Cables are Generally used up to
A. 33 KV
B. 66 KV
C. 132 KV
D. 220 KV
Answer: B. 66 KV
5. Extra Super tension Cables are Generally used up to
A. 66 KV
B. 132 KV
C. 220 Kv
D. 33 KV
Answer: B. 132 KV
6. Belted Cables are used up to
A. 11 KV
B. 22KV
C. 66 KV
D. 33KV
Answer: A. 11 KV
7. Screened Cabled are used up to
A. 11 KV
B. 22KV
C. 66 KV
D. 33KV
Answer: C. 66 KV
8. Pressure Cabled are used up to
A. Above 11 KV
B. Above 22KV
C. Above 66 KV
D. Above 33KV
Answer: C. Above 66 KV