EELCTRICAL POWER CABLES:

Q1) What is the difference between a wire and a cable?

A wire is a single conductor(material most commonly being copper or aluminium) while cable is two or more insulated wires wrapped in one jacket. Multiple conductors that have no insulation around would be classified as a single conductor.

Q2) What are the most important parameters while selecting a cable?

There are various types of cable available but the choice of particular type depends on the operating voltage and service requirement. However, a cable must fulfill some of the basic requirement. Those requirements are as follows

Short Circuit Rating:

It happens frequently that the conductor size necessary for an installation is dictated by its ability to carry short-circuit current rather than sustained current. During a short –circuit, there is a sudden inrush of current for a few cycles followed by a steadier flow of current for a short period until the protection switchgear operators, normally between 0.1-0.3 seconds.

Current Carrying Capacity:

Current carrying capacity is an important aspect is the selection of the optimum size of conductor. The safe current carrying capacity of an underground cable is determined by the maximum permissible temperature rise. The cause of temperature rise is the losses that occur in a cable which appear as heat.

Voltage Drop:

The allowable maximum voltage drops from source to load is another aspect of power cable conductor design.

As per ohm’s law V = IR. The first is the choice of material used for the wire. Copper is a better conductor than and will have less voltage drop than aluminum for a given length and wire size. Wire size is another important factor in determining voltage drop. Larger wire sizes (those with a greater diameter) will have less voltage drop than smaller wire sizes of the same length.

Q3) How cables are classified on the basis of voltages?

·        LT cables: Low-tension cables with a maximum capacity of 1000 V

·        HT Cables: High-tension cables with a maximum of 11KV

·        ST cables: Super-tension cables with a rating of between 22 KV and 33 KV

·        EHT cables: Extra high-tension cables with a rating of between 33 KV and 66 KV

·        Extra super voltage cables: with maximum voltage ratings beyond 132 KV

Q4) Why do we use stranded conductors? What are strands?

Instead of having one solid conductor, we usually have multiple strands in a cable. Strands are basically thin wires of small cross sectional area.

Basic, reason of using stranded conductor is to make the conductor flexible. If we use a single solid conductor. It does not have sufficient flexibility. It becomes difficult to transport a single solid conductor of long length over the distance. To eliminate this drawback, conductor is formed by using several thin wires of small cross section. By making the conductor stranded, it becomes flexible. Which makes stranded conductor suitable to be coiled easily to transport it over long distance.

Generally, in successive layer, the stranding is done in opposite direction to preceding layer. This mean, if the strands of one layer are twisted in clockwise direction, the strands of next layer will be twisted in anticlockwise direction and so on ‘x’ is number of layers in conductor.

Q5) What is skin effect in cables and how stranding is helpful in reducing it?

Skin effect is the tendency of an alternating electric current (AC) to become distributed within a conductor such that the current density is largest near the surface of the conductor, and decreases with greater depths in the conductor. Current passes through the skin of the conductor and not from the centre.

By using stranded conductors surface area increases and therefore a better current carrying capacity for AC

Q6) What are the components of an underground cable?

An underground cable consists of conductor, insulation, sheath, bedding, Armour and serving

Conductor > Insulation > Metallic Sheath > Bedding > Armour > Serving.

An easy way to remember this is CIMBAS

1)     Cores or Conductors

The number of core or conductors in a cable depends upon its use. For example, a three core cable is used for three phase system. The conductors are made of copper or aluminum and are stranded to provide flexibility.

2)     Insulation:

A suitable thickness of insulation is provided on each core or conductor so that the conductors of underground cable may withstand the operating or designed voltage. The thickness of insulation on the core increases as the operating or design voltage is increased. Normally the insulation materials are made of impregnated paper, varnish cambric or rubber mineral compound.

3)     Metallic Sheath:

Metallic sheath surrounds the insulation on the core. It is provided to protect the insulation from moisture, gases, oil, liquids etc. present in the soil and atmosphere. It is normally made of lead or aluminum.

4)     Bedding:

Bedding surrounds the sheath. It is made of fibrous materials like jute or hessian tape. The purpose of bedding is to protect the sheath from mechanical injury due to armouring. You can think it like a shock absorber.

5)     Armouring

Armouring is provided over the bedding to protect the cable from mechanical injury during its handling and lying. It consists of one or two layers of galvanized steel wire or steel tape. In some cable, armouring is not provided.

6)     Serving:

To protect the armouring from atmospheric condition, a layer of fibrous material like jute is applied on the armouring. This layer is called serving

Thus, we observe that the main working part of underground cable are core / conductor and Sheath. Other parts of cable, just protect the conductor and sheath from mechanical injury or chemical attack. This does not mean that, they are not important. Bedding, Armouring and Servings are very important parts of cable else cable won’t work as its insulation will degrade.