The ranges of transmission lines as given below.

Short transmission lines: below 80 km

Medium transmission lines: 80 km to 160 km

Long transmission lines: above 160 km

In radial distribution system, different feeders radially came out from the substation and connected to the primary of distribution transformer.

A parallel distribution system has two ends feeding and an alternative of parallel line. So, in case there is a fault, the isolator can isolate the faulty part and let the healthy system operate.

When an alternating potential difference is applied across two conductors whose spacing is large as compared to their diameters, there is no apparent change in the condition of atmospheric air surrounding the wires if the applied voltage is low.

When the applied voltage exceeds a certain value (critical disruptive voltage), the conductors are surrounded by a faint violet glow called corona.

Materials used for conductor in transmission lines must have following properties.

• High conductivity
• High tensile strength
• Light weight
• High resistance to corrosion in weather conditions
• High thermal stability
• Low coefficient of thermal expansion
• Low cost

Materials use for transmission lines are:

• Copper
• Aluminium
• Cadmium – Copper alloys
• Phosphor bronze
• Galvanized steel
• Steel core copper
• Steel core aluminium

The voltage limit for solid type cables is 66 kV due to the following reasons:

(a) As a solid cable carries the load, its conductor temperature increases and the cable compound (i.e., insulating compound over paper) expands. This action stretches the lead sheath which may be damaged.

(b) When the load on the cable decreases, the conductor cools and a partial vacuum is formed within the cable sheath. If the pinholes are present in the lead sheath, moist air may be drawn into the cable. The moisture reduces the dielectric strength of insulation and may eventually cause the break- down of the cable.

Skin effect is the tendency of an alternating electric current 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.

The electric current flows mainly at the skin of the conductor, between the outer surface and a level called the skin depth.

The skin effect depends on the following factors.

a) Frequency

b) The diameter of the conductor

The sources of heat generation in the cables are:

• Dielectric losses in cable insulation
• Losses in the conductor
• Losses in the metallic sheathing and armouring

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 aluminium is provided over the insulation as shown in figure. It has a minimum dielectric stress in a cable.

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 armouring.

Armouring: Over the bedding, armouring is provided which consist 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.

• When suspension string is used to sustain extraordinary tensile load of conductor, it is referred as strain insulator
• When there is a dead end or there is a sharp corner in transmission line, the line has to sustain a great tensile load of conductor or strain
• A strain insulator must have considerable mechanical strength as well as the necessary electrical insulating properties

The transmission lines which have length less than 80 km are generally referred as short transmission lines.

Distributor fed at both ends:

• In this type of feeding, the distributor is connected to the supply mains at both ends and loads are tapped off at different points along the length of the distributor
• Here, the load voltage goes on decreasing as we move away from one feeding point say A, reaches minimum value and then again starts rising and reaches maximum value when we reach the other feeding point B, the minimum voltage occurs at some load point and is never fixed; It is shifted with the variation of load on different sections of the distributor
• If a fault occurs on any feeding point of the distributor, the continuity of supply is maintained from the other feeding point
• In case of fault on any section of the distributor, the continuity of supply is maintained from the other feeding point
• The area of cross section required for a doubly fed distributor is much less than that of a singly fed distributor
• It is more economical

Sag in overhead Transmission line conductor refers to the difference in level between the point of support and the lowest point on the conductor. Due to sag between two supports, conductor takes the form of catenary.

• Aluminium is light in weight and is having good conductivity.
• But it is having very low tensile strength.
• To make it suitable to be used as conductor for transmission line, we need to increase its tensile strength.
• To increase the tensile strength, steel is used as a core of conductor.
• A good example of steel core aluminium is an ACSR (Aluminium Conductor Steel Reinforced) conductor.
• ACSR is have high tensile strength, good conductivity and economical.
• ACSR conductor widely used in transmission line and as an inner conductor of coaxial cable.
• In ACSR conductors, there is no requirement of insulation between aluminium and steel conductors.

• Booster transformer is most economical for regulating the voltage in distribution feeder
• Booster transformer is used towards the end of a power line to raise the voltage to the desired value
• It is used for controlling the voltage of a feeder at a point far away from the main transformer
• It is used in railways for eliminating the flow of stray current

• Aluminium is light in weight and is having good conductivity
• But it is having very low tensile strength
• To make it suitable to be used as conductor for transmission line, we need to increase its tensile strength
• To increase the tensile strength, steel is used as a core of conductor
• A good example of steel core aluminium is an ACSR (Aluminum Conductor Steel Reinforced) conductor
• ACSR is have high tensile strength, good conductivity and economical
• ACSR conductor widely used in transmission line and as an inner conductor of coaxial cable
• In ACSR conductors, there is no requirement of insulation between aluminium and steel conductors