SAG in electrical power transmission.

What is  Sag.?

In electrical power transmission and mechanical design of overhead transmission line.

SAG.

A perfectly flexible wire of uniform cross-section, when string between the two supports at the same level, will form a catenary. However, if the sag is very small compared to the span, its shape approximation a parabola.

 The difference in level between the point of support and the lowest point on the conductor is known as sag 

The factors affecting the sag in an overhead line are given below.

1. Weight of the Conductor,

 This affect the sag directly. Heavier the conductor, greater will be the sag. In locations where ice formation takes place on the conductor, this will also cause increase in the sag.

2. Length Of the Span.

This also affect the sag. Sag is directly proportional to the square of the span length Hence other conditions, such as type of conductor, working tension, temperature etc. remaining the same a section with longer span will have much greater sag.

3. Working Tensile Strength.

The sag is inversely proportional to the working tensile strength of conductor if other conditions such as temperature, length of span remain the same. Working tensile strength of the conductor is determined by multiplying the ultimate stress and area of cross section and dividing by a factor of safety.

4. Temperature.

All metallic bodies expand with the rise in temperature and, therefore. The length of the conductor increases with the rise in temperature, and so does the sag.

Reference from .

Transmission and distribution of electrical power by-J.B.Gupta.

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The Part of Electrical Distribution system

The Electrical Distribution system  further be divided into following.

Feeders, Distributors and Service Mains.

1.Feeders.

Feeders are the conductors which connect the stations (in some cases generating stations) to the areas to be fed by those stations.

Generally from feeders no tapping is taken to the consumers therefore current loading of a feeder remains the same along its length. 

It is designed mainly from the point of view of its current carrying capacity 

2.Distributors. 

Distributors are the conductors from which numerous tapping’s for the supply to the consumers are taken. 

The current loading of a distributor varies along its length. Distributors are designed from the point of view of the voltage drop in them.

3.Service Mains.

Service mains are the conductors which connect the consumer's terminals to the distributor.

Distributor is subject to the legal requirement that the voltage at the consumers terminals should be maintained within +/- 6% of the declared (or rated) voltage.

However, there is no such legal restriction on a transmission line and the voltage can vary as much as 10% or even 15% due to variations in loads.

Reference from.

Transmission and distribution of electrical power by- J.B.Gupta

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COMPARISON BETWEEN OVERHEAD AND UNDERGROUND ELECTRICAL SUPPLY SYSTEMS

Transmission and distribution of electric power can be carried out by overhead as well as underground systems. Comparison between the two is given below:

1.Public Safety: Underground system is more safer than overhead system.

2.Initial Cost: Underground system is more expensive. For a particular amount of power to be transmitted at a given voltage the underground system costs almost double the cost of overhead system.

3.Flexibility: Overhead system is more flexible than underground system. In overhead system new conductors can be laid along the existing ones for load expansion. In case of underground system new conductors are to be laid in new channels.

4.Working Voltage: The underground system cannot be operated above 66 kV because of insulation difficulties but overhead system can be designed for operation upto 400 kV or higher even.

5.Maintenance Cost: Maintenance cost of underground system is very low in comparison with that of overhead system.

6.Frequency of Faults or Failures: As the cables are laid underground, so these are not easily accessible. The insulation is also better, so there are very few chances of power failures or fault as compared to overhead system.

7.Frequency of Accidents:The chances of accidents in underground system are very low as compared to overhead system.

8.Voltage Drop: In underground system because of less spacing between the conductors inductance is very low as compared to overhead lines, therefore, voltage drop is low in underground system.

9.Appearance: Underground system of distribution or transmission is good looking because no wiring is visible. Due to its good looking, inspite of its higher cost it in adopted in modern cities like Chandigarh(Punjab) Faridabad.Ahmedabad.(Gujarat)

10.Fault Location and Repairs: Though there are very rare chances of occurring fault in underground system, but if occurs it is very difficult to locate that fault and it repair is difficult and expensive.

11.Charging Current: On account of less spacing between the conductors the cables have much capacitance, so draw higher charging current.

12.Jointing: Jointing of underground cables is difficult so tapping for loads and service mains is not conveniently possible in underground system.

13.Damage Due to Lightning and Thunder Storm: Underground system is free from interruption of service on account of thunder storm, lightning and objects falling across the wires.

14.Surge Effect: In underground system surge effect is smoothened down as surge energy is absorbed by the sheath.

15.Interference to Communication Circuits: In underground system there is no interference to communication circuits.

Mostly the high voltage transmission is carried out by overhead system due to low cost. However, distribution in congested areas and in modern cities, like Chandigarh, Ahemdabad is carried out by underground cables.

The overhead line as a mean of transmitting electrical power over long distances is chep and efficient (a 400 kV double-circuit line may transmit 2,000 MW over a distance of 160km in either direction with an efficiency of 98%).

Reference from.

Transmission and distribution of electrical power by- J.B.Gupta.

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