Question

THESE QUESTIONS ARE FROM THE SUBJECT POWER SYSTEM FUNDAMENTALS

1. Name the important components of an overhead transmission line.

2. Discuss the various conductor materials used for overhead lines. What are their relative advantages and disadvantages ?

3. Discuss the various types of line supports.

4. Why are insulators used with overhead lines ? Discuss the desirable properties of insulators.

5. Discuss the advantages and disadvantages of (i) pin-type insulators (ii) suspension type insulators.

6. Explain how the electrical breakdown can occur in an insulator.

7. What is a strain insulator and where is it used ? Give a sketch to show its location.

8. Give reasons for unequal potential distribution over a string of suspension insulators.

Answer 1

1. Overhead Transmission lines are used for transmission of electrical power.The main components are

a) Conductors: conductors are lines which carry power from one end to other end.

b) Line Supports: supports may be towers or poles which carry electrical conductors.Generally made up of steel, wood, RCC.

c) Insulators :these are connected to poles or towers which provide insulation between conductor and ground as well as provide support to the conductors.Generally made of porcelain or glass.

d) Cross Arms :Cross arms holds the insulators to the poles or towers.Generally made of steel.

2. Various conductor materials used for conductors are copper, aluminium, steel-cored aluminium, galvanised steel and cadmium copper.

a) Copper :It has higher current carrying capacity than any other material.it is an ideal material for conductors

Advantages: 1. High electrical conductivity per unit cross section area and high Tensile strength.2. High durability and has high scrap value.

Disadvantages: 1.High cost 2. Less available material.

b) Aluminum: it is alternatve material for copper but has less conductivity then copper.

Advantages: 1.specific gravity is less so aluminum is light weight compare to copper 2.cheaper and more available then copper.

Disadvantages: 1.less tensile strength and high coefficient of linear expression so sag is more 2. Conductivity is 60% of copper conductivity.

C) Steel core aluminum: As aluminum has lower tensile strength the sag is more so over come this effect aluminum is reforced with steel core as steel has higher tensile strength.The conductor obtained is called ACSR (aluminum conductor stell reforced).

Advantages:1.Tensile strength is high compared to aluminum 2. Less sag and used for bigher spans.3.due to smaller sag ,the towers or poles with less height is used.

Disadvantage: as conductor surface is not uniform the corona loss will be more in ACSR conductors.

Galvanised steel: As steel as high tensile strength this type of conductors are used in extreme weather conditions where stress on conductors is high.

Advantages : High tensile strength so used for long spans .

Disadvantages: 1.As steel has high resistance so there will more power losses so used for short line section where abnormally high stress is present. 2. Not suitable for transmission of power over long distance.

Cadmium copper: It is a copper alloyed with cadmium to increase the tensile strength of copper . Adding 1-2% of cadmium the strength is increased by 50%.

Advantages : More tensile strength so used for exponentially long spans.

Disadvantages: high cost of cadmium so rarely used.

3.Various types of line supports are wooden poles,steel poles ,RCC poles and steel towers.selection of poles depends on span length ,voltage level, cost,cross section area.

1. Wooden poles :Made of seasoned wood and suitable for moderate cross section area and short span length. There are cheaper and used in distribution lines of voltage level 20kv ,in rural areas.problem with this type is ground of poles rottens fast so life span(20-25 years) is less for this type of poles .

2. Steel poles: Has high tensile strength , longer span length, more life span.The type of poles are painted or galvanized to avoid rusting and increasing life span. Used in distribution lines of cities.these are of 3 types 1. Rail poles 2. Tubular poles 3.Rolled steel joints .

3.RCC poles : these are made of concrete and used in distribution lines of voltage level upto 33kv .These have longer life and very little maintenance is required.But cost is more.provides good insulation between conductor and ground.

4.Steel towers : Used in high voltage and long distance transmission lines.Has higher mechanical strength and can withstand in extreme weather conditions. Used for long spans.For different voltage level there are different designs of steel towers.

4. Insulators in transmission lines are used for two purposes one is to support line conductor and other is to provide insulation between conductor and towers or poles.these are attached to line Supports.

The proporties of insulators are

1.The insulator material should have high Resistance in order avoid leakage currents to earth.

2.The insulator should have high mechanical strength in order to support heavy conductors.

3.Material should have high permitivity and high dielectric strength.

4.Insulator should have high ratio of puncher strength to flashover

5.Insulator material should be non porous , free from impurities so that permittivty is not lowered.