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DEPARTMENT OF TECHNICAL EDUCATION
ANDHRA PRADESH
Name : G.Venkateswara Rao
Designation : Lecturer
Branch : Electrical & Electronic Engg.
Institute : G.P.T,Hyderabad
Year/semester : V Semester
Subject : Power systems-II
( Transmission & Distribution)
Subject code : EE-504
Topic : Transmission lines
Sub topic : Need for transmission & supply systems
Duration : 50 min
Teaching Aids : ppt, diagrams.

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RECAP
• Electrical energy (power) is essential ingredient for
the Industrial and all round development of any
country.
• It is converted form of energy and generated centrally
in bulk then transmitted economically over long
distances.

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On completion of this period, you would be able to
know
OBJECTIVES
•The need of Transmission lines, choice of supply
systems and relative advantages of D.C and A.C
systems.

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NEED FOR TRANSMISSION LINE
• Generating stations, transmission lines and distribution
system are the main components of an electric power
system.
• Generating stations and a distribution systems are
connected through transmission lines, which also connect
from one grid to another.

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• Electrical power generated at a voltage of 11 to 25 kv,
which is stepped up to the transmission levels in the range
66 to 400 KV or higher.
• The need of distribution systems and transmission lines is
to transmit the electric Power from the generating station to
the Consumer (Industry or Domestic).

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FIG.1

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TRANSMITTING THE ELECTRICAL POWER
Types of systems:
• D.C (Direct current) system
• A.C (Alternating current) system

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TYPES OF D.C SYSTEMS
• D.C Two wire system
• D.C Two wire with mid point earthed system.
• D.C Three wire system

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TYPES OF A.C SYSTEMS
1. Single phase A.C system
• Single phase two wire.
• Single phase two wire with mid point earthed.
• Single phase three wire

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2. Two phase A.C system
• Two phase three wire
• Two phase four wire
3. Three phase A.C system
• Three phase three wire
• Three phase four wire

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D.C Two wire system
D.C SYSTEMS

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D.C Two wire with mid point earthed system

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D.C Three wire system with mid point earthed

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A.C SYSTEMS
Single phase two wire:

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Single phase two wire with mid point earthed:

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• Single phase three wire with mid point earthed:

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Two Phase Three wire:

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Two phase four wire:

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Three phase three wire:

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Three phase four wire:

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Advantages of D.C Transmission:
• Requires only two conductors.
• No inductance & capacitance effect.
• No phase displacement & surge problem.

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• No skin effect & less corona loss.
• Reduced interference with communicating circuits.
• Potential stress on the insulation is less.

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• Loss due to charging current eliminated.
• Used in under ground cables also.
• Voltage regulation is better & No stabilizer required

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Disadvantages of D.C Transmission:
• Power can’t be generated at high voltages
• Voltage can’t be stepped up
• Cost of converting and inverting stations are high
• Requires costly switch gear

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Advantages of A.C Transmission:
•Voltage can be stepped up and stepped down.
•Electrical power generated at high voltages.
•Maintenance of substation is easier &cheaper
• Cost of switch gear is less

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• More conductor material is required.
• Power loss due to charging current.
• Construction of transmission lines are difficult.
• Synchronizing problems.
Disadvantages of A.C Transmission:

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• Due to skin effect line resistance will increase
• Due to corona Spacing between conductors should be
kept more.
• In cables alternating current causes sheath loss

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SUMMARY
• The need of transmission and distribution lines.
• Choice of supply system.
• Different types of D.C & A.C systems.
• Advantages & disadvantages of both systems.
We have discussed about

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1. In India ------ system is adopted for
transmission of electric power.
1. 2 phase 2 wire
2. 1 phase 2 wire
3. 3 phase 4 wire
4. 3 phase 3 wire
Quiz

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2. D.C transmission is ----- to A.C transmission.
1. Poor
2. Best
3. Good
4. superior

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3. Primary transmission is done by 3 phase --- wire
1. 2
2. 5
3. 4
4. 3

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4. ---- voltage is used for power transmission
as a matter of economy.
1. Low
2. High
3. Medium
4. All of the above

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FREQUENTLY ASKED QUESTIONS IN
EXAMINATIONS
1. What is the need of transmission lines?
2. What are the supply systems? Draw the
diagrams of each system.
3. Distinguish between D.C and A.C supply system?

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