1. Page 1 of 17
Power Transmission & Distribution
Engr. Asad Muneer
Lecture # 1
Introduction of
Distribution Systems
2. Page 2 of 17
Learning Objectives
• Typical AC Supply System.
• Elements of Distribution System.
• Primary AC Distribution System.
• Secondary AC Distribution System.
• Radial Distribution System.
• Ring Main Distribution System.
• Inter-connected Distribution System.
• Overview of Voltage Levels.
• Review Questions.
4. Page 4 of 17
Generation
• 3-phase alternators
• 11 KV (may be 6.6 KV, 22 KV, 33 KV)
• Step up transformers
Primary Transmission
• 132 KV, 220 KV, 400 KV, 500 KV, 750 KV
• 3-phase, 3-wire system
• Steel Towers
Secondary Transmission
• Receiving stations
• Received HV.
• Step down transformers
• 3-phase, 3-wire system
• 33KV or 22 KV
5. Page 5 of 17
Primary Distribution
• Substation
• Received 22 KV or 33 KV.
• Step down transformers
• 11 KV, 6.6 KV, 3.3 KV
• 3-phase, 3-wire system
• To feeders
Secondary Distribution (Distributers) •
Step down distribution transformers
• 11 KV to 400V or 230 V.
• 3-phase, 4-wire system
• To service mains
6. Page 6 of 17
Components of Distribution System
Distribution Substation (DS):
• Receives 22 KV or 33 KV.
• Step down transformers.
• Reduced to 3.3 KV/6.6 KV/11 KV for Feeders.
Feeders:
• Conductors (lines) of large current carrying capacity.
• They connect sub-station to area where power is to be distributed.
• No tapping’s are taken on feeders.
• Current in feeder remains same.
• Design consideration of feeders is current carrying capacity while voltage drop of
feeder can be compensated at substation.
7. Page 7 of 17
Distributers:
• Conductors used to distribute supply to consumers
from sub-station.
• Tapping’s are taken for supply on distributers.
• AB, BC, CD and DA are distributers.
• Current in distributer is not same because of tapping’s at
different places.
• Design consideration of distributers is voltage drop across
it.
• Voltage variations limit at consumer’s terminal is + 6 of rated voltage.
Service Mains:
• Small cables which connect distributers to consumer’s terminals.
8. Page 8 of 17
Primary AC Distribution System
• PDS provides power to various distribution sub-stations.
• Sub-stations are located near consumer localities and contain step-down transformers.
• Operates at voltages somewhat higher than general utilization voltages.
• Primary distribution voltages may be 11 KV, 6.6 KV and 3.3 KV.
• Due to economic considerations, PDS is 3-phase, 3-wire system.
9. Page 9 of 17
Secondary AC Distribution System
• SDS provides electricity to consumer utilities.
• SDS is 400/230 V, 3-phase, 4-wire system.
10. Page 10 of 17
DC & AC Radial Distribution System
11. Page 11 of 17
• In Radial Distribution System, Distributer is connected to substation on one end only
with the help of feeder.
• As Feeders, Distributors and Service Mains radiate away from Sub-station, hence
name given as radial system.
• Feeder OC supplies Distributer AB at point A.
• Distributer AB is fed at one end only i,e., point A.
Advantages:
a) Simplest as distributer is fed at only one end.
b) Low initial cost.
c) Useful when generation is at low voltage.
d) Preferred when substation is located at the centre of load.
Disadvantages:
a) The end of the distributer near to substation gets heavily overloaded.
b) As consumers are dependent on single feeder and distributer, any fault on any of these
two causes interruption in supply to all consumers.
c) When load on distributer changes, the consumers at the distant end of distributor face
voltage fluctuations.
13. Page 13 of 17
• In Ring Main System, Feeder covers the whole area of supply in the ring fashion and
terminates at the substation from where it was started.
• Primaries of distribution transformers form a loop, hence the name given.
• Substation supplies to closed Feeder LMNOPQRS.
• Distributors are tapped from different points M, O and Q of Feeder through distribution
transformers.
Advantages:
a) Less voltage fluctuations at user’s end.
b) As each Distributor is fed from two Feeders, continuity of supply is maintained in case
of fault in any Feeder. If fault occurs at any point “F” of section SLM of Feeder, then
14. Page 14 of 17
section SLM of Feeder can be isolated for repair and supply can be maintained to all
consumers via Feeder SRQPONM.
15. Page 15 of 17
Inter-Connected System
• In Inter-connected System, Feeder ring is energized by two or more than two
substations.
16. Page 16 of 17
• Feeder ring ABCD is supplied by S1 and S2 at points D & C respectively.
• Distributors are fed at points O, P, Q and R via distribution transformers.
• It increases service reliability.
• More efficient system. Feeders can be fed according to load demand.
Overview of Voltage Levels
S # System Voltage Levels
1 Generation 6.6 KV, 11 KV, 22 KV, 33 KV
2 Primary Transmission 66 KV, 132 KV, 220 KV, 400 KV, etc.
3 Secondary Transmission 11 KV, 22 KV, 33 KV
4 Primary Distribution 6.6 KV, 11 KV
5 Secondary Distribution 230 KV, 400 KV
17. Page 17 of 17
Review Questions
Q. 1 what do you understand by distribution system?
Q. 2 Define the terms: feeder, distributor and service mains?
Q. 3 Discuss Radial, Ring main and Inter-connected distribution system?
Q. 4 Discuss design considerations in distribution systems?
Q. 5 What are voltage levels in primary and secondary distribution systems?
Q. 6 How dc voltage is achieved for distribution?
Q. 7 Differentiate AC and DC distribution systems?
Q. 8 Discuss power and distribution transformers?
Q. 9 What are advantages of high transmission?
Q. 10 What are main elements of distribution system?
