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Typical AC Power Supply system (Generation, Transmission and Distribution) scheme and Elements of Distribution System

Typical AC Power Supply system (Generation, Transmission and Distribution) scheme and Elements of Distribution System (a complete note With Diagrams)

The lines network between Generating Station (Power Station) and consumer of electric power can be divided into two parts.

  • Transmission System
  • Distribution System

We can explore these systems in more categories such as Primary transmission and secondary transmission. Similarly primary distribution and secondary distribution. This is shown in the below image (One Line or Single Line diagram of Typical AC power System Scheme).

It is not necessary that the entire steps which are sown in the above image must be included in the other power schemes. There may be difference. For example, there is no secondary transmission in many schemes, in some (small) schemes there is no transmission, but only distribution.

Also Read: Nuclear Power. Why is It The Last Option in Most Countries?

The following parts of a typical power supply scheme are shown in figure 1.

Typical AC Power Supply system (Generation, Transmission and Distribution) scheme and Elements of Distribution System (a complete note With Diagrams)

  1. Generating Station
  2. Primary transmission
  3. Secondary transmission
  4. Primary Distribution
  5. Secondary Distribution

Following is detail of the above sections

Generating Station:

The place where electric power produced by the parallel connected three phase alternators/generators is called Generating Station (i.e. power plant).

The Ordinary  power plant capacity and generating voltage may be 11kV, 11.5 kV 12kV or 13kV. But economically, it is good to step up the produced voltage from (11kV, 11.5kV Or 12 kV) to 132kV, 220kV or 500kV or greater (in some countries, up to 1500kV) by Step up transformer (power Transformer).

Primary Transmission:

The electric supply (in 132kV, 220 kV, 500kV or greater) is transmuted to load center by three phase three wire (3 Phase – 3 Wires) overhead transmission system.

also read: Power Factor

Secondary Transmission:

Area far from city (outskirts) which have connected with receiving station by line is called Secondary transmission. At receiving station, the level of voltage reduced by step-down transformers up to 132kV, 66 or 33 kV, and Electric power is transmit by three phase three wire (3 Phase – 3 Wires) overhead system to different sub stations. So this is a Secondary Transmission.

Primary Distribution:

At a sub station, the level of secondary transmission voltage (132kV, 66 or 33 kV) reduced to 11kV by step down transforms.

Generally, electric supply is given to those  heavy consumer which demands is 11 kV, from these lines which caries 11 kV ( in three phase three wire overhead system) and they make a separate sub station to control and utilize this power.

In other cases, for heavier consumer (at large scale) their demand is about 132 kV or 33 kV.  they take electric supply from secondary transmission or primary distribution ( in 132 kV, 66kV or 33kV) and then step down the level of voltage by step-down transformers in their own sub station for utilization ( i.e. for electric traction etc).

Secondary Distribution:

Electric power is given by (from Primary distribution line i.e.11kV) to distribution sub station. This sub station is located near by consumers areas where the level of voltage reduced by step down transformers 440V by Step down transformers.

These transformers called Distribution transformers, three phase four wire system ((3 Phase – 4 Wires)). So there is 400 Volts (Three Phase Supply System) between any two phases and 230 Volts (Single Phase Supply) between a neutral and phase (live) wires.

Residential load (i.e. Fans, Lights, and TV etc) may be connected between any one phase and neutral wires, while three phase load may be connected directly to the three phase lines.

Elements of Distribution System

Secondary distribution may be divided into three parts

  1. Feeders
  2. Distributors
  3. Service Lines or Service Mains

Elements of Distribution SystemFeeders:

Those Electric lines which connect Generating station (power station) or Sub Station to distributors are called feeders.Remember that current in feeders (in each point) is constant while the level of voltage may be different; the current flowing in the feeders depends on the size of conductor. Fig 2.

Also Read: Power Factor Improvement


Those taping which extracted for supply of electric power to the consumers or those lines, from where consumers get electric supply is called distributors.As shown in fig 2.Current is different in each section of the distributors while voltage may be same. The selection of distributors depends on voltage drop and may be design according voltage drop. It is because consumers get the rated voltage according rules.

Good to Know: the main difference between Feeder and Distributor is that Current in Feeder is same (in each section) in the other hand, Voltage is same in each section of Distributor

Service Lines or Service Mains:

The Normal cable which is connected between Distributors and Consumer load terminal called Service Line or Service Mains.Here is a a complete Typical AC Power Supply system scheme, in other words, the above whole story in below image.Typical AC Power Supply system scheme 

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  1. this page is awsum. and wasim khan u r genius

  2. good mashallah

  3. wasim khan can u please clarify that in the primary transmission where there are three phase and 3 wires, why there are 6 lines instead of 3? thanks

  4. nicely for imagination…..

  5. Great write up as always..I wd like to get something straight: on distribution feeders, u said the current flowing depends on the size of the conductors..I think u meant to say the current flowing DETERMINES the conductor size. All in all great work. Keep it up. We all have room for improvement ☺

  6. My brother suggested I might like this web site. He used to be entirely right. This post truly made my day. You can not believe just how much time I had spent for this info! Thank you

  7. three phase has only three wires or not?<br />

  8. three phase has only 3 wires or more..?

  9. In T&D (Transmission and Distribution) system, why use 11KV,22KV, 33Kv for 11 series only.plz explain friends

  10. pls write me some notes and FAQs on electrical power generation , transmission and distribution

  11. Ahmad Md Daud

    Sometime we come across 3 phase 415 V 50 hz electrical power supply. As noted in the above note said about of 400 V line phase supply. Is there any different between these two types of voltage?? What is the significant?

  12. what is the best way for connection generation 600- 1000 MW if these generators near load center is it possible to connect them at 132 kv or to 400 kv

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