Single Line Diagram of AC Power Transmission

AC Power Transmission Line System

In this post, single line diagram of power system is given. It consists of generating station, primary transmission line, secondary transmission line, primary distribution system and secondary distribution system.

Generating Stations

• The electrical power is generated in the thermal / hydro / nuclear power station. 
• The generation of 11 kV is done by alternator. 
• The generated power is transmitted to different areas which are far away from generating station. 
• The generated voltage is step up for transmission due to following reasons. As the generating voltage increases

      ( 1 ) Conductor volume ( weight ) decreases

     ( 2 ) Efficiency of transmission line increases

     ( 3 ) Percentage voltage drop decreases.

Primary Transmission 

• The step up transmission step up the generating voltage from 11 kV to 132 kV / 220 kV. 
• It is transmitted by three phase, three wire supply system.

Secondary Transmission

• The primary transmission line terminates at the receiving substation. 
• The transmission line voltage steps down from 132 kV / 220 kV to 66 kV at the receiving end substation. 
• The electrical power is transmitted at voltage level of 66 kV to different substation. 
• This is called as secondary transmission.

Primary Distribution

• The secondary transmission line terminates at the substation where the voltage is reduced from 66 kV / 11 kV by step down transformer. 
• The 11 kV voltage is transmitted to different area of city by transmission line. 
• The primary transmission is done through 3 – phase, 3 – wire system which supply power to HT consumers.

Secondary Distribution

• The electrical power from primary distribution line is delivered to pole mounted distribution transformer. 
• The pole mounted delta / star transformer step down the 11 kV to 440 V.
•  The secondary of transformer star connected with neutral which supplies three phase four wire. 
• The residential consumer gets single phase supply through one phase and neutral ( Phase R and N, Phase Y and N, Phase B and N ). 
• The three phase load is directly connected to three phase supply.

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