Single Line Diagram of AC Power Transmission

Generating station

• 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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