Thursday, January 21, 2010

Electric Power Supply Systems

Supply Systems

The convergence of electric power from a Power Station to consumer’s premises is known as Electric Supply Systems.

This post illustrates the basic movement of power from the alternators at the generating station to your home/premises. I have omitted the complicated processes because its meant to give Non-Engineers a glimpse of how power is transmitted.

Generating Station

3-Phase alternators in parallel - The common generation is normally 11kV or 33kV (For advanced countries).

For economical reasons power is stepped up to 132kV or even more depending upon length of transmission lines and amount of power to be transmitted. It is stepped up to 132kV with the help of 3-phase transformers.

The transmission of power at high voltages has several advantages including saving conductor material and high transmission efficiency...

It may be advisable to transmit power at the highest voltage possible, however up to a limit because of insulation problems and increased cost of switchgear and transformer equipment.

Primary Transmission

Electric power at 132kV is transmitted by 3-Phase, 3-wire overhead system to the outskirts of the city. The reason for transmission of power at high voltages is because the amount of power that has to be transmitted through transmission lines is very large and if this is transmitted at 11kV or 33kV, the line current and power loss would be very large.

Power Loss: P = I2R

Therefore this voltage is stepped up to 66kV, 110kV, 132kV, 220kV, 400kV by Sending End Sub-Stations. High voltage transmission lines transmit electrical power from the Sending End Sub-Stations to the Receiving End Sub-Stations.

Secondary Transmission

Primary transmission line terminates at the Receiving Station (RS) which is usually lies at the outskirts of the city. At the Receiving Station voltage is reduced to 33kV by a step-down transformer. From this station power is transmitted at 33kV by 3-Phase 3-wire system overhead to various Sub-Stations.

Primary Distribution

Secondary transmission line terminates at the Sub-Station(S.S). Here voltage is reduced from 33kV to 11kV, 3-Phase, 3-wire. The 11kV lines run along the important roads of the city. This forms the primary distributions. Big consumers (Demand >50kW) are generally supplied power at 11kV for further handling with their own sub-stations.

Secondary Distribution

Power from primary distribution lines (11kV) is delivered to the distribution Sub-Station (D.S.S) or D.S. These substations are located near consumers’ localities. They step-down the voltage to 415V, 3-Phase 4 wire for secondary distribution. Voltage between any 2Phases is 415V (Line Voltage) and between a Phase and Neutral is 240V (Phase Voltage)

The following illustration summarizes the above explanation.


  1. Why generation level is 11KV , & transmission voltage is 66kV, 110kV, 132kV, 220kV, 400kV
    is there any reason or international standard

  2. Yes there is a reason, the 11kV,66kV etc are standards which vary much in countries and there generating capability. In advanced power engineering, you will find out that these values are arrived upon various calculations and economic considerations. The 11-33kV is much preferred for generation because it ensures a balanced 3phase and is also economical.