Correct Answer - Option 4 : Single-phase ac supply using a 3-wire system.
Primary transmission :
The electric supply (132 kV, 220 kV, 500 kV or greater) is transmitted to load center by three-phase three-wire (3 phase - 3 wires) overhead transmission system.
Secondary transmission :
At the receiving station, the level of voltage reduced by step-down transformers up to 132 kV, 66 or 33 kV and electric power is transmitted by three-phase three-wire (3 phase - 3 wires) overhead system to different substations.
Primary distribution :
At a substation, the level of secondary transmission voltage (132KV, 66 or 33KV) is reduced to 11 kV (in a three-phase three-wire overhead system) by step down transformers.
Secondary distribution :
- Electric power is given to (from primary distribution line (i.e.) 11 kV) distribution substation.
- This substation is located nearby consumers area where the level of voltage reduced by step down transformers is 415 V.
- In a 3 phase four wire system (3 phase - 4 wires), there are 415 volts (Three-phase supply system) between any two phases and 230 volts (single-phase 2-wiresupply) between neutral and any one of the phases (lives) wire.
- Residential load (i.e. Fans, light, and TV, etc) may be connected between any one phase and neutral wires, while three-phase loads may be connected directly to the three-phase lines.
- A three-phase 415V, supply is used for supplying small industrial and commercial loads such as garages, schools, and blocks of flats. A single-phase 230 V supply is usually provided for individual domestic consumers.
