This type of distribution system is used for feeding balanced three-phase loads only e.g., three-phase motors. As there is no return path for out-of-balance current, in the case of unbalanced star loads, the load will get unequal voltages in its phases. In case of extreme unbalance, phase voltage across a load may become almost equal to line voltage which will be hazardous.
Three-phase four-wire system:
This system can feed both balanced three-phase loads as well as unbalanced three-loads and single-phase loads. The neutral wire provides a path for out-of-balance current. Therefore, even though currents are unbalanced, phase voltages are balanced.
Typical AC Power Supply Scheme:
- The large network of conductors between the power station and the consumers can be broadly divided into two parts which are the transmission system and distribution system.
- Each part can be further subdivided into two which are primary transmission and secondary transmission and primary distribution and secondary distribution.
- In a certain power scheme, there may be no secondary transmission and, in another case, the scheme may be so small that there is only distribution and no transmission.
Generating Station:
- Here electric power is produced by 3-phase alternators operating in parallel.
- The usual generation voltage is 11 kV ( It may be 6·6 kV or even 33 kV in certain cases).
- For economy in the transmission of electric power, the generation voltage (i.e., 11 kV) is stepped up to 132 kV (or more) at the generating station with the help of 3-phase transformers.
Primary Transmission:
- The electric power at 132 kV is transmitted by a 3-phase, 3-wire overhead system to the outskirts of the city.
- This forms the primary transmission.
Secondary Transmission:
- The primary transmission line terminates at the receiving station which usually lies at the outskirts of the city.
- At the receiving station, the voltage is reduced to 33 kV by step-down transformers.
- From this station, electric power is transmitted at 33 kV by the overhead system to various sub-stations (SS) located at the strategic points in the city.
(iv) Primary Distribution:
- The secondary transmission line terminates at the sub-station (SS) where voltage is reduced from 33 kV to 11kV, 3-phase, 3-wire.
- The 11 kV lines run along the important roadsides of the city.
- It may be noted that big consumers (having demand more than 50 kW) are generally supplied power at 11 kV for further handling with their own sub-stations.
Secondary Distribution:
- The electric power from the primary distribution line (11 kV) is delivered to distribution sub-stations (DS).
- These sub-stations are located near the consumers’ localities and step down the voltage to 400 V, 3-phase, 4-wire for secondary distribution.
- The voltage between any two phases is 400 V and between any phase and neutral is 230 V.
- The single-phase residential lighting load is connected between any one phase and neutral, whereas 3-phase, 400 V motor load is connected across 3-phase lines directly
- Three phases four-wire system is the common distribution system for domestic supplies because it gives a choice of single-phase voltage as well as three-phase voltages and allows unbalances too.
- 3 Phase 3 wire system includes all the phases i.e., R Y and B while in 3 phase 4 wire system an additional neutral N is present.