# Three phase and single phase AC Distribution system

In the previous post we have seen that how electricity flows from generating station to the consumer, you can read that post here. In this tutorial, we are going to learn about three phase and single-phase AC Distribution system.

Three phase AC Distribution system has two stages:

1. Primary Distribution (3-phase 3-wire system)
2. Secondary distribution (3-phase 4-wire system)

Before going to discuss the two stages of distribution system, we should learn a little about two types of configurations of three phase system.

Three phase system has two types of configurations:

1. Delta connection
2. Star connection

## What is Delta and Star connection system?

In three phase system the 3 windings can be connected in two different ways as shown in the figure below.

The Delta connection has no common or neutral point because the ends of all three windings are connected to each other. This connection forms the 3-phase 3-wire system.

The star connected system has a neutral or common point because the three ends of windings are connected to each other but another 3 ends are joined together which makes the neutral point. A neutral wire is taken from this point along with other three phase wires hence, it forms 3-phase 4-wire system.

## What is primary and secondary distribution?

### Primary AC Distribution system:

Primary distribution of three phase power starts from distribution Substation at voltage level of 11kV or 33kV generally.

The power is taken from distribution Substation to the pole mounted substation by using 3-phase 3-wire system as shown in figure below.

Industries and large factories take direct connection from these lines to fed three phase loads.

### Secondary AC distribution system:

The pole mounted substation steps-down the voltage level from 11kV to just 415V for secondary distribution purpose.

The primary side of distribution transformer placed at pole mounted substation is Delta connected but the secondary side is Star connected. Hence, a neutral wire is also present in secondary distribution system which also makes it 3-phase 4-wire system.

The voltage between any two-phase wires is 415 volts but the voltage between one phase wire and neutral wire will be only 230 volts which is used for single phase supply system.

Users with heavy loads or high-power demands takes the three-phase power directly, but normal residential users with normal load demand uses the single-phase supply.

The single-phase supply is given by taking one phase connection out of three phase wires and one neutral connection.

Let us assume that a Colony has 15 houses, then supply to 5 houses will be given from red phase and neutral wire, next 5 house will use yellow phase and neutral wire, and then the blue phase and neutral wire will complete the supply to last 5 house.

In this way Three phase and single phase distribution system works.