1- 3 phase 4 wire AC system:

Three phase four wire system is used for power supply distribution in cities and villages as widely. In primary side is used delta winding and secondary side is used star winding. Star winding is so important for getting neutral point so neutral point is important for equipment utilization. Without neutral we can not utilize the equipment.
The star connection in secondary side output is important so delta to star is called three phase four wire system.

As per star winding, one side three phase R, Y, B winding points is connected as junction point and other side all three phases is connected to separate R, Y and B phase. The junction of three phase is called neutral and it is connected to the earth point.

So this type three phase and neutral is called as three phase four wire system.

2- 3 phase 4 wire system voltage calculation:

Distribution transformer are used in 3 phase 4 wire system. It is called another name as step down power transformer where input supply is connected to HT Power and output voltage is connected with utilisation panel.

Input Voltage: 11KV or 33KV

Output Voltage: 415V

3- Circuit diagram of 3 phase 4 wire system:

According circuit diagram power transformer primary is connected in delta winding and secondary out put winding is designed with star connection. In star connection 4 output connection is received where 3 for phase and one is neutral.Neutral is used in power distribution for utilization.

4- Advantage of three phase four wire.

The advantages of this system are below mentioned.

1. Neutral: We can get neutral from secondary star junction point for utilization of equipment.
2. Load balancing:: If the load is unbalance on secondary side then it will affect to primary side. The primary winding is already connected to the delta connection then unbalance load will distribute into all three phases as equal.
3. Harmonic rays: Star junction point is connected to the earth point for removing harmonic filter from AC current.

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