Neutral wire functions in 3 phase - 4 wire system

The power from generating stations is transmitted over long distances through transmission lines to various receiving stations. The power is then distributed to various sub-stations located at various places and localities. The voltage is ultimately stepped down to 400/230 volts i.e. 400 volts for bulk consumers and 230 volts for general domestic consumers.

The connections of the windings of the transformers installed in the sub-station are in delta on the primary side and in star on the secondary side.
 
The distribution is generally single phase two wire and three phase four wire. The voltage between any phase wire and neutral is 230 volts and between any two phase wire is 400 volts.

The supply to houses, small offices, shops other premises requiring small loads is taken from the distribution mains at 230 volts with the help of one phase and one neutral wire.  
 
Where supply is to be given to a large establishment like hotels, offices, hospitals the system of three phase four wire supply is adopted.  It consists of three phase wires and a neutral.

 
The function of neutral wire in 3 phase 4 wire system is to serve as a return wire for general domestic supply system. The neutral is paired to each of the single phase load. The potential of the neutral point can be very well understood from the following Figure.

In the above diagram, the alternator is connected to the load by three phase four wire system. The neutral points in both alternator and the load are joined together. The neutral wire serves as common return to all the three phases acting outward from N1.

Therefore, the total neutral current is the vector sum of the three line currents. Under balanced conditions the vector sum is zero and therefore, the neutral current is zero. In this case, there is no question of voltage drop along neutral and the potential of N2 is same as that of N1.
 
This makes clear that if the supply system is reverted to three phase three wire system the neutral conductor can be removed without any change in potential distribution of the network. In that case, the potential of N2 will still be equal to that of N1. This is why, main transmission network is three wire system.
 
The three phase loads are balanced and do not contribute to neutral current therefore the neutral conductor can be removed.
 
But balance of load on every phase is difficult in the case of single phase loads. Due to this imbalance some neutral current always flows. Therefore, the neutral wire is very essential in this case.

Balancing of Phases in 3 Phase 4 Wire System

 
Balancing of phases means distribution of single phase lighting loads equally on 3 phase 4 wire supply line conductors so that the line currents on all the phases are approximately equal.
 
The difference in load will cause an out of balance current to flow through neutral wire. The impedance of three conductors will be the same and unequal current flowing through them will give rise to unequal voltage drops with the possibility of the voltages at the loads being unbalanced. However to attain absolutely equal distribution in such cases is not possible and as a result a small current in the neutral may exist.
 
To obtain fairly equal distribution of load in three phase wires, the residential buildings should be connected sequentially, where three phase supply is given to the large buildings such as hotels, schools, commercial buildings etc., it is important that equal distribution of the load on all the phases should be the main objective.
 
The ‘balancing’ provides most effective utilization of the generator and the transformer. For instance, a 100 kVA transformer can satisfactorily take 33.3 kVA single phase load on each of its phases. If it is connected on only one phase supply, it will then be overloaded.

Why is the neutral earthed?

 
The purpose of earthing of neutral is shown in Figure.

In Figure A, an 11 kV / 230 V transformer is fed from 11 kV line is shown. The secondary of this transformer is not earthed in this case.
 
If insulation between HT and LT windings fails due to any reason, 11 kV supply voltage will appear at 230 V terminals of the transformer. It will be a very dangerous situation for both equipment connected to that line and the operator.
 
Now look at the Figure B, the secondary of the transformer is earthed in this case. If the 11 kV voltage appears at secondary terminals then excessive current will flow through the path shown in the Figure and fuse will blow.
 
Hence the earthing of the neutral wire of a distribution transformer at the sub-station is very necessary form the safety point of view.
 

Voltage between Neutral and Earth

 
Very little voltage between neutral and earth may exist as the neutral is solidly connected to earth at the sub-station and it may rise if earthing of sub-station is not working properly.
 
Under faulty conditions, for instance the fuse or the circuit breaker protecting the feeder does not trip in the case of earth fault on one of the lines, the neutral may attain a much higher potential with respect to the earth.
 
Under such conditions, there will be heavy voltage drop across the sub-station earth due to fault current which may give quite severe electric shock.