Polyphase Systems

The neutral on a single phase system that you encounter in domestic power supplies is the neutral of the three phase supply being delivered to your locality. A pure single phase system does not have a neutral.

The potential of the neutral is zero only if the load is balanced on all the three phases. A balanced three phase system would not have any current in its neutral, but, an unbalanced one would have current in the neutral. Normally, the neutral would carry some amount of current as the load cannot be exactly balanced. The neutral does not carry this current to the generator per se, as there is no neutral wire on transmission lines. Transmission lines are configured in delta, and hence do not have a neutral wire.

 

In a commercial 3-phase distribution system, the neutral carries no current, except under fault conditions. If a hot wire is shorted to ground for example, neutral current will be present. During unfaulted conditions, the 3 hot leads carry all load current, balanced or unbalanced.

The benefit of 3 phase is that only 3 wires are needed. A 4th wire carrying current defeats the purpose of 3 phase. If every transformer has at least one delta connected set of windings, or a 3 legged E type core, than the 3 phases remain balanced even with unbalanced loading, without neutral current.

 

Actually, most residential systems are supplied a single phase center-taped 230volt feed from an autotransformer on the pole. The center-tape is grounded at the pole and in the residents panel, the neutral is connected to this center-taped ground. It carries current but the potential will be close to zero volts.

 

In a three-phase wye connected system the neutral may or may not be present depending on the way the load is connected. If the load is to be operated from the phase voltage (i.e. connected between any phase and neutral) a neutral wire will be needed. If the load is operated from line voltages (i.e. connected between any two phases) it will not be needed.

If the load is unbalanced, and a neutral wire is used, then the current in the neutral is the phasor sum of the three line currents.

In the special case of a balanced load, the phasor sum equals zero for the neutral.

Keep in mind that the stuff you have read in all of these posts depends on how the system is connected.

CS