If the three armature coils of a 3-phase alternator are not interconnected but are kept separate, as shown in Figure below, then each phase or circuit would need two conductors, the total number of conductors, in that case, being six. It means that each transmission cable would contain six conductors which will make the whole system complicated and expensive.
Hence, the three phases are generally interconnected which results in substantial savings of copper. The general methods of interconnection are:-
- Star or Wye (Y) connection and
- Mesh or Delta (Δ) connection.
Star or Wye (Y) Connection
In star connection, three similar ends (start or finish) of the three windings are joined together at a common point. This point is known as star point or neutral point. The three conductors meeting at star point are replaced by a single conductor known as neutral conductor (or neutral). Star connection is also known as Y or Wye connection.
If the voltage of a star-connected alternator is applied across a balanced load, the neutral wire will carry three load currents which are exactly equal in magnitude but 120o out of phase with each other. Hence, their vector sum is zero.
i.e. IR + IB + IY = 0,
or IN = 0.
The neutral wire, in that case, may be omitted although its retention is useful for supplying lighting loads at low voltages.
Voltages and Currents in Star Connection
The potential difference between any terminal (or line) and neutral (or star) point gives the phase voltage (VPH). But the potential difference between any two lines gives the line-to-line voltage or simply line voltage (VL).
In balanced star connection line voltage = √3 x phase voltage (in magnitude) and line voltage leads the phase voltage by 30o.
The current in each winding is known as phase current (IPH) and the current flowing in each line is called line current (IL). In star connection line current is equal to phase current.
While considering the current distribution in a three-phase system, always keep the following two points in mind:
- Arrows placed alongside the phase currents IR, IY, and IB indicate the direction of current when they are assumed to be positive and not the direction at a particular instant, and at no instant, all the three currents will flow in the same direction. It is so because the three currents have a phase difference of 120o.
- The current flowing outwards in one conductor is equal to the sum of currents flowing inwards in the other two conductors. It means that each conductor provides a return path for the currents of the other two conductors.
Advantages and Applications of Star Connection
- Three-phase generators are usually star-connected. It is so because only 1/√3 of the line voltage will appear on every phase winding of the alternator. It means in star connected generator, the number of coil turns required per phase is less than for a delta-connected generator.
- Star connection provides two voltages i.e. phase voltage and line voltage. Hence lighting loads are connected across the three phases whereas power loads like three-phase motors are connected across lines. Therefore, this system (three-phase four-wire system) is widely used to supply electric power to domestic as well as commercial and industrial users.
- Another advantage of the star connection is that the neutral of the generator can be earthed. In that case, the potential difference between each line and earth is equal to phase voltage i.e. VL/√3.
Hence if by a fault line conductor is earthed, the insulator will have to bear a voltage of VL/√3 only. But in the case of delta connection, the insulator will have to bear full line voltage VL. It will increase the possibility of insulator breakdown.
Balanced Star Connection
A balanced star connection is one in which the three-phase voltages are equal in magnitude but displaced 120o from one another. In a balanced star-connected system, three-line voltages will also be equal in magnitude but displaced 120o from one another.
Star Connected Lighting Loads
In Figure, a star-connected lighting network in a three-story house is shown. For such a load, it is essential to have a neutral wire in order to ensure uniform distribution of load among the three phases despite random switching on and off or burning of lamps.
It is seen from the Figure, that network supplies two flats on each floor of the three-story residence and there is a balanced distribution of lamp load among the three phases. There are house fuses at the cable entry into the building which protects the two mains against short-circuits in the main cable.
At the flat entry, there are apartment (or flat) fuses in the single-phase supply which protects the two mains and other flats in the same building from short circuits in a given building.
There is no fuse on the neutral wire of the mains because blowing of such a fuse would mean a break in the neutral wire. This would result in unequal voltages across different groups of lamps in case they have different power ratings or numbers.
Consequently, filaments in one group would burn dim whereas in other groups they would burn too bright resulting in their early burn-out.