Star Connection Definition
In this method of interconnection, three similar ends a’, b’, c’ of the three windings are joined together at a common point. This point is known as star point or neutral point N.
The three conductors meeting at star point are replaced by a single conductor known as neutral conductor (or neutral).
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. Ia + Ib + Ic = 0,
or IN = 0.
In the case of balanced loads, the neutral may be omitted to give us three phase three wire system. The three phase four wire system is widely used to supply electric power to domestic as well as commercial and industrial users because it provides two different values of the supply voltage.
In star connected systems (i.e. alternators or transformers) the potential difference between any line (or terminal) and the neutral is called phase voltage (VPH) but the potential difference between any two lines gives the line voltage (VL).
In a balanced Star Connection
VL = √3 Vph (in magnitude)
But line voltage leads the phase voltage by 30o.
IL = Iph (in magnitude and phase)
Power, Pthree phase = √3 VLIL cos φ
Power, Pthree phase = 3 VphIph cos φ
While considering the current distribution in three phase system, always keep the following two points in mind:
- Arrows placed alongside the phase currents Ia, Ib, and Ic 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 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 alternator. It means in star connected generator, a 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.
- Another advantage of 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.