Interconnection of the three phases

INTERCONNECTION OF THE PHASES

The three phases are interconnected to

(i) achieve the economy.

(ii) reduce the number of conductors and space.

(iii) reduce the complexity of the circuit

There are two types of interconnection They are

(i) Star or wye (Y) connection and (ii) Delta (Δ) or Mesh connection.

1. Star or Wye (Y) Connection

In star connection, one set of terminals (either start or end) is connected together at a cominon point N and the other three terminals are connected to external load through three conductors called lines. The external load may be either star connected or delta connected. The point N is called neutral point or neutral. A wire brought from neutral is called neutral conductor. The three line conductors and neutral wire provide 3 phase 4 wire system.

Usually three windings of a 3 phase alternator (A.C. voltage source) are connected in star. The neutral point is connected to ground.

 (a) Line voltage and Phase voltage in star system

The voltage across-each coil is called phase voltage denoted by Ep or E. It is also the voltage between each line and neutral.

ERN, EYN and EBN are three phase voltages. In a balanced supply system, these voltages are equal in magnitude and displaced by 120°. (electrical)

| E_RN | = |E_YN | = | E_BN | = | Eϕ | ... (i)

If E_RN is taken as reference for RYB phase sequence, we can express

E_RN  = Eϕ ∠ 0° … (ii)

E_YN = Eϕ ∠ - 120° … (iii)

E_BN = Eϕ ∠ 120° … (iv)

The voltage between any two lines is called line - to line voltage or simply line voltage denoted by E_L. Here E_RY, E_YB and E_BR are the line voltages.

The phase and line voltages in star connection are shown in Fig. 4.5. The phasor diagram is shown in Fig.4.5.

It is seen that in a star system E_L ≠ E ϕ.

(b)Relation between E, and E, in a balanced star connected 3 phase system - Derivation:

Thus in the star connected alternator, the line voltage is equal to √√3 times phase voltage. Also the phase difference between the line voltages is 120°.

Note: (i) The phasor sum of phase voltages is zero.

i.e., E_RN + E_YN + E_BN = 0

Similarly (ii) Phasor sum of line voltages is zero.

i.e., E_RY + E_YB + E_BR = 0

2. Delta (Δ) or Mesh Connection

If the three coils are connected such that, the start of one coil is connected to the finish of the next coil, the result is delta connection. The fig.4.6 shows a delta connection. Here R' is connected to Y, Y' to B and B' to R. Thus a closed mesh is formed, hence the name mesh connection. The supply lines in delta connection are taken from the three junctions as indicated in the fig.4.7. Thus a delta connection gives 3 phase 3 wire supply system. The phases are connected between the line terminals. The voltage between any two lines is the line voltage E_L.

Hence, in a balanced delta connected system

The phasor sum of line voltages