Electrical Engineering ⇒ Topic : Main points of Star or Wye Connected System

The following points may be noted :

(1) The three phase voltages (i. e. E_RN, E_YN, and E_BN) are equal in magnitude but displaced 120° from each other. The same is true for line voltages (i.e. V_RY, V_YB and V_BR). Such a supply system is called balanced supply system.

(2) It can be shown See Art. (A) that in case of balanced star-connected supply system, the magnitude of line voltage is ,√3 times the magnitude of phase voltage i.e.

                                    Line voltage =   √3 x Phase voltage                                                                      ........ in magnitude

Thus Y-connected balanced supply system enables us to use two voltages viz, phase voltage and line voltage.

(3) In star connection, the lines are in series with their respective phases. Therefore, magnitude of line current is equal to the magnitude of phase current i.e.

                                Line current = Phase current                            ............. in magnitude

(4) For balanced loads, all line currents (or phase currents) are equal in magnitude but displaced 120° from each other.

(5) For 3-phase, 4-wire star-connected system, the current I_N in the neutral wire is the phasor sum of the three line currents. For a balanced load, I_N= 0. If the load is not balanced, the neutral wire will carry current equal to the phasor sum of the three line currents.

                                                                                                 figure (a)

Note. The arrowheads alongside currents (or voltages) indicate their directions when they are assumed to be positive and not their actual directions at a particular instant. At no instant will all the three line currents flow in the same direction either outwards or inwards. This is expected because the three line currents are displaced 120⁰ from one another. When one is positive, the other two might both be negative or one positive and one negative.Thus, at any one instant, current flows from the alternator through one of the lines to the load and returns through the other two lines. Or else current flows from the alternator through two of the lines and returns by means of the third.