Electrical Engineering ⇒ Topic : Unbalanced 3-Wire Star-Connected Load
Unbalanced 3-Wire Star-Connected Load
Fig. (a) shows an unbalanced *3-wire star load connected to a balanced 3-phase supply. Note that supply star point N (normally at zero potential) is isolated from the load star point N'. Since the Y load is unbalanced and N is isolated from N', the potential of N' will be different from N. The result is that (i) load phase voltages will not be equal to the supply phase voltages e.g., VRN is not **equal to VRN' (ii) the three load phase voltages will be unequal and will differ in phase by unequal
angle. The magnitude of each load phase voltage will depend upon the degree of load unbalance.If the load impedance in one or more phases changes, the potential of N' also changes, causing the load phase voltages to change in magnitude and in phase. The load neutral N' is sometimes called the floating neutral because its potential changes according to the imbalance of the load.
If the load unbalance is severe, the potential of N' w.r.t. N may vary considerably and the load phase voltages may become highly unbalanced in magnitude as well as phase. Such a condition is definitely undesirable. Some loads will operate inefficiently because of low voltage, while other equipment may be damaged due to overvoltage. Because of the variations on an unbalanced 3-wire Y load, individual single phase loads should not be connected in Y unless they form a balanced 3-phase load (or nearly balanced 3-phase load) at all times.
The following points may be noted carefully
(i) The phasor sum of the three unbalanced line currents is zero i.e.,
IR+ IY +IB = 0 . .... ...phasor sum
This is expected because the unbalanced line currents meet at load star point N' and they cannot pile up there. According to Kirchhoff's current law, their phasor sum must be zero.
(ii) Only load phase voltages change in magnitude and in phase. However, line voltages remain
balanced i.e., they are equal in magnitude and 1200 apart in phase.
Triangular phasor diagram. The behaviour of unbalanced 3-wire star load can be best explained from the triangular phasor diagram shown in Fig. (b). Here Nis the neutral (normally at zero potential) of the supply and is located at the centre of the equilateral triangle RYB. The point N' is the load neutral. Due to load unbalance, N' has some potential w.rt. N and is shifted to some other position. The distances RN, YN and BN are equal but 120° apart from one another and represent the supply phase voltages. However, distances RN', YN' and BN' (shown dotted for clarity) are unequal and represent the supply line voltages as well as the load line voltages and are balanced i.e., equal in magnitude and 120° apart in phase.
It may be seen that magnitude and phase of load phase voltages (i.e., RN' YN' and BN') depend upon the position of load star point N'. The greater the potential of N w.r t. N, the greater the distance NN' and more unbalanced will be the load phase voltages.
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