Phase Sequence (Y CONNECTED GENERATOR)

The phase sequence can be determined by the order in which the phasors representing the phase voltages pass through a fixed point on the phasor diagram if the phasors are rotated in a counterclockwise direction. For example, in Fig. 1 the phase sequence is ABC. However, since the fixed point can be chosen anywhere on the phasor diagram, the sequence can also be written as BCA or CAB. The phase sequence is quite important in the three-phase distribution of power. In a three phase motor, for example, if two phase voltages are interchanged, the sequence will change, and the direction of rotation of the motor will be reversed. Other effects will be described when we consider the loaded three-phase system. The phase sequence can also be described in terms of the line voltages. Drawing the line voltages on a phasor diagram in Fig. 2, we are able to determine the phase sequence by again rotating the phasors in the counterclockwise direction. In this case, however, the sequence can be determined by noting the order of the passing first or second subscripts. In the system of Fig. 2, for example, the phase sequence of the first subscripts passing point P is ABC, and the phase sequence of the second subscripts is BCA. But we know that BCA is equivalent to ABC, so the sequence is the same for each. Note that the phase sequence is the same as that of the phase voltages described in Fig. 1. If the sequence is given, the phasor diagram can be drawn by simply picking a reference voltage, placing it on the reference axis, and then drawing the other voltages at the proper angular position. For a sequence of ACB, for example, we might choose EAB to be the reference [Fig. 3(a)] if we wanted the phasor diagram of the line voltages, or ENA for the phase voltages [Fig. 3(b)]. For the sequence indicated, the phasor diagrams would be as in Fig. 3. In phasor notation,