Phase Sequence in Electrical Systems

In Electrical Systems, sometimes without identification of phase sequence it is impossible to proceed further. Both students and practicing engineers often find it confusing.  There are some important situations where identification of phase sequence is a must. These are when:

  • One synchronous generator is to be synchronized to the grid.
  • Two systems are to operate in parallel.
  • Two transformers are to operate in parallel.
  • Connecting two different lines originating from the same source.
 In a three phase system the voltage or current sinusoid attain peak values periodically one after another. The sinusoid are displaced 120 degrees from each other. So also phasors representing the three sinusoids for voltage or current waves of three lines are phase displaced by 120 degrees.

Now the question arises what is the sequence ? In which order the voltage or current waves attain the peak values cyclically. In the diagram just look at the ABC Anti Clock Wise phase sequence. Here the phasors are rotating in anti clock wise direction. An imaginary viewer(see figure-A, left ) will encounter first phase A, then B, then C again A, then wise.The sequence is ABCABCABC.......  or ABC Anti Clock Wise sequence .

You might imagine about the possibility of phase sequence ACB Anti Clock Wise. Yes it can be! In this case the phasors rotating the same anti clock wise direction the imaginary viewer(see figure-A, right )  will encounter first phase A, then C, then B. So here the sequence continues like ACBACBACB......  .

You might think that in anti clock wise rotation, do the other sequences possible?

you may think why not BCA or BAC or CBA or ......?

From the above ABC sequence if you start from B then you can see that BCA is nothing but the same ABC sequence. Similarly BAC and CBA sequence are the same as ACB, only we started from other phase.

Note:  If you studied permutation and combination maths then it is easier to appreciate the case.

Hence it is clear that for anti clock wise rotation there are two possible phase sequences ABC or ACB.

Why anti clock wise ? yes it is the convention mostly used. Just recall the school maths when you always measured the trigonometric angle starting from positive x-axis in anticlockwise direction and called it positive angle and in clock wise direction the angle is negative. Accordingly the sine wave is drawn. This is the reason why anti clock wise rotation is so prevalent.

However the Clock Wise ABC and ACB phase sequence can also be used. The phase sequence identification is purely a convention. It helps in identifying the sequence in which three phase voltage or current attain  the peak values.

The Anti Clock Wise ABC is equivalent to Clock Wise ACB. Just analyze by placing the imaginary viewer and rotating the phases in respective directions.

When any two of the three phase conductors connecting to the three phase induction motor is interchanged the phase sequence of the supply to motor is changed. This results in the rotation of motor in opposite direction. Actually this is the principle used in mechanical phase sequence detectors. The same direction of rotation means same phase sequence. These days solid state sequence detectors are increasingly used.

In some regions of the world other letters may be used for phase sequence, like L1L2L3 or RYB. 
It is really confusing when synchronization of two different systems are considered.

In real world, before synchronization or paralleling, the two sides phase sequence is identified by using the same sequence detector. If the same direction of rotation is observed for both sides by the detector, then they are marked accordingly for same sequence. Of course only same sequence is not sufficient. It is also ascertained that terminals of same phase are connected together.