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 B.....like 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.


14 comments:

Anonymous said...

"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."

Anti clock wise is positive becasue while moving anti clock wise, increase in angle increases in magnitude, hence it is taken as positive. just refer to how Phasor of sinA is drawn in circular real and imaginary frame

Anonymous said...

if anti colck wise ABC is equivalent to anticlock wise ACB ,then why do we need both? is there any significance?

Anonymous said...

i wrote anti clock wise ACB,but it would be clock wise ACb ,plz explain that qus and furthermore if you kindly elaborately tell me the reason of only ABC and ACB sequence
i know about permutation and combination,there would be 6 arrangement possible but we only take 2;i guess a explanation but i need your answer ,kindly provide answer ASAP

Anonymous said...

HOW TO DETERMINE IF A PHASE CHANGED IN HV , HOW WILL THAT IMPACT THE LV ROTATION

mon said...

Thank you for your answer.Now I knew.

mon said...

Thank you for your answer now i knew what is phase sequence.

Ahmed Khaled said...

Really Thanks For Very Simple Explanation , I Failed to Get Something About This Any Where Else , Thanks Again , And I Want From You One More Favour , I Want You To Help Me With a Good Book In "Power System" That Explains Everything Clearly As You Do , Thanks

Anonymous said...

i couldn't understand what do you want to say....

KenG said...

A generated 3 phase is meant to have 3 waveforms. Seen on an oscilloscope, it will be apparent that the 3 waveforms are spaced 120 degrees apart, and there is a definite sequence. If the waveforms are identified as A, B & C in the sequence as they appear on the oscilloscope in time, then the supply conductors supplying these waveforms must necessarily be identified as A, B & C respectively. The phase sequence now represented by ABC is now the positive phase sequence, in the sense that A is ahead in time of B, and B in ahead of C, then recycles for C to be ahead of A as the waveforms continue into time.
Once the first supply is so labelled with A,B, C, any 3 phase motor whose terminals are marked A, B, C are connected to the respective supply terminals A, B, C that rotates in the direction it has been manufactured to do (eg turn a pump or fan in the correct direction) will be regarded as being correctly wired. If not, two conductors of the motor must be swapped over and the two terminal labels swapped with the conductors. Now if the motor is reconnected to the supply, the motor will turn in the manufactured direction.
The essential requirement in all this is that a phase sequence indicator must be tested on a known supply wired up for the socket terminals A, B, C to truly provide the positive phase sequence A, B, C in time sequence. If the phase sequence indicator reads correct that it is positive ABC, then and only then can it be used to be check other supply sockets for correct wiring of the socket(s) for positive phase sequence.
KG

Mr. Belize said...

"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." If it is just a convention then how can this be? I thought that if you say anticlockwise or clockwise you are effectively saying that the positive degrees will be in that direction for the phasor diagram you are drawing. Furthermore, ABC clockwise and ABC anticlockwise should be the same thing. Let me know if I am misuderstanding.

Mr. Belize said...

"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." If the direction is just a convention then how can this be. I thought that when you say anticlockwise or clockwise you are referring to where the positive degrees will be placed on your vector diagram. For clockwise, 90 degrees is placed to the right of 0 but to the left for anticlockwise. Therefore, for both ABC clockwise and ABC anticlockwise, B lags A by 120 degrees and C lags A by 240 degrees. Furthermore, they are the same thing, just a mirror image of one another with the degrees being mirrored as well. Let me know if I have misunderstood the comment. Thanks

Baba Pola said...

what are the effects of incorrect phase sequence to the three-phase compressor?

Baba Pola said...

what are effects of incorrect phase-sequence to the compressor?

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