Phase sequence/rotation confusion

[elec_eng]

This confuses me a lot. Is the A-B-C (CCW) phase sequence/rotation the same as the A-C-B (CW)? By looking at the phasor diagram, I thought they are the same but I want to make sure.

I have a utility with A-B-C (CW) phase sequence and I need to parallel generators that have A-C-B(CW) sequences. Here is what the generator phasor diagram says

"When electrical energy is generated, the phasing is clockwise A-B-C"

So Basically do they have the same phase sequence? I don't have to worry about swapping any phases in the field?

 

[GoldDigger]

As I understand it, there is no ambiguity as to which phase voltage is leading another phase voltage, because the same point in the waveform comes up 1/3 of a period earlier on the leading waveform.
And in standard graphical plots, if the zero phase angle is chosen as the x axis, a positive angle will be in the counter-clockwise direction from that axis.

The problem comes in whether you consider a phasor at a small negative angle from the x-axis as being ahead of the x-axis in time (i.e voltage vector is considered as rotating counter-clockwise) or a phasor at a small positive angle from the x-axis is considered as being ahead in time (i.e. the voltage vector rotates clockwise.)

You can avoid that question entirely by simply stating in the time domain that A peaks first, followed by B, then C.
If you do that, the only room for CW and CCW becomes in whether the motor (for load phasing) or the generator (source phasing) happens to be rotating CW or rotating CCW.

It is true that in the time domain (what peaks first) the order A-B-C is the exact opposite of A-C-B.
A-B-C is the same as B-C-A and C-A-B. You can remember this by the fact that each of the triplets can be found in the same infinitely repeating sequence A-B-C-A-B-C-A-B-C..... It is just a question of where you start.
By the same token, C-B-A, B-A-C, and A-C-B all come from the sequence C-B-A-C-B-A-C-B-A...

That said, I am not really sure how to interpret two specification one with CW and one with CCW in it, but I can authoritatively say that A-B-C (CW) and A-C-B (CW) are NOT compatible and you will have to interchange exactly two leads when you will be switching back and forth between the two sources.

If the utility wires are A, B, and C and the generator wires are a, b, and c, one way to connect them would be A-a, B-c, and C-b.

 

[xptpcrewx]

"When electrical energy is generated, the phasing is clockwise A-B-C"

This is a very ambiguous statement. Do you have a vector diagram on the machine?

Also, you would want to perform phasing/commissioning checks prior to interconnecting the two systems. Put a rotation meter on the generator if you can. If rotation is ok then there's nothing to worry about since the generator will synchronize to the source.

 

[LMAO]

This confuses me a lot. Is the A-B-C (CCW) phase sequence/rotation the same as the A-C-B (CW)? By looking at the phasor diagram, I thought they are the same but I want to make sure.

I have a utility with A-B-C (CW) phase sequence and I need to parallel generators that have A-C-B(CW) sequences. Here is what the generator phasor diagram says

"When electrical energy is generated, the phasing is clockwise A-B-C"

So Basically do they have the same phase sequence? I don't have to worry about swapping any phases in the field?

Not entirely sure what your question is but ABC phase sequence is not the same as ACB. Swapping any 2 phases will reverse rotation. ABC=CAB=BCA is opposite of ACB=CBA=BAC.

 

[kwired]

Not entirely sure what your question is but ABC phase sequence is not the same as ACB. Swapping any 2 phases will reverse rotation. ABC=CAB=BCA is opposite of ACB=CBA=BAC.

I was thinking along the same line. He seems to be wanting to parallel two generators - Whatever is actually identified as A, B, or C is somewhat meaningless though there is a good chance they all are based from same reference in each machine. What is most important is same phase sequence from each unit being connected together in the paralleling effort.

If you had two units with three output leads but no marking on them - getting same sequence connected together from each machine is all you really would care about, plus additional regulation methods to assure they are fully in phase with one another.

 

[Jraef]

A-B-C is CLOCKWISE (CW), A-C-B is COUNTERCLOCKWISE (CCW), they are OPPOSITE, not the same. If you connect a CW generator in parallel to a CCW source, something is going to fail catastrophically! Your generator supplier is simply telling you that the factory wiring of the generator is CW. If you need it to be CCW, you make that change in the way you connect it.

Paralleling two generators without being able to fully grasp the concept of phase rotation is an indicator that perhaps this should be done by someone else with experience.

 

[xptpcrewx]

Clarification

Clarification

A-B-C is CLOCKWISE (CW), A-C-B is COUNTERCLOCKWISE (CCW), they are OPPOSITE, not the same. If you connect a CW generator in parallel to a CCW source, something is going to fail catastrophically! Your generator supplier is simply telling you that the factory wiring of the generator is CW. If you need it to be CCW, you make that change in the way you connect it.

Paralleling two generators without being able to fully grasp the concept of phase rotation is an indicator that perhaps this should be done by someone else with experience.

For the record, all phasors are thought of as rotating CCW so calling it CW is very ambiguous. Instead, system phase rotation for ABC is considered positive rotation; and ACB is considered negative rotation.

Note: Clockwise and Counter-Clockwise really only come into play when talking about motor/shaft rotation as seen from one end of the machine...

 

[iceworm]

This confuses me a lot. ...
So Basically do they have the same phase sequence? I don't have to worry about swapping any phases in the field?

Yes, the first time through one of these is pretty brutal.

Add 2 sets of corner grounded Open Delta pts feeding the synchronizer - and it can be a mess.

The end test is a Chance phasing set across the gen CB - measuring A-A and B-B. (Unless the gen is at 480V, then one can use a couple of DVMs.) With the gen speed (frequency) as close a practicable to the utility, the two sets should be going up and down together. That tells you the rotation is correct.

Then one checks the pt inputs to the synchronizer.

And finally check the sync scope goes to zero when the phasing sets read zero.

I've done this on my own maybe a half-dozen times (in 40 years) And it still bothers me the first time I close the gen CB.

So, to answer. Check the attachments.
The first is my method for determining phase rotation.
The next are a few pages from Nema MG-1 (2016)

Hope this helps.

the worm

 

[Cow]

I wish some degree of field experience was a prerequisite to getting an engineering license.

I have nothing else to add here.

 

[xptpcrewx]

Phase sequence/rotation confusion

I wish some degree of field experience was a prerequisite to getting an engineering license.

I have nothing else to add here.

It absolutely already is. Your beef isn’t with that as much as it is with the quality and application of that experience.


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[iceworm]

I wish some degree of field experience was a prerequisite to getting an engineering license.

I have nothing else to add here.

Cow -
With all due respect, we all started out in the same place - no experience. Yes, even the electricians.

We got a guy looking for technical understanding in a place that reports to have technical expertise.

And he never said he had an engineering license.

 

[kwired]

For the record, all phasors are thought of as rotating CCW so calling it CW is very ambiguous. Instead, system phase rotation for ABC is considered positive rotation; and ACB is considered negative rotation.

Note: Clockwise and Counter-Clockwise really only come into play when talking about motor/shaft rotation as seen from one end of the machine...

and if you don't specify which end of the machine is the base of reference, you still have 50-50 chance of being correct.

 

[xptpcrewx]

Phase sequence/rotation confusion

and if you don't specify which end of the machine is the base of reference, you still have 50-50 chance of being correct.

True but when speaking about system rotation it is purely electrical and not mechanical/physical. In other words, the direction of shaft rotation should already be fixed/specified/determined by the prime mover. The OP first needs to figure out which torque is being applied by the prime mover (CW or CCW), then we can talk about what electrical sequencing to expect for that machine. Once that is known, system rotation can later be discussed.

 

[Cow]

Cow -
With all due respect, we all started out in the same place - no experience. Yes, even the electricians.

We got a guy looking for technical understanding in a place that reports to have technical expertise.

And he never said he had an engineering license.

With all due respect, that doesn't make my point any less valid.

 

[xptpcrewx]

With all due respect, that doesn't make my point any less valid.

It actually does because you are wrong and clearly don’t understand what an engineering license is.

 

[Chamuit]

This confuses me a lot. Is the A-B-C (CCW) phase sequence/rotation the same as the A-C-B (CW)? By looking at the phasor diagram, I thought they are the same but I want to make sure.

I have a utility with A-B-C (CW) phase sequence and I need to parallel generators that have A-C-B(CW) sequences. Here is what the generator phasor diagram says

"When electrical energy is generated, the phasing is clockwise A-B-C"

So Basically do they have the same phase sequence? I don't have to worry about swapping any phases in the field?

No matter what the diagrams say, you should always field verify rotation.

 

[Tony S]

True but when speaking about system rotation it is purely electrical and not mechanical/physical. In other words, the direction of shaft rotation should already be fixed/specified/determined by the prime mover. The OP first needs to figure out which torque is being applied by the prime mover (CW or CCW), then we can talk about what electrical sequencing to expect for that machine. Once that is known, system rotation can later be discussed.

Commissioning a new plant with over 100 drives I had the contractor confidently tell me that if you connect a motor L1 L2 L3 it will go clockwise looking at the shaft. Our drawing office hadn’t told him the whole works phase rotation was reversed. He wasn’t happy, even less happy when I wouldn’t let him reverse the drives at the MCC.

 

[xptpcrewx]

Commissioning a new plant with over 100 drives I had the contractor confidently tell me that if you connect a motor L1 L2 L3 it will go clockwise looking at the shaft. Our drawing office hadn’t told him the whole works phase rotation was reversed. He wasn’t happy, even less happy when I wouldn’t let him reverse the drives at the MCC.

People struggle with the phasing/connection sequencing concept a lot. They think system rotation is just like a damn motor and it’s only a CW or CCW thing, so they look for the easiest physical place to start swapping phases. Then they get angry when you tell them swapping two phases there isn’t the solution and actually causes more problems....

 

[StarCat]

On the Matter of the best Engineers.....

On the Matter of the best Engineers.....

I wish some degree of field experience was a prerequisite to getting an engineering license.

I have nothing else to add here.

The Best ones are those who do go into the field and lay hands on tools and systems.
The best HVAC instructor I ever had was a Utility Electrical Engineer.
The other class that has been referred to I find mostly insufferable. They might be called " Theoretical " Engineers.
Also people who put things right in the " Field " can and should be classed as " Service Engineers." As is often the case, they see, deal with, and solve matters that designers have never considered and do not know about in real terms. This often times happens with the enlistment of " other " Engineers than the ones who have created the problem. This is a case of the best mechanics and the best on the university side getting things done that no one else is held accountable for.

 

[RumRunner]

This confuses me a lot. Is the A-B-C (CCW) phase sequence/rotation the same as the A-C-B (CW)? By looking at the phasor diagram, I thought they are the same but I want to make sure.

I have a utility with A-B-C (CW) phase sequence and I need to parallel generators that have A-C-B(CW) sequences. Here is what the generator phasor diagram says

"When electrical energy is generated, the phasing is clockwise A-B-C"

So Basically do they have the same phase sequence? I don't have to worry about swapping any phases in the field?

No, anytime you have two different AC power sources, there will always be two different phase sequences.

Your generator could be running faster or slower that will have effect on the frequency of the output power.

Besides, your generator prime mover-- being mechanical in nature-- whose speed could be unstable to provide sustainable synchronization-- especially when load is applied or removed.

Your worries about what rotation would be needed in reference to the grid-- is just as important in terms of how important phase sequence is.

There are four conditions that must be met in order to have a successful power synchronization set up.

1. Phase Sequence

2. Voltage Magnitude

3. Frequency

4. Phase Angle.

Each power generation source should have the same characteristics.

A synchroscope did help in synchronizing two power sources in the old days, but it requires human intervention. The operator has to throw the switch at the right moment when the synchroscope stops spinning CCW.
Throwing the switch on at the wrong time and you could end up with a very expensive metal junk.

All of the above problems were overcome by the arrival of inverters that convert DC to AC through the use of solid state HV thyristors.

In today's technology, there is such a thing called asynchronous HVDC link back to back that is used to connect two different power sources of different voltages, frequencies, and phase angles. This is a technology that is getting popular worldwide. It involves tying regional grids to provide more reliable power source.

We, Southern Californians have been using this technology for decades in our PACIFIC INTERTIE dc link from a hydro plant that provides supplemental power supply from station located 850 miles from an Oregon hydro-station.
The station provides about 500 kilovolt -- 3100 megawatts of DC power to a substation nearby where I live.
We need all the power we can get during the hot summer months.

My references above may not collocate with your original query, nevertheless, they involve similar basic fundamentals as applied in synchronizing two dissimilar power set up.

These principles can be applied also in lower voltages like yours.

I suggest you read some abstract papers regarding power systems synchronization technology.

You may need a lot of homework and not rely on mysticism.
Save your career and even make more money.

As a side note:
I have posted a comment about this topic a while back on another thread that you may find helpful . . . try searching the archives.