How to recognize phase A? 5

[pisimatza]

Hi
I am disputing with a colleague:
I say : Phase "A" exists. every 3-phase system has a phase "A"
He says: Phase "A" is random. Important is just the rotation ABC, or BAC. You cant determine which is A.
I believe: every circuit has a phase A, that can be measured, and labeled as A.
If there is another source: transformer, Generator, etc: That has also a phase "A".
If you want to connect the 2 circuits(even through buscoupler), you have to connect A to A.

Question: if i am true, how can one measure which one is phase A ?

 

[marks1080]

measure all there phase voltages.

During Positive sequence phase 'A' will lead Phase 'B' will lead Phase 'C'.

Note: not all utilities refer to the three phases as A, B and C. Where I am now we use Red, White and Blue (which I am still trying to get used to)

Bottom line: Phase 'A' does exist, as does 'B' and 'C'. It is crucially important that you know which is which to determine wheter or not your system is running properlly.

There has been an on going debate regarding system sequence components and whether or not representing a system as either positive, negative or zero sequence is a true representation of what is physically happening in real world or whether or not it is just a convienient representation. Personally, I believe that it is a true representation.

 

[pisimatza]

Great. I measure all 3 voltages. And i will have voltage information.
But how do you measure which is A?

 

[roydm]

If you want to connect transformers in parallel you need more than just rotation.
Like Marks1080 i learned on Red Yellow Blue, there was no possibility of making a mistake with that system, however I also agree with pisimatza unless you trace it all the way back to the point of generation how do you know for sure if your red wire is A,B or C?
Roy

 

[apprenteng]

The notations given are all for convenience. When you are working and you need for example to parallel to circuits or transformers, the notation will make it easier.Whether A,B, C or R,W, B Or R,Y,B. Depending where you come from.. You acn use the many instruments available and you will definately measure the magnitude of phase voltages and their phase angles. What matters is the consistence of the system.

For some locations with lots of Dy transformers, I see the phase next to the neutral bushing denoted as A or Red. Since lines traverse long distances, it is only fair that a system of reference is developed.

 

[Marmite]

There's more to it that just rotation. Imagine what happens if you try to parallel 2 feeders where the phasing has been rolled. eg ABC/BCA. Same rotation, big bang.
A is traditionally the reference phase. You need a reference so that you can express other quantities with respect to it. Phase A is important on interconnected systems, but if you were synchronising a generator onto the system, only the rotation would matter (in this context).
Regards
Marmite

 

[LionelHutz]

Well, your theory is not correct.

Say that you now have the 3-phase wires labeled 1 2 and 3 and that measuring them you find you get a positive sequence when they are in the order 1 2 3.

You will then also get a positive sequence when the phases are ordered 2 3 1 or 3 1 2. There is nothing you can measure that will tell you if the wire 1, 2 or 3 should be marked as 'A' phase.

If you want to use parallel feeds you always have to measure the phasing between all 3 wires to ensure they are in phase with each other and that you won't be creating a fault when you turn on the second feed.

 

[Sideswiper]

To make things more confusing, in some places Phase 1 is Phase A... while in other places Phase 1 is Phase C.

I'd say to really know what Phase A is you have to know the standards of the utility and how they define Phase A.

 

[jghrist]

To know which is A, you have to trace the phases all the way back to a source where the phase is known. If all you need is to close one circuit into another circuit (where the two circuits are synchronized), then you can measure the voltages between the two circuits to determine which are the same phases without knowing which is phase A.

 

[edison123]

pisimatza

Good discussion point. While the rotation sequence (1,2,3 or 1,3,2) would matter inside a system (of having motors and generators), the naming sequence would be definitely important while paralleling with other systems, as marmite says.

I have seen protection & relay testers use what is called 'phasing in' where they connect a voltmeter (via PT's) between the incoming system to the main system and check for zero voltage (?). They do that for all three phases.

 

[rbulsara]

If you have only single source, you cannot tell which is A just my measuring any quantities.

Designations A B C or 1, 2, 3 are arbitrary but once in place you need know physical locations (terminations) at the source if you need to synch with another "fixed" source like another utiltiy feeder. This can be done by chekcing out volatage between phases of the two sources.

If the one of sources is not fixed such as a geneator all you need to know is the rotation of the two sources are same.

 

[pisimatza]

Back to topic:
So:
There is no method to find which one is A?
Just to trace the phases all way back to generator.
And more: it doesn't matter which one is A. If u have to connect 2 circuits, just to be sure that there is no phase difference between them.

I have found something, but it is too scientific, it is not a common device:

http://www.clarke-hess.com/2500.html

If you see on display, angle 120 deg is shown. This means is phase A, or not?

 

[slavag]

Hi Pisimatza.
I would like ask you: What is a phase A?
Lets start from here.
for my pinion, you can see it only on the generator.
Phisicaly, it's some abstract declaration. important for us only rotation.
Regards.
Slava

 

[jghrist]

If you see on display, angle 120 deg is shown. This means is phase A, or not?

Not.
This just means that the measured voltage is 120° ahead of the reference voltage.

 

[marks1080]

It is completely arbitrary what you call the phases... Whether it's A, B and C or Red, White and Blue. The rotation, however, is extremely important. For an ABC system, the rotation should be A followed by B followed by C (using conventional labelling).

If you were to see BAC rotation on such a system that would be a good indication of a fault, or other non-ideal system operation.

Look up the Two-Lamp method for determining phase sequence. It's easy to understand, and set up.

 

[wolf39]

It's true: Phase A exists. Phase A is always the phase the utility in question has assigned to be phase A. The same is the case for phases B and C, and with this decision the utility has determined the phase rotation sequence as well, once and forever. And there is no way to argue with the utility. Every single three phase device (generator, transformer, etc.) commissioned at a later stage has to follow this regulation.

Wolf

http://www.hydropower-consult.com

 

[cranky108]

Phase A is a lable, just like 1, or red, it is just a lable used as a reference. (No more, no less).
Depending on how you lable the three phases determines the phase rotation, ABC, or CBA. (ABC is easer to work with for some reason).

Consiter this: From a new customer point of view, the utility delivers three phases, and a neutral. To see that a motor will rotate in the proper direction you would do a bump test.
The electrtition will add colored tape to the four black wires as a reference (So which is A, or does it matter)?

 

[peebee]

Asking which phase comes first is like asking if the chicken or the egg comes first.

 

[rcw retired EE]

Look at it on a scope. If it is a sine wave it is phase A, If it is a cosine wave it is phase B. (Tongue in cheek.)

You can't tell the difference between a sine wave or a cosine wave. One starts at voltage zero the other at voltage maximum. Sine or cosine just depends on when you start the stopwatch o trigger the scope.

Same with phasing, you can tell what the phase sequence is (the sequence that the three wires reach their positive voltage maximum) but not which wire is called phase A at the other end.

Instruments are available to inject a signal onto a phase wire at a substation that can be detected out on the line to match the distribution wire to the substation nomenclature. Other fancier instruments use GPS timing and signal injection to measure the phase angle of any voltage or current in the power system against the reference phase at a main substation.

Hydro One in Ontario has a system something like that. But they don't call it A phase, its red, white, blue (?). In our plant it is ABC so we did a conversion A =white, B= blue and C = red. Inside the plant A phase is always on the left when facing the front of the equipment. Helps when starting motors the first time if we keep it straight.

 

[burnt2x]

y'all,
I just seem to forget which standard it is, but i understand there is a convention as to how to lay conductors such that others who will inherent a certain system will find it easy to determine which is phase A, B, and C (line1, line2, and line3).
It is really a matter of convention as the voltages measured will likely be the same as wellas the phase angles between them.
On bus bars, if arranged from left to right, the first busbar on the left is assigned as Phase A, then Phase B at the center, and the rightmost busbar, Phase C. etc...
Will post back if can find the corrrect doc!