Maintaining phase rotation before and after transformer replacement

[DrDrreeeaaa]

Hi All

We have a site where the existing 7.6 kV/ 433V 200kVA transformer is being replaced with a 750kVA transformer. I am told by the site electricians that the phase rotation of the existing system is 'not right' (they can't tell me any more detail than that)

Now, we are replacing the TF with a new unit and we have to make sure that the phase rotation of the new installation results in the same motor turning direction as the old rotation.

I have thought about this a bit and I think I have it straight but I am an engineer, not a technician, I have never measured phase rotation etc so I fear I am missing the more practical part of the issue.

This is what I was thinking:

1)Measure existing site phase rotation using rotation meter that will tell us if the rotation is CW or CCW.

CW denotes R, W, B, R, W, B....
CCW denotes R, B, W, R, B, W...

2)The new transformer has the intended phase rotation direction marked on the nameplate, it shows:

R, B, W, R, B, W...

So essentially if our phase rotation test comes out to CW, we have to swap the B and W phase (for example) between the LV side of the TF and the MCC. If the existing phase rotation is CCW, we simply connect R, W, B to R, W, B

Haha I am explaining this in an elementary way, only because I am trying to account for every eventualuty that may occur on site when this is done

Would appreciate any info. Thanks.

 

[GTstartup]

I've never seen a transformer with a phase rotation marked on it. Don't know why it would care. Maybe somebody could educate me.

I would measure phase rotation before, and then after at the same place and using the same instrument. Then swap two phases if it is wrong. As a final check after it is energized, bump one motor to make sure it is correct.

I think this is pretty much what you posted, sorry - didn't have time to study all your R,B,W,s etc

 

[jghrist]

I've never seen a transformer that changed the phase rotation. If it's an ANSI transformer, H1,H2,H3 will have the same phase rotation as X1,X2,X3. If bushings are marked differently - U,V,W will have the same rotation as u,v,w - A,B,C will have the same rotation as a,b,c - 1U,1V,1W will have the same rotation as 2U,2V,2W.

Unless either the old or the new transformer has special (and exceedingly weird) connections, if you connect the new transformer the same as the old, you won't change the phase rotation. Check it anyway before connecting loads.

If the phase rotation of the existing system is "not right", do all the motors run backwards? I would be extremely cautious about changing the phase rotation of an existing system.

 

[hacksaw]

this is not rocket science or an engineering question; your electricians know how to sort that out, you need to know the desired rotation (the engineering part)

all motors have to be checked for rotation

 

[DrDrreeeaaa]

Hi guys

I understand the 'triviality' of putting the rotation on the nameplate - I have attached the nameplate for you to have a look at.

I am not an expert in this area, however, from what I have read on this site I speculate the following:

The existing transformer has Dyn1 vector group (known, from nameplate), the phase connections on the HV and LV side were adjusted to bring it to Dyn11 - and somewhere in the mix the phase rotation was changed.

Yes the information I have from site is rather sketchy, i.e. 'phase rotation is not right'... the motors do not run backwards at the moment, however, I think the phases have been swapped at the final distribution level for each motor.

Yes, we will be doing a rotation test before and after, and bumping motors - however, it is not as simple as just swapping two of the phases as the incomers are relatively large and dragging them over to the other pole of the CB after the cables are cut would create phase clearance issues.

If we have to swap two phases at each motor, my customer is going to be asking me why we didnt figure this all out before hand so that there wouldn't be a problem - as you said it's not rocket science.

Yes I have worked before with electricians who take care of this themselves however in this case I am not so lucky

 

[jghrist]

Seems to me that you have a choice:

1. With great difficulty, swap phases on the primary so that the phase rotation is "right" (according to the electrician's sense of what is "right"). Then you have to swap cables at every motor back to what they were before being swapped to make them run the right direction.

or

2. Connect the primary the same as on the existing transformer. Leave all the motor leads as they are.

 

[jghrist]

I've never seen a transformer with a phase rotation marked on it.

If you are going to label your phases G,H,J and R,W,B then marking the phase rotation is a good idea because it is not intuitively obvious that W follows R and B follows W.

 

[DrDrreeeaaa]

Forgot to mention the new TF has Dyn11 vector group

 

[waross]

You should be more concerned with the polarity. Replacing an additive polarity transformer with a subtractive polarity transformer will reverse all the motors. Changing vector groups does not affect rotation but it is an issue when paralleling transformers.

Bill
--------------------
"Why not the best?"
Jimmy Carter