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Transformers Paralleling
3

Transformers Paralleling

Transformers Paralleling

(OP)
I have two identical 3 phase dry type transformers I am trying to parallel:
1.5 MVA 15kv/480v Z=7.2% Dy11
They are being fed from the same source. I measured voltage across A-A B-B and C-C before closing the tie breaker and they measured 47 V. Voltages between phases are all the same, 480v. Is it safe to go ahead and parallel them? Or is any issue like a phase shift?
Thank you all for your help.

RE: Transformers Paralleling

Something is not adding up.  You have 10% voltage difference between transformer secondaries but state the all the phase-to-phase voltages are 480V.  

I would not parallel sources with 10% voltage difference unless I could explain why the difference exists and what I expect to happen when I parallel them.  

You might want to re-check your voltages and verify your measurement technique.  

 

David Castor
www.cvoes.com

RE: Transformers Paralleling

What about tap changer position?

RE: Transformers Paralleling

(OP)
Thanks for your help dpc.
I went back to rechecked everything and still the same.
Another piece of information about the scheme: This is an ungrounded wye system with a ground fault detection scheme.  Neutral is connected to ground through a 28 ohm resistor bank and a CT to measure ground current. A VT is connected across the resistor that feeds a 59N relay.
 

RE: Transformers Paralleling

(OP)
Thanks for your help nawao.
Tap changer is in central position (3) on both transnformers.

RE: Transformers Paralleling

What is the source? Be careful that they are not two separate utility feeds, even in sync.

Does 47V diff remain constant when you hold the VM?
What is the load on the two transformers?. Load (phase angle) difference will also show up as some difference.

There was a thread on similar subject here a few days ago, it is common to see large phase angle difference on Main-tie Main even, if the loading of transformers is much different.

I had to set some sync check relays to 20%-25% to allow the closing of a tie. But I was very sure of that they are only due to different loadings and otherwise they were in sync.

If you are sure of phasing and the source being the same, it should be OK to close the tie. Just look for circulating current after that. Or at least be prepared to see one of the three breaker opening.

AT ANY RATE, HAVE AT LEAST ONE OTHER EXPERIENCED ENGINEER AT THE SITE REVIEW THE SET UP AND MAKE SURE THAT YOU ALL AGREE BEFORE ANY SWITCHING.

Rafiq Bulsara
http://www.srengineersct.com

RE: Transformers Paralleling

If possible, try putting some load on the lesser loaded transformer and see if the voltage difference narrows. (or shed some load on the other..). This will tell you if loading is affecting the phase angle difference.

Rafiq Bulsara
http://www.srengineersct.com

RE: Transformers Paralleling

Is your equipment rated for the short circuit level of the paralleled transformers, approximately 50 kA?

Using rough approximations to calculate circulating currents: the 10 % voltage difference and the impedance of the two units in series = 2 x 7.2% = 14.4 %  Per unit current = 0.1 Vpu / 0.144 Zpu = 0.7 per unit circulating current. 1500 kVA Rated Amps = 1800 Amps. There will be up to 1250 amps circulating between the two transformers leaving only 30% of the KVA rating for load current.

Is a 30% increase in capacity worth it to parallel the transformers?  (This number is not totally correct. Circulating current will add to load current in one unit and counteract it in the other. Also, this rough analysis ignores phase shifts and phasor addition/subtraction of voltages and voltage drops).

As mentioned, check your tap changer settings.  

Phase shifts inherent in the transformer design may be creating the voltage difference. Try measuring all phase-neutral voltages plus A-A', A-B', A-C', B-C' and the other voltage combinations. Check for any N-N' voltage.  If the voltage's are phase shifted A-B' will not equal A-C'. The test results may give you an idea of what needs to be changed to get a better voltage match at no load.

Note that the voltage match may change with load.
 

RE: Transformers Paralleling

Check the voltage between XO and XO (Neutrals).

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

RE: Transformers Paralleling

I'm struggling to understand the transformer configurations.  One side is ungrounded wye, which is, and what is the other side connection?
"Check the voltage between XO and XO (Neutrals)."
If the high side is wye, be sure it is grounded.  Ungrounded wye high sides can create unusual secondary voltages.

Jim T

RE: Transformers Paralleling

Rubulsara

Can you please explain how different loading on the transformers can cause this voltage difference between the two trasformer ouputs?

Is this due to the internal voltage drop across the transformers internal imedance that will be effected by the different load currents and therefore be different for each unit?

RE: Transformers Paralleling

Danyboymx...

You never explicitly said so, but is the 47 V phase to phase difference between transformers consistent on all three phases?

Are either one or both transformers loaded?

One assumes that you wouldn't be attempting to parallel these units unless the grounding and neutral connections were identical on both units...

Finally, was a true-RMS meter used to measure the phase to phase voltage between transformers?  It is always possible that one transformer serves loads which are rich in harmonics exaggerating the phase to phase voltage reading.

RE: Transformers Paralleling

Quote:

Is this due to the internal voltage drop across the transformers internal imedance that will be effected by the different load currents and therefore be different for each unit?
Yes.
10% voltage difference seems a lot if it is caused by unequal loading.  You would need to know each load current (magnitude and angle) and the X/R ratio of the transformers to calculate the difference caused by voltage drop.
 

RE: Transformers Paralleling

(OP)
potteryshard...

Yes! 47 V consistent on all of them.

Only one transformer was loaded.

Today I am planning on verify neutral grounding and recheck everything again.
I am also going to measure phase voltages to ground and check what kind of load we have on the one that is loaded.
Test equipment we are using is RMS like Fluke 87-V.

Thanks for your input.
 

RE: Transformers Paralleling

jghrist

Why do you need the X/R ratios of the (2) transformers?  Can you just use the load currents in conjunction with the p.u. impedance of each transformer to calculate the voltage drop?

Or are you looking specifically for voltage angle differences caused by the load current flowing through the transformers, with the X/R ratios being used to supply a magnitude and angle of the ouput voltage?

RE: Transformers Paralleling

Quote:

Or are you looking specifically for voltage angle differences caused by the load current flowing through the transformers, with the X/R ratios being used to supply a magnitude and angle of the ouput voltage?
Yes.

RE: Transformers Paralleling

rockman:

I am a stating what I have observed and done. I like to think that it is related to differences in power factors and voltage regulation tolerances. I will let others explain the math.

Most operators LV M-T-M installations do not even know about this as most of them do not even have a sync check relay. Phasing is checked during installation/commissioning, typically when both xfrmers are unloaded. During operation they close them, without thinking (this is permanent M-T-M arrangements that fed from the same sources).

I am not approving anything here as I am not at the site.

 

Rafiq Bulsara
http://www.srengineersct.com

RE: Transformers Paralleling

During normal operation at close to unity power factor the resistive voltage drop of the transformer is the predominating factor. The reactive voltage drop is at right angles and has little effect on the terminal voltage.
During short circuit testing the transformer impedance is the only impedance in the circuit.
The only times that the transformer impedance can be used to determine the internal voltage drop of a transformer is during a short circuit at the transformer terminals and in the special case where the external load has the same X:R ratio as the transformer.
The transformer manufacturer will supply a regulation figure which will indicate the transformer internal voltage drop at a specified power factor. PU regulation is always a lower number than PU impedance.  

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

RE: Transformers Paralleling

Quote:

The reactive voltage drop is at right angles and has little effect on the terminal voltage.
But it will have a significant effect on the voltage difference between the secondaries of two transformers when one is loaded and one isn't.  The two terminal voltage magnitudes may be very nearly the same, but the measurement between the secondaries can be significant if most of the voltage drop is reactive.
 

RE: Transformers Paralleling

I agree with you jghrist but we have about 9% difference here. I would expect voltage drops, real and/or reactive to be less than the 7.2% impedance.
On the other hand if there are any harmonics causing a current through the grounding resistor, then less than 2 amps would cause a 47 Volt offset between the phase voltages.
Hence my suggestion:

Quote:

Check the voltage between XO and XO (Neutrals).
This point may be discussed and argued, or it may be resolved with a simple voltage measurement.

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

RE: Transformers Paralleling

waross

Why do you say the reactive voltage drop at a right angle has little effect on the voltage drop?  Is this because the reactive component is much less than the resistive component?

Can you please explain?

RE: Transformers Paralleling

Quote:

Why do you say the reactive voltage drop at a right angle has little effect on the voltage drop?  Is this because the reactive component is much less than the resistive component?
Look at the attached diagram.

At unity power factor (ø=0), I·X would be vertical.  VDR would equal I·R.  Unless the voltage drop is large, the voltage angle (between VR and VS) is small and VS is nearly equal to VR + VDR.  Voltage drop (the difference in the magnitudes of VS and VR) would not depend on the vertical component VDX.

A measurement between the two secondaries, however, would not measure the difference in magnitudes of VS and VR, but would measure the magnitude of VD.
 

RE: Transformers Paralleling

1. You said you have two identical transformers, right?
2. Your setup is one energized and loaded, while the other is later energized at the high side with the purpose of paralleling with the other which is already online by closing the main secondary breaker.
3. You verified 47V difference on each phase voltage between the two transformers secondary.

Understandably, the loaded trafo will have a terminal voltage which is lower than the unloaded trafo. I suggest you de-energize the 15kV main breaker of the incoming transformer and then tie the secondary bus of both transformers (480V side). Complete the paralleling process by closing the high side breaker of the incoming transformer. Hope that helps.

RE: Transformers Paralleling

I think many of the responses are ignoring the OP.  It states that the measured voltages across each transformer secondary are 480 V.  So if this is accurate, the issue is not simple voltage drop across the loaded transformer.   A phase angle difference could explain the difference between the transformer, but it's hard to come up with a VOLTAGE phase angle difference that large at the transformer terminals on two identical transformers.   

David Castor
www.cvoes.com

RE: Transformers Paralleling

An unloaded transformer has only exciting current. Exciting current is non linear and as such has a high percentage of harmonics. Harmonic current through the grounding impedance will cause a voltage drop across the resistor. This voltage drop may cause a shift all the line to line voltages referenced to ground.
I have seen strange voltage ratios on unloaded wye transformer secondaries until a load was connected.
This used to show up a lot when we used d'Arsonval type analog meters. The distorted waveform did not have the same form factor as a sine wave and this introduced a lot of metering errors.
On the other hand, have you double checked once more that both transformers are on the same tap setting?

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

RE: Transformers Paralleling

Excellent thread with many good points.

waross

Can you please explain the line to line voltage shift with respect to ground reference for current flow through the neutral resistor?  This is an interesting point.

RE: Transformers Paralleling

How to explain? If the transformer voltages are equal, then you should be able to superimpose the vector diagrams. If you then shift one diagram in any direction so that there is a 47 Volt difference between A-A, B-B, and C-C, there will also be a 47 Volt difference between X0-X0. A current (mostly harmonic from the magnetizing current) in the grounding resistor may cause a voltage drop across the resistor that will shift all the voltages.
Again, have you checked the voltage between X0 and X0. This may prove or disprove this speculation.
If you are using a voltmeter that responds to average values rather than RMS values, you may expect significant metering errors when measuring distorted waveforms. Most analog voltmeters responded to average values which were scaled to RMS by the meter face.

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

RE: Transformers Paralleling

Bill, not necessarily.  The two vectors could be rotated (not sure how, but it could happen) so that X0=X0', but all the A-A', B-B', C-C' voltages are off by 47V due to teh phase shift.

RE: Transformers Paralleling

If your energized transformer is carrying a small amount of neutral current, in this case 1.7A (so 1.7A x 28 Ohms = 47V), but not enough to trip 59N relay, then all of your phase voltages will be displaced by the 47V w/ respect to the unloaded transformer that is grounded with no neutral displacement. Did you measure the neutral VT secondary voltage  to check this?

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