Transformers Paralleling 3

[Danyboymx]

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.

 

[dpc]

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

http://www.cvoes.com

 

[nawao]

What about tap changer position?

 

[Danyboymx]

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.

 

[Danyboymx]

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

 

[rbulsara]

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

 

[rbulsara]

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

 

[rcw retired EE]

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.

 

[waross]

Check the voltage between XO and XO (Neutrals).

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

 

[jtester]

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

 

[rockman7892]

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?

 

[potteryshard]

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.

 

[jghrist]

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.

 

[Danyboymx]

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.

 

[rockman7892]

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?

 

[jghrist]

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.

 

[rbulsara]

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

 

[waross]

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

 

[jghrist]

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.

 

[waross]

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:

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