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.
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
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
RE: Transformers Paralleling
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
Tap changer is in central position (3) on both transnformers.
RE: Transformers Paralleling
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
Rafiq Bulsara
http://www.srengineersct.com
RE: Transformers Paralleling
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
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Transformers Paralleling
"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
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
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
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
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
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
RE: Transformers Paralleling
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 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
RE: Transformers Paralleling
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:
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
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
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
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
David Castor
www.cvoes.com
RE: Transformers Paralleling
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
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"Why not the best?"
Jimmy Carter
RE: Transformers Paralleling
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
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
RE: Transformers Paralleling