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.

 

[rockman7892]

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?

 

[jghrist]

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.

 

[burnt2x]

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.

 

[dpc]

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

http://www.cvoes.com

 

[waross]

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

 

[rockman7892]

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.

 

[waross]

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

 

[davidbeach]

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.

 

[SMONR]

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?