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ungrounded wye-delta
2

ungrounded wye-delta

ungrounded wye-delta

(OP)
the subject is 3-100kVA, ungrounded wye-delta transformer bank, 2400/4160Y-240/480V. During a spot check-up on this bank, we were surprised with the line-ground voltages 302V/320V/198V that were measured between the secondary terminals and the tank equipment grounding (as ground point). thinking it was due to grounded load or bad connections, the transformers were isolated, and terminals were cleaned. megger, winding resistance and TTR all turns good for these transformers. but after the cleaning, the same voltage magnitudes were measured when the transformer was commissioned. by the way, line-to-line voltages are balanced at around 480V. 4160V side is connected to upstream bus with LRG.

Has anyone experienced the same case or someone perhaps know what's going on with our transformers?

Appreciate your inputs.

RE: ungrounded wye-delta

What were you expecting to measure to ground on an ungrounded system?  The voltages depend on the relative capacitance to ground of the secondary system and the impedance of the meter.
 

RE: ungrounded wye-delta

Yes, you have no solid ground to reference only capitance to ground as jghrist states.

RE: ungrounded wye-delta

Not just the capacitance. The admittance of the meter now affects the very thing you are trying to measure. Note that the measured voltages fail not only to meet in the center at 277, they fail to meet at all.

RE: ungrounded wye-delta

The readings are a reflection of different leakages to ground of the transformers.
The leakage is so little that it appears that the meter loading is dropping the values slightly so that they don't meet.
I raised the 302 voltage up to 320 and did a quick check with the help of Mr. Pythagoras and the result was below 480 Volts. The error that I introduced should have put the result above 480 Volts.
Conclusion, you have significant meter loading.
Please consider grounding one of the 240 Volt center taps, unless there is an urgent need to run ungrounded.

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

RE: ungrounded wye-delta

waross,
Your suggestion does not define whether the secondary system is grounded three-phase or single-phase.
This is a problem for analysis and calculations.
A more comprehensive solution would be to ground secondary system by grounding transformer with neutral resistor
to limit earth current to 10A.
This current allows you to give alarm at first stage and trip at second stage in a delay of 1 (one) hour!.
This is enough time to restablish the system without loss of service continuity.

RE: ungrounded wye-delta

The information given was for a delta connection of 240/480 volt transformer secondaries at 480 Volts. Each of the three 240/480 Volt transformer secondaries  will have a center tap available which may be used for an acceptable ground point even if it is not perfectly symmetrical.

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

RE: ungrounded wye-delta

waross,
because is not perfectly symmetrical there is no model representation of transformer in order to calculate short circuits. It will be necessary to made tests in the field.
 

RE: ungrounded wye-delta

(OP)
thanks for all your inputs.

waross,
could you kindly explain what do you mean by "significant meter loading"? We used the same DMM with the adjacent transformer bank (ungrounded wye-delta) and measured balanced voltages (line-ground).

actually, one transformer was used for 240V service (but still not grounded)and that circuit got grounded. we isolated that circuit but still getting the same that voltage unbalance in the secondary.

I am aware that we can't get a well-balanced line-ground voltage on delta side, but would it really be that far between phases?

RE: ungrounded wye-delta

2
I think it was referring to the fact that your meter has some non-infinite impedance, which could affect voltage to ground if other impedances to ground were same order of magnitude or higher (although dmm is very high impedance). I don't see any clear indication that meter is affecting these measurements (if they were all below 277, that would be a clear indication).  On the other hand, there definitely appears to be an imbalance in impedance (for example stray capacitance) to ground on the low side and everything connected to it.   

By the way, what is connected to the low side? Is this transformer feeding some cables and loads?   The cause of the unbalance certainly can lie with them.  One long cable run closer to ground. Also ground indicating lights on MCC's.... if one burned out you'd probably have 1 high and 2 low voltages (you have 2 lows and a high... maybe 2 burned out).  Also of course if you have single phase loads, you may have different ones on each phase and it wouldn't be surprising for associated stray capacitance to be different.  

=====================================
(2B)+(2B)'  ?

RE: ungrounded wye-delta

Why do you care what the line-ground voltage is?

RE: ungrounded wye-delta

Hi Pete;
Can you calculate what the line to line voltages must be to correspond to phase to ground readings of 302 V, 320 V, and 198 V ?
To me the interesting point is not that the voltages to ground are not equal, it is that the line to ground readings are too low for a 480 Volt system.
This may be meter loading.
Or,
I have seen significant waveform distortion on unloaded wye:wye transformer banks but I have never had the opportunity to put a scope on an unloaded delta.
This also may be a combination of waveform distortion and metering errors.
The tip off to waveform distortion is that averaging meters, (both analog and digital, will not indicate the correct voltage with many distorted waveforms.
If this is a case of distorted waveforms, try your readings again with some load on the transformer. If this is a waveform issue you will probably get close to the same ratio of line to ground voltages but the voltages will be increased so as to be congruent with a 480 Volt delta.
 

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

RE: ungrounded wye-delta

Bill - I think that line to line voltages of 467vac (just 2.5% low) separated by 120 degrees, could give the readings 198, 302, 320 to ground.

=====================================
(2B)+(2B)'  ?

RE: ungrounded wye-delta

Thanks Pete. I agree that may be a little too much error for meter loading with a digital meter.
I suspect a distorted waveform may be at play. The nature of the distortion I have observed, will certainly introduce that much error when averaging meters are used. The shape of the wave form may fool some true RMS algorithms also but not sure.

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

RE: ungrounded wye-delta

Sorry, not 2.5% low but 2.9% low.

Here is one scenario where balanced line to line voltages 2.5% low (469 instead of 480) can give the measured line to ground voltages.
                
The sum of the line to ground voltages is not zero=ground, but is some other point, call it n for neutral.  We can write the balanced line to neutral voltages as:
Van=    269    Angle    0    degrees
Vbn=    269    Angle    120    degrees
Vcn=    269    Angle    -120    degrees
(269 is 2.5% below nominal 277 volt).

Now let us postulate that ground voltage is displaced from the 0 point as follows:
Vgn=    72.4    Angle     9.6    degrees

We can compute Vag = Van – Vgn, Vbg = Vbn-Vgn, Vcg = Vcn-Vgn  to give:
Vag=    198.0    Angle    -3.5    degrees
Vbg=    302.0    Angle    133.0    degrees
Vcg=    320.0    Angle    -130.0    degrees

198, 302, 320 matches the measurements.  That doesn't prove anything.   For all we know the voltage could be exactly 480 and the meter loading is are influencing the measurement slightly. Or lots of other possibilities.

As you point out, the meter loading can't be the whole story because it affects all phases the same so cannot introduce an unbalance.

jghrist asks why do we care?  Good question for the op.  I would say in general we should investigate signs of unbalanced impedance to ground on an ungrounded system... it may have a perfectly benign explanation, but it can also be indication of a ground on one phase which should be resolved.

=====================================
(2B)+(2B)'  ?

RE: ungrounded wye-delta

Quote:

but it can also be indication of a ground on one phase which should be resolved.
Obviously not a hard ground in this case.

=====================================
(2B)+(2B)'  ?

RE: ungrounded wye-delta

And sorry there were typos about the scenario... should've been: 2.9% low: 466 line to line, 269 line to neutral.  In spite of those typos, the other numbers were correct.

=====================================
(2B)+(2B)'  ?

RE: ungrounded wye-delta

(OP)
thanks again for the valuable inputs.

jghrist, its been a normal practice for us to measure line-to-line and line-to-ground voltages during routine check of power distribution. and its been this case where we found unbalanced line-to-ground voltage with no grounded phase at the load side. the curiosity with these weird voltage readings are of course related to whether we need to have the transformers repaired, though I have mentioned that DLRO, Megger and TTR yields acceptable results.

electricpete, the transformer bank feeds some 480 and 220V, but the unbalance line-ground voltages were measured at the transformer secondary, with all the loads disconnected. so our speculation at this point is that the problem is "within" the transformer.

waross, I'll try to clamp a PQ meter to check for the waveform and vector arrangement.

more inputs would be much appreciated.

RE: ungrounded wye-delta

The distorted waveform that I have seen on wye:wye banks looks like the positive half of a sine wave, but a little shorter than the expected 8.3 milliseconds. Then there is about 1 millisecond of zero Volts.  Then there is a corresponding negative half cycle. That sure messes up the reading of the old d'Arsonval based meters. I suspect that an algorithm that does not properly account for the zero intervals may influence a supposed "True RMS" meter.
Try taking the measurements with a little load on the transformer bank.

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

RE: ungrounded wye-delta

Quote (electricpete):

Obviously not a hard ground in this case.
nerb31, add a small load (light bulbs?) from one leg to ground and compare the measured voltages to those of the unloaded case. This (with a bit of math) will tell you what the magnitude of the leakage and/or capacitance to ground is that's causing the original unbalance.

odlanor, fault currents should still be calculable (with a bit more work) for the grounded center tap case by modeling the voltage source with the appropriate symmetrical components.

RE: ungrounded wye-delta

The low side winding of a given phase physically sits between the high side of that phase, and the core.   So as first cut, the low side winding voltage tries to float somewhere between the adjacent high voltage winding and ground, also meeting the constraint that the three low side phase to phase voltages are relatively balanced.

CLH probably has more insulation and smaller capacitance than CLG.  However if we assume it is not too much smaller, than it is easy to imagine that the high side has more influence on the low winding voltage than the adjacent ground (because voltage difference between low side and high side is bigger than voltage difference between low side and ground).  i.e. VLH/XLH > VLG/XLG so more capacitive current flows between low and high than low and ground, so high has bigger influence on voltage.

Still with everything balanced on the high side, meeting the constraint that the 3 low side phase to phase are balanced, then I think it would put the lv windings balanced around ground. BUT what if there is a small imbalance on the supply side.... I think probably that can result in relatively bigger imbalance to ground on the low side delta winding.    What do you guys think?

In any case, I don't think it's a concern because you have ruled out transformer anomaly with your testing.   

=====================================
(2B)+(2B)'  ?

RE: ungrounded wye-delta

Attached is a quick attempt to explore what I suggested above... whether a small  unbalance on high side voltages can result in relatively large unbalance on low side voltages to ground.

I assumed all CHL and CLG were 1uF (probably higher than actual, but I think all that matters is their ratio).  To get rid of startup transients, I added a 1E5 ohm resistor in parallel with each cap (time constant 0.1 sec) and examined the solution starting after 10 time constants (1 second).

I assumed
VBAL~VAH
VCBL~VBH
VACL~VCH

I used 2400 peak phase-to-ground hi-side and 480 peak phase-to-phase low-side as nominal values (should've multipled by sqrt2.... sorry... doesn't change the conclusions).

I used 1 ohm internal impedance for each low side voltage source so it wouldn't hiccup at the thought of unbalanced delta loop which did not close.

The ltspice model is shown on slides 1 and 2.

Initially I increased VAH and VBAL magnitude by 1% above nominal.  Result shown in slide 4 is that phase to ground voltages deviate 1.8% high and 0.7% from nominal.

Then as shown in slides 5 and 6, I changed VAH and VBAL phase angle by 2 degrees from their nominal position. That resulted in spread of voltage 262 to 292.

What it proves, I'm not sure.  It did not show as much influence of high side unbalance on low side as I would have guessed, even though we chose CHL = CLG instead of more realistic CHL<CLG which would have given even less effect from high side to low.    I am less inclined to believe high-side unbalance is causing your problem after seeing these results then I was before. (fwiw...I'm not sure how good a model this really is).
 

=====================================
(2B)+(2B)'  ?

RE: ungrounded wye-delta

Except that the high voltage is wye connected and the inner part of the winding may be connected to the wye point and there may not be a large deviation from ground on that part of the winding.

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

RE: ungrounded wye-delta

My model showed each winding as a single lumped capacitance at terminal voltage, which is a simplification from reality. I agree the further details of winding configuration can be important.   If the neutral side of HV winding sits physically adjacent to LV winding, that will greatly reduce the effective CHL as you say.  Likewise, if the neutral side of LV winding sits physically adjacent to the core, that will greatly reduce effective CLG.   For all I know, these two effects tend to cancel since we are only interested in the ratio of CHL and CLG.    I'm sure there are people on this forum that could give a better guess at the ratio CHL/CLG than me....

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(2B)+(2B)'  ?

RE: ungrounded wye-delta

RE: ungrounded wye-delta

Quote (electricpete):

Likewise, if the neutral side of LV winding sits physically adjacent to the core, that will greatly reduce effective CLG.
Scratch that.  There's no neutral on a delta winding winky smile

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(2B)+(2B)'  ?

RE: ungrounded wye-delta

This needs to be checked with three voltmeters simultaneously to eliminate the effects of meeter loading.
If the voltages are not congruent with the delta, look at the waveforms with a scope.

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

RE: ungrounded wye-delta

It doesn't seem logical to me that meter loading would cause an apparent unbalance when sequentially measuring one phase to ground on an otherwise balanced system.   The meter would create the same change in each measurement.

=====================================
(2B)+(2B)'  ?

RE: ungrounded wye-delta

.... however I'm in full agreement with getting more data if you want to try to find an explanation for your measurements.

 

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(2B)+(2B)'  ?

RE: ungrounded wye-delta

I also liked PHovnanian's suggestion to add some trial impedances to ground to help quantify things.

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(2B)+(2B)'  ?

RE: ungrounded wye-delta

I'm not concerned that the voltages to ground do not balance exactly. That's life in the real world. There is an another issue when the voltages to ground are not consistent with the line to line voltages.

Put the load on the transformer and see if the voltages to ground are consistent with the line to line voltages.

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

RE: ungrounded wye-delta

Pete,

Start with a simple idealized system, perfectly ungrounded, admittance to ground equal to zero. Attach a voltmeter with a non zero admittance from one leg to ground. The current through the meter is zero, since there is no current path (Note that this changes when three meters are connected simultaneously). Zero current through a finite impedance measures zero voltage. Do this sequentially and you will find each voltage to ground is zero, while the line to line measurement still measures nominal. The impedance of the meter pulls the ground up to match the potential of each corner when measured sequentially. Same effect on each phase as you said, but the effect suggests unbalance.

The real world is slightly more complicated. We have unbalanced capacitances and inductances involved and DVMs with high impedances. If you draw an equilateral triangle with sides of 480 and draw circles of radius 302/320/198 around each corner, you will see the circles do not intersect at a point, each being too short to reach both of the others at a single point. Again the same effect is seen on each phase, pulling the ground potential away from where it would reside if the meter were not present. This was what I expected.

As Schrödinger pointed out, the very act of observing affects what is observed.

In school, if we failed to account for meter effects, grades were effected. It might be insignificant, but you better include the impedance in your diagram and explain how it's insignificant. In this case, it makes a difference.  

 

RE: ungrounded wye-delta

Hi Steve –  Everything you said sounds fairly straightforward to me, and at the same time, no conflict that I can see with anything I said.

We have two symptoms that we have discussed.  
1 – the circles don't meet / the measured delta voltages don't close
2 – the line-to-ground voltage measurements are unbalanced.

I believe that I addressed #1 above in detail on 10 Apr 11 0:58 by proving that the circles will meet if the line-to-line voltage is 2.9% low, which certainly seems credible to me. (call it scenario 1A).  In that same post, I acknowledged that meter loading could be an alternate explanation for symptom #1  (call it scenario 1B)....  ("For all we know the voltage could be exactly 480 and the meter loading is influencing the measurement slightly") )

My more recent comments had nothing to do with symptom 1... I was addressing symptom 2: "It doesn't seem logical to me that meter loading would cause an apparent unbalance when sequentially measuring one phase to ground on an otherwise balanced system.   The meter would create the same change in each measurement. "   

This was based on my perception that Bill was addressing problem #2... apparently I misunderstood Bill's meaning and was projecting my own opinions onto his comments. Personally I think we have two very plausible explanations for #1 (scenario 1A and scenario 1B above), but I remain a little bit curious about 2.  (why would we have unbalanced capacitances inside the transformer?.... and what are the magnitudes of associated impedances).

 

=====================================
(2B)+(2B)'  ?

RE: ungrounded wye-delta

Not unbalanced capacitances but unbalanced impedances.
 

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

RE: ungrounded wye-delta

Which impedances do you have in mind?

I think possibly the high side magnetizing impedances could be slightly unbalanced since the center phase has slightly lower reluctance. That effect would be easy to check by comparing the hi-side currents to check for unbalance.

Any other impedance you had in mind?

=====================================
(2B)+(2B)'  ?

RE: ungrounded wye-delta

Resistance leakage. This may be less than capacitive leakage but may be at a closer phase angle to the meter impedance.

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

RE: ungrounded wye-delta

Lower reluctance on center phase? On a three phase transformer maybe? The OP was speaking of a bank of single phase (3-100 kVA) transformers. I'd say the unbalance comes from variations in the three units.

RE: ungrounded wye-delta

My mistake, I was thinking 3 phase transformer. I was just trying to figure out what other impedance Bill was referring to.  I had pretty much ruled out resistive myself since it was stated megger reading was ok, and I don't think resistive leakage current wouldn't be anywhere near capacitive leakage current unless something was extremely out of whack. Look at typical doble power factor test... insulation power factor tends to be around power factor of 0.01 or 0.02 capacitive, suggesting it is dominated by capacitance.

At any rate, I'm just throwing in my thoughts fwiw.  I personally don't know whether it is worth pursuing further or not... have never seen any other measurements on lv side of completely unloaded delta transformer to know if this is normal.

=====================================
(2B)+(2B)'  ?

RE: ungrounded wye-delta

Steven's comments have unspoken question for original poster (if not already asked): are these 3 identical transformers, or not?

=====================================
(2B)+(2B)'  ?

RE: ungrounded wye-delta

Hi Pete. I'm not sure but I suspect that the meter is almost in phase with the resistive component of the meter while the capacitive component is at 90 degrees. The meter may act to lower the total resistive component and so the the resistance may be significant even when it is quite a bit higher than the capacitive component.
But I don't know for sure. Comments?

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

RE: ungrounded wye-delta

Hi Bill
My thoughts fwiw:  I focus on the unbalanced voltage to ground.

Since dc insulation resistance tests are good,  then when energized with ac, the resistive part of insulation leakage current is much less than capacitive current.   

I suspect (*) the meter resistive current is also much less than capacitive insulation current. In that case the capacitive current of the insulation will still dominate, and the voltage measurements to ground should be balanced (assuming capacitance is balanced**)

* Even the meter resistive current were comparable to or greater than capacitive current, as long as meter doesn't change behavior between measurements (for example no change in scale), then it should still result in similar measurement for each phase.

** Steve might have figured it out... if the transformers in the bank are different.

=====================================
(2B)+(2B)'  ?

RE: ungrounded wye-delta

My concern is that the voltages are too low to "fill the delta". I have seen similar twice and both times it was waveform issues.
I am thinking that the true RMS meter (If it is a true RMS meter) may be correctly calculating the RMS values of the positive half cycles and of the negative half cycles but is not incorporating a few milli seconds of zero voltage between the positive half cycles and the negative half cycles. I remember seeing that wave form on an unloaded transformer bank, but it was a long time ago and I don't remember whether the distortion was line to ground or line to line. It did create noticeable errors with d'Arsonval (average responding) meters.
Once some load was placed on the transformer banks the error disappeared.  

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

RE: ungrounded wye-delta

(OP)
thanks again for the inputs guys, sorry it took me long to reply, as I was dragged again in another problem in the Plant.

The transformers are identical units.

the discussions are great, though I haven't digest all... and still haven't get the chance to clamp a PQ meter!

RE: ungrounded wye-delta

Thanks Bill.  I don't tend to view the lack of completed delta as a noteworthy symptom because it seems that there are very simple plausible explanations (voltage 2.9% low, or meter loading).  In contrast, I can't come up with any simple explanation for the unbalance.

I didn't really understand what you were suggesting about distortion.  I know the delta winding can carry the 3rd harmonic currents (which are zero sequence).

Also, the 3 single-phase units would have a path for zero sequence flux unlike a 3-legged 3-phase transformer, which does not.  So they have lower reluctance for zero sequence, higher impedance for zero sequence. That means the ratio of zero sequence voltage to current is higher on the secondary of these 3 single phase than a single-phase 3-leg transformer.  I'm not sure if that means higher voltage or lower current on secondary.....  I guess that would be either a neutral (indifferent) or aggravating factor in terms of 3rd harmonic voltages on the secondary.

But I can't make any connection between 3rd harmonic delta voltages on secondary and the symptom of unbalanced voltage to ground.  

I'm interested if you have any thoughts or explanation for whether the distorted waveforms may be related or have a common cause to the symptom of unbalanced voltages to ground.

At any rate, just because I don't understand it doesn't mean it's wrong. I have no doubt you have much much more experience on this type of problem than me and your insight is very valuable.  

=====================================
(2B)+(2B)'  ?

RE: ungrounded wye-delta

I've told this story before but I'll have another go. I was working as an instructor and sent some students into the shop to set up a wye:wye transformer bank with small dry type transformers and to measure the voltages on the secondary and verify the 1.73 ratio between line to line and line to neutral. It was obvious that the ratio was not 1.73 !!
I put a 'scope on the transformer output and saw the cause of the problem.
A normal sine wave starts at zero degrees, peaks at 90 degrees and crosses zero at 180 degrees. It reaches a negative peak at 270 degrees and returns to zero at 360 degrees.
This was not a sine wave.
The wave form was similar to a sine wave, no peaks or obvious harmonics, but the wave form started 5 or 10 degrees after zero electrical degrees. The waveform returned to zero about 5 or 10 degrees before 180 electrical degrees and did not cross zero. The waveform stayed at zero amplitude until 5 or 10 degrees after 180 electrical degrees and then repeated in the negative direction.
Although the shape of the half waves appeared similar to sine waves, the ratio of average to RMS was no longer 1.1   Once a load was applied to the transformers the voltages corrected and the correct ratios between line to line and line to load were observed.
I took advantage of my flub to teach a short lesson about possible errors when using d'Arsonval meters on distorted wave forms.
I saw this effect once again in the field at a flood water pumping station. I did not have the luxury of a scope but when a load was applied the voltage ratios became correct.
I am wondering if the sampling and calculation algorithms used in some DMMs may be confused by the milliseconds of zero voltage.
Pete, nobody worries about unbalanced voltages to ground on a floating system until one phase goes close to zero volts to ground. A fraction of a percent difference in water content in the oil may be responsible, someone may have used some hardware with slight surface corrosion to construct one of the transformers.
When impedances are in the G-ohm range don't expect them to be perfectly matched.

 

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

RE: ungrounded wye-delta

Thanks Bill.   I agree that if one tries to apply single-frequency sinusoidal (phasor) analysis to highly non-sinusoidal waveforms, the results will not match the rules we have fashioned for sinusoidal systems. (create the appearance of not closing the delta).

There are certainly some floating (ungrounded) systems where unbalanced voltages to ground are used as means for ground monitoring, and deviations are investigated. We have quite a few at our plant.  

fwiw, I am not convinced of the logic that you used to explain why you consider this ground voltage a normal expected condition.   In this case there are no external loads, which eliminates a lot of potential sources of unbalance. The transformer impedances (capacitive) depend primarily on geometry and construction features, not on insulation condition.  A change in moisture conditions can change the power factor and resistive leakage, but as discussed above, the resistive current is miniscule compared to capacitive (pf ~0.01 capacitive).  If you had enough resistance that it became important relevative to the capacitive balance, it should show up on the megger reading (it did not).  

=====================================
(2B)+(2B)'  ?

RE: ungrounded wye-delta

Quote:

There are certainly some floating (ungrounded) systems where unbalanced voltages to ground are used as means for ground monitoring, and deviations are investigated. We have quite a few at our plant.  
But I agree it is not normally investigated until one phase-to-ground voltage drops close zero as you said. I didn't read your post closely before writing these particular words.

=====================================
(2B)+(2B)'  ?

RE: ungrounded wye-delta

"Identical units" are not all that identical. They may have the same turns ratio and an impedance within 14% (ANSI allows the nameplate impedance to deviate from the actual impedance by 7%), and similar losses. Manufacturing variations will ensure capacitances vary.  

RE: ungrounded wye-delta

For what it's worth, Steve's explanation sounds reasonable to me.

nerb31 - If you wanted to investigate further (*), you could request a Doble test (or similar test using AVO equipment). That would tell you capacitance CHG, CLG, CHL for each single-phase transformer, and would provide all the info needed to solve the voltage divider problem to confirm capacitive unbalance is causing this.  It also provides further evaluation of insulation condition to corroborate your megger... may be more sensitive for certain types of faults.  A more thorough investigation of the transformer overall.

(* I'm not saying this is required)


 

=====================================
(2B)+(2B)'  ?

RE: ungrounded wye-delta

Quote (electricpete):

you could request a Doble test.... That would tell you capacitance CHG, CLG, CHL for each single-phase transformer, and would provide all the info needed to solve the voltage divider problem to confirm capacitive unbalance is causing this.
Correction - the effective capacitance depends not only on the physical configuration but voltage distribution as Bill pointed out. For example if the neutral end of HV winding is next to LV winding, then the effective CHL would be lower than what you would measure with a Doble test.  

=====================================
(2B)+(2B)'  ?

RE: ungrounded wye-delta

I guess I found a model representation of transformer.
See Case 3-Delta-Delta Bank, a´Mid-tap Grounded, Secondary Phase c-to-Ground Fault, page 199,
Symmetrical Components for Power Systems Engineering - Blackburn

RE: ungrounded wye-delta

I don't think capacitive elements such as winding to ground and winding to winding would be included in that model because in most cases you have much smaller impedances (load impedances and fault impedances) which make the capacitive impedances negligible.  On the other hand, for this thread discussing the case of unloaded delta winding, I don't think those capacitances can be neglected.  

=====================================
(2B)+(2B)'  ?

RE: ungrounded wye-delta

Hello Electricpete, May I just ask how you arrived with the value Vgn=    72.4    Angle     9.6    degrees?

Because in my case, my line to ground voltages are 38V, 73.7V, and 92.1V. My line to line voltages are 54.4V, 53.2V, and 53.3V. The transformer is 4160/480 wye-delta. However, we injected 480V at the primary for testing purposes.I wonder what is the expected Vgn for this to case? Thanks!

RE: ungrounded wye-delta

Gents (and ladies of course),

I was called to investigate why when a standby generator was used at a local hospital expensive damage occured to light fittings and other electrical equipment on single phase circuits.

It turned out that the generator star point was disconnected. The problem therefore was easily solved.

Try as I might I cannot recall the formula for calculating the phase voltages on a 3 phase system with differing 1 phase loads where the neutral had become disconnected.

Any thoughts ?
 

RE: ungrounded wye-delta

It's called a voltmeter. Too many loads exhibit non linear impedance with voltage changes. Just as you think you have a handle on that, you will discover that your calculated voltage swings are being influenced by saturation of transformers and lighting ballasts. Another factor is the generator reactance. With fixed excitation the terminal voltage of a generator will typically drop almost 50% from no load to full load. Normally the voltage regulator takes care of this, but with unbalanced loading and a floating neutral, the output of the generator will depend on the type of AVR. Single phase sensing or three phase average sensing. Also the phases that a single phase AVR is connected to in relation to the loading of the phasing will have an effect.
If you attack the problem from the viewpoint of generator reactance, your results may be invalidated by saturation and the nonlinearity of many loads.
Pete may be able to give us a method for determining the voltages of transformer fed resistive loads. Basically it consists of constructing an unbalanced wye inside a balanced delta. The length of each side of of the wye will be inversely proportional to the resistance.
The answer to your question may be non-trivial.
 

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

RE: ungrounded wye-delta

I agree with Bill.  You would need exact information on all the loads connected to each phase at the time of the damage as well as detailed information on the generator and AVR.  Then you could analyze the system with a transient program like EMTP.  Basically, the line-neutral voltage could be anywhere from zero to full line-to-line voltage.  

RE: ungrounded wye-delta

And, due to saturation effects, the neutral point can be continuously moving so that you can have three "line-neutral" voltages that won't form a single point.

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