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






RE: ungrounded wye-delta
RE: ungrounded wye-delta
RE: ungrounded wye-delta
RE: ungrounded wye-delta
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
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"Why not the best?"
Jimmy Carter
RE: ungrounded wye-delta
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
Bill
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"Why not the best?"
Jimmy Carter
RE: ungrounded wye-delta
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
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
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.
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(2B)+(2B)' ?
RE: ungrounded wye-delta
RE: ungrounded wye-delta
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
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"Why not the best?"
Jimmy Carter
RE: ungrounded wye-delta
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RE: ungrounded wye-delta
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
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"Why not the best?"
Jimmy Carter
RE: ungrounded wye-delta
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.
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RE: ungrounded wye-delta
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RE: ungrounded wye-delta
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RE: ungrounded wye-delta
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
Try taking the measurements with a little load on the transformer bank.
Bill
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"Why not the best?"
Jimmy Carter
RE: ungrounded wye-delta
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
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.
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(2B)+(2B)' ?
RE: ungrounded wye-delta
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).
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RE: ungrounded wye-delta
Bill
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"Why not the best?"
Jimmy Carter
RE: ungrounded wye-delta
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RE: ungrounded wye-delta
http://
As far as capacitance calculations, there is a lot to it. Entire Chapter 7 here devoted to it. It looks like good reading for anyone who has time... not me at the moment.
h
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RE: ungrounded wye-delta
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RE: ungrounded wye-delta
If the voltages are not congruent with the delta, look at the waveforms with a scope.
Bill
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"Why not the best?"
Jimmy Carter
RE: ungrounded wye-delta
RE: ungrounded wye-delta
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RE: ungrounded wye-delta
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RE: ungrounded wye-delta
Put the load on the transformer and see if the voltages to ground are consistent with the line to line voltages.
Bill
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"Why not the best?"
Jimmy Carter
RE: ungrounded wye-delta
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
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).
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(2B)+(2B)' ?
RE: ungrounded wye-delta
Bill
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"Why not the best?"
Jimmy Carter
RE: ungrounded wye-delta
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?
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RE: ungrounded wye-delta
Bill
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"Why not the best?"
Jimmy Carter
RE: ungrounded wye-delta
RE: ungrounded wye-delta
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.
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RE: ungrounded wye-delta
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RE: ungrounded wye-delta
But I don't know for sure. Comments?
Bill
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"Why not the best?"
Jimmy Carter
RE: ungrounded wye-delta
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.
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RE: ungrounded wye-delta
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
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"Why not the best?"
Jimmy Carter
RE: ungrounded wye-delta
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
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.
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RE: ungrounded wye-delta
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
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"Why not the best?"
Jimmy Carter
RE: ungrounded wye-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).
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RE: ungrounded wye-delta
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RE: ungrounded wye-delta
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)
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RE: ungrounded wye-delta
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
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RE: ungrounded wye-delta
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
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
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
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"Why not the best?"
Jimmy Carter
RE: ungrounded wye-delta
RE: ungrounded wye-delta