ungrounded wye-delta 2

[nerb31]

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.

 

[waross]

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

 

[electricpete]

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.

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(2B)+(2B)' ?

 

[waross]

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

 

[nerb31]

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!

 

[electricpete]

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.

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(2B)+(2B)' ?

 

[waross]

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

 

[electricpete]

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).

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(2B)+(2B)' ?

 

[electricpete]

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.

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(2B)+(2B)' ?

 

[stevenal]

"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.

 

[electricpete]

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)




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(2B)+(2B)' ?

 

[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.

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(2B)+(2B)' ?

 

[odlanor]

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

 

[electricpete]

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.

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(2B)+(2B)' ?

 

[drmalabanan]

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!

 

[Freeportpower]

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 ?

 

[waross]

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

 

[jghrist]

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.

 

[davidbeach]

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.