Delta-Wye transformers: zero sequence effects 1

[alce]

We have carried out two tests on a three-phase delta-wye 3-legged core transformer, to see the effect of a zero sequence current on the transformer voltage.


A).- System conditions for test 1
- Delta open ( we open one of the phases) so the zero current can not flow.
-We supply the wye side with a zero sequence current source.
- We measure the voltage wye side voltage.

Observations: The measured voltage is NOT sinusoidal. The peaks of the waveform have been deformed, and present a "sag".



B).- System conditions test 2
Same as before, but the Delta side of the transformer is closed so the zero sequence current can flow.

Observations: The measured voltage is still not sinusoidal, and the "sag" have been moved 30% (it is not longet at the peak values).

Why is the voltage distoted? After some tests, we found that the magnitude doesn't depend on the zero current magnitude.

Does anybody know where I can find some information on this issue?

Thanks

Alejandro

 

[jbartos]

Suggestions marked ///\\alce (Electrical) Oct 30, 2003
We have carried out two tests on a three-phase delta-wye 3-legged core transformer, to see the effect of a zero sequence current on the transformer voltage.
///Which industry standard has been adhered to?\\
A).- System conditions for test 1
- Delta open ( we open one of the phases) so the zero current can not flow.
///If the delta is closed, the zero sequence current would not flow either.\\\
-We supply the wye side with a zero sequence current source.
///Is wye grounded or ungrounded?\\- We measure the voltage wye side voltage.

Observations: The measured voltage is NOT sinusoidal. The peaks of the waveform have been deformed, and present a "sag".
///Probably, the voltage has higher magnitude causing the waveform to be distorted by saturation in the transformer core\\

B).- System conditions test 2
Same as before, but the Delta side of the transformer is closed so the zero sequence current can flow.
///Zero sequence current is not supposed to flow in the delta winding\\Observations: The measured voltage is still not sinusoidal, and the "sag" have been moved 30% (it is not longet at the peak values).

Why is the voltage distoted? After some tests, we found that the magnitude doesn't depend on the zero current magnitude.
///Possibly the supply voltage is too high or due to asymmetry of the supply voltage.\\Does anybody know where I can find some information on this issue?
///Reference:
William D. Stevenson, Jr., Elements of Power System Analysis, 3rd Edition, McGraw-Hill Book Company, 1975
See zero sequence electrical equivalent circuits.\\

 

[alce]

-We haven't followed any standard
- The zero sequence current flows in a delta winding, and it does it within the windings when the loop is closed. By the way, in all our test the delta side was unloaded

- Is doesn't matter is Wye was grounded or not. The zero current source was connected between the three phases and the center point of the Wye side.

- I don't think there is saturation in the core, since the zero sequence current magnitude was half the rated transformer current.

- The current source presents a pure sinusoidal waveform

 

[jghrist]

I agree with Eiichi Haginomori who responded to your query in the ATP/EMTP forum. He pointed out that in a 3-legged core transformer, there is nowhere for the zero-sequence flux to travel except through air and the transformer tank. This is likely the cause for the voltage distortion.

 

[alce]

When the flux closes through the air there is a very small saturation (it behaves as an air-gap inductor). Besides, the voltage measured at the wye terminal is very small. Therefore, saturation is out place.

The question is, what causes the voltage distorsion. Should the distorsion be realted to the zero sequence current magnitude? In what way?

 

[rbulsara]

I am a little bit miffed. What is the puropose of this test?? What you are tyring to do with this data?

In the real world, zero seq current will flow when a primary side voltage is available and a L-N fault takes place on the secondary.

A better test will be simulate a L-N short on one of the secondary phases with 3 phase voltage applied and see what happens to the primary voltage.

What is the ZSC 'source'? What is the voltage betn L-N terminals of the secondary when this 'current' is flowing?
Is primary open circuited during test?
What are the rated voltages?

Trasformer is designed to transform "voltage per turn" not current to voltage.

 

[alce]

The goal of the test it to determine the effects of a zero sequence current component on the voltage. The reason is that we want to use capacitor banks to compensate undervoltage conditions. We would like to know if we should control the phases independenly or together ( only positive sequence).

If we use individual phase control, it could happend that one of the phase is injecting more current than the others ( more cap banks on). We know that if that happend we are creating positive and negative sequence in the Delta side of the transformer, but we would like to know if the zero sequence ( the one that ciculates in the Delta windings) has any effect on the voltage since we are using a 3 legged transformer.

The transformer values are:

- S=15 kVA
- Vs (Wye side)=208
- Vp (Delta side)=2815V
This is a very small transformer compare with the one we will be using in the field by a factor of 100, but it can give us a good understanding of what is going on.

Regarding the unload circuit, yes. All the test were carried out with the Delta side unloaded

 

[alehman]

Sinusoidal current yielding non-sinusoidal voltage implies non-linearity, so what could be the cause? Saturating iron jumps to mind. I wonder if you are somehow saturating the iron somewhere in the tank.

I would be cautious of testing zero sequence behavior on a small scale. You may see something completely different in a larger transformer since the behvior depends on the surroundings. Do both your test transformer and the real one have the same insulation system (dry vs. oil)?

 

[alce]

Yes,
both transformers are dry-type.
Saturation could case the voltage dip. However, the recorded voltage magnitude is only 10 volts peak.
I think the voltage is too small to have saturating iron.

 

[CarlPugh]

Magnetization of core steel is non linear. That is when you double the applied volts/turn, the exciting curent usually does not double.
Also if you are using a transformer with laminations, there is the possibility that in some locations the flux density is doubled.
Try exciting a single phase transformer with a sine wave current, the output voltage will not be a sine wave.

 

[alce]

The only way the saturation in the core could have any influence on the voltage waveform is if the iron reluctance
excess the reluctance of the air path.

For zero sequence current "the total flux through air will be the summation of the three individual equal fluxes in the three transformer legs and consequently through the air path reluctance. For some transformers, this air reluctance
which can be assumed as constant value is much larger than reluctance of the iron path 'leg'. However, if the flux increases, the reluctance of the iron will increase 'while the air path reluctance remains constant'. The iron
path reluctance when reaching saturation can reach high reluctance values". Sami G. AbdulSalam)

This is the only case the core could have an effect on the voltage.

 

[alehman]

You said the test current was half of the rated winding current. Since flux is proportional to ampere-turns, I would expect to see high density in the air and surrounding iron.

Intuitively, saturation in the core seems unlikely because flux in the phase legs would be half of design and should be much less in the cross members since all phase legs would have the same polarity.