3rd harmonic in generator current energizing a transformer 5

[stason]

Hello,

I am performing some simulations in PowerFactory where I energize a transformer (13 MVA, 57/10,5 kV, YNd11) on the LV side (delta side) from a 7 MVA synchronous generator (with the star point solidly grounded).

The inrush current causes 2nd and 3rd harmonics to appear in the generator's currents.

If I look at the simulated zero-seqence current of the generator, it equals zero. I figured this could not be true, since the star point of the gen is connected, which would allow the 3rd harmonic currents to flow in the gen's neutral, which they also do. Is this correct? And if it does flow in the neutral, might the ground current relay trip?

Besides, will the 2nd harmonic current in the generator make the differential relay think it could be an internal fault?

Thank you in advance!
Regards,
stason

 

[electricpete]

For completeness, the current solution should also be divided by a factor of w that I forgot, but doesn't affect the conclusion.

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[Bronzeado]

Sorry folks! I did not make myself clear.
The offset I talk about is the negative part that appears in the current waveform, which it supose to be zero.

electicpete, could you, please, explain that offset?

Regards,

Herivelto

 

[electricpete]

I was trying to explain why the graph posted 19 Apr 10 12:10 has much larger positive component than negative component.

The answer to that question has to do with angle of voltage at time of closing as was demonstrated for simple inductive circuit.

I would be glad to elaborate on that, but I’m not sure I’m answering the question you asked....

The offset I talk about is the negative part that appears in the current waveform, which it supose to be zero.

Which current waveform (posted what date)?
I didn’t see any where the negative is larger than the positive. (where?)
Why would part of the waveform be zero?

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[Bronzeado]

electricpete,

Thank you very much for your interest. I think this phenomenon is not so trivial.

I am not talk about "components" but about the Phase X current waveform itself posted on 19 April 10 12:52 by jghrist. In that waveform, a typical transformer inrush current, we can see a negative part which does not appear normally in computer simulations.

Best regards,

Herivelto Bronzeado

 

[davidbeach]

Then your computer simulations are wrong. That's what inrush looks like.

 

[electricpete]

Attached is a simulation I did in excel vba using simple R/L circuit... the result in tab "plot sheet" looks very similar to the one posted by jghrist.

My question to you is why would you expect it to look differently? It is not a hard matter to construct the curve yourself by mentally integrating (assuming you start with simple model... my preference is to start with simple models... do you think something is missing from simple R/L model that would affect the results?).

My suspicion is that you may be used to viewing transients where the positive part goes much higher so the plot scale gets compressed and the negative portion of the cycle is hard to see.

By the way, you can tweak the parameters in the spreadhseet and re-run the simulation if you'd like.... I think the instructions are self-explanatory.

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[electricpete]

Another thought – is it possible you had a load on the secondary of transformer during energization? That would certainly change the waveform dramatically different than the R/L model I suggested.

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[electricpete]

The last plot closed at an angle midway between zero and peak of the voltage.

Here is a plot that closes at voltage zero crossing which gives max offset. I also extended the plot range to steady state.


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[electricpete]

I guess there is one difference between my simulation plot and jghrist’s post... mine have a rounded negative portion of the wave and his have a flat portion. I don’t know why that is. Any ideas?

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[DHambley]

A simple way to visually see where the 3rd harmonic currents are going is to simply sketch them on a peice of paper with a pencil. Forget the equations for a moment.
1. Draw 3 separate horizontal lines, then 3 waveforms 120deg apart.
2. Now draw in a 3rd harminic for each one, starting a 0 degress for each one.
3. Now compare all of your 3rd harmonics. See how they are all in phase? They can't all flow into the neutral without current flowing in the neutral.

 

[Bronzeado]

Hi davidbeach. I used to say that the only certainty I have about simulations is that they are not the reality! By the way, do you have any ideia about how to explain this phenomenon?

electricpete, your your simulation looks good. However, the jghrist's current waveform has a flat "zero" offset. I am sending attached another inrush current waveform that shows the same behaviour. The question is why this offset occurs? Please, explain physically, if possible.

Regards,

Herivelto Bronzeado

 

[Bronzeado]

electricpete,

As I said, my guess is that this offset may be caused by the an offset magnetizing of the core during the transformer inrush.
In a three-phase transformer with three limbed core, this phenomenon can beseen in the neutral current, indicating that this should be caused by zero sequence magnetic flux (induction), which path goes outside the core.

Regards,

H. Bronzeado

 

[Bronzeado]

electricpete,

In your simulation, the transformer seems to be over-excited in the steady-state.

Regards,

H. Bronzeado

 

[electricpete]

I didn't give a huge amount of thought to the particular paramater values set for that simulation other than to try to recreate the posted waveform. I am under the impression that power transformers at no-load would be partially into saturation and some waveform distortion would be evident.

As load increases the current would look more sinusoidal due to:
1 - exciting branch voltage decreases due to voltage drop across leakage reactance. Therefore not as much saturation effects
2 - In addition to magnetizing current we now have load component which is shifted in phase relative to magnetizing component and will have less distortion.

At least that's my thought. It's been 10 years since I've looked at transformer waveforms.

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[stason]

The fact that the inrush current can become negative while the flux stays above zero during the inrush means that each the flux and the current have their own DC offset magnitudes. That I just discovered from looking at some simulations.

 

[stason]

The physical explanation for the DC offset would be the law of the flux linkages, which states that the flux just before the switching event and right after it has to be the same and can't change instantaneously, if that was the question, Bronzeado.

 

[electricpete]

Bronzeado – I see now your earlier comments and questions were deeper than the simple aspects I was discussing. My apologies. I can imagine there can be zero sequence flux during energization, but it’s beyond my ability to visualize what that would look like in the current waveforms or to connect it with the flat shape on the negative side of the A phase current waveform.

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