Flux of a transformer decays to zero when left connected to load 5

[stason]

thread238-222884

Hello,

I have read in a certain thesis that the residual flux of a transformer which is de-energized but is left connected to a load will decay to zero. We are talking about immediate decay.

I cannot explain to myself how would a passive load be able to do that.

Does anyone have an explanation?

Thank you in advance,
best regards
stason

 

[Skogsgurra]

If you have a capacitor connected when you switch off the primary, you may have an oscillation that - if Q s high enough - takes the transformer core through several cycles and thus demagnetizes the core. I think that I promised Bronzo to make a test run with such a set-up. Never got time to do it. Will try again. Stay tuned!

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Skogsgurra]

Here it is. Not particularly well done. But shows that deflux can be achieved with a parallel capacitor. Maximum inrush reduced from 73 A (quite often and repeatedly tripping breaker) to rare events with 30 A and not tripping breaker.

Take care. The resistor that I used to minimize the capacitor inrush got very hot and is no more. End of day - end of week.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

Thank you for another contribution to my continuing education, Gunnar. LPS
Can you give a rough and ready estimate of a range of capacitor current versus magnetizing current to successfully demagnetize a transformer?

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

 

[stason]

oh yes, a capacitor. That makes sense. Thank you!

 

[Bronzeado]

Well done Gunnar, thanks!

Bill, to successfully demagnetize a transformer with a capacitor, it should create a resonance with the transformer "magnetizing inductance".

Regards,

H. Bronzeado

 

[stason]

A somewhat different question regarding sympathetic inrush of transformers connected in parallel or in series.

Literature says the offset current of the already running transformer is of opposite polarity to that of the one being energized. Is that because the voltage drop across the system resistance make the voltage applied to the already energized transformer smaller so that the flux starts to decrease?
Thanks.
Regards

 

[stason]

Let me express it better:

The inrush current of the transformer being energized is predominantly homopolar so that the voltage drop affects only one half-cycle. Let's say the fist half-cycle is positive, then the voltage drop makes this positive value to decrease. The waveform of the opposite half-cycle (the negative one) of the voltage stays undisturbed. If the whole cycle is integrated the area is not equal to zero but is a certain negative value. This is what makes the running transformer saturate in the negative direction if the one being switched on draws inrush current with positive peaks. Can someone confirm this theory?

 

[Bronzeado]

Dear stason,

Another way explained by Blume:
The "dc voltage drop" across the system resistance caused by the inrush current creates a "dc voltage" of opposite polarity on the busbar voltage to which the transformer already in operation is submitted, saturating gradually the transformer.

Please, find attached a self explanation schematic obout the sysmpathetic inrush phenomenon that occurs in paralelled transformers.

Best regards,

H. Bronzeado

 

[stason]

Hi Bronzeado,

thanks. I found a paper by you and Yacamini dating back to 1996 with the thorough explanation. I find it pretty good!

Regards

 

[Bronzeado]

Thank you stason!

I think this phenomenon (sympathetic interaction between transformers) still not being considered as it suppose to be.

Regards,

Herivelto Bronzeado

 

[stason]

Hi folks,

a kind of a different question:

what is the flow path of the decaying DC offset component in the transformer current? It also appears at the generator. Does it only flow in the generator stator or also the rotor? And what is the return path to the transformer if the generator neutral is not earthed?

Thanks
Regards

 

[waross]

The DC offset is a component of the asymmetrical AC fault current. It flows in the same conductors as the AC current. It flows until the energy causing it has been dissipated in the resistance of the current path.

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

 

[stason]

Hi Bronzeado,

I have another thought about the sympathetic inrush. Maybe you could contribute to it?!

If I have several (let's say 3) transformers in parallel and one of them is being energized driving the other two into sympathetic interaction, will the sympathetic inrush last longer because there are now 2 transformers in parallel to the one being energized?

I am thinking that those two will only get saturated half as bad as each of them would if it were the only one hanging in parallel to the oncoming transformer. The asymmetric currents at the common point must be equal before they start decaying, and since the symp. inrush currents of the two running transformers add up at the point (they both get saturated in the same direction) each of them should contribute by half of the inrush current caused by the oncoming transformer.

So the question is: will the decay be faster because the transformers got saturated only half as bad?
Or will the decay be slower since the symp. inrush current of each of them is only half that large causing a smaller voltage drop across the winding resistance contributing to the decay?

Thanks in advance!
Regards
stason

 

[stason]

Or maybe these two effects will exactly compensate each other so that the decay has the same rate?!

 

[Bronzeado]

Hi stason,

Remember that we are talking about a non-linear phenomenon so we cannot think linearly in this case. Anyway, your thought is in the right direction.

In the past, when I was studying at the Univerdity of Aberdeen, Scotland (almost 20 years ago), I did a quick investigation on the sympathetic interaction with more than a transformer in operation. I will try to find where are those results to post in this forum.

Back to your question, let's think together, as this situation is not trivial (due to the non-linearities involved) so I cannot answer it straight (sorry!).

Let's consider that the transformers in operation have the same saturation curve. So they will saturate similarly and generating sympathetic currents with iqual shapes and values which will be added at the point of commom coupling. Of course, they will present a saturation condition different from that if just one transformer was in operation. Even so, we cannot say that "the transformers got saturated only half as bad" as you said.

Also, we cannot consider these transformers as an equivalent transformer with half of the original transformer resistance as the superposition principle cannot be applied on non-linear systems (transformer saturation).

Based on the above, I have just a feeling (not a confirmation) that the sympathetic interaction will die way slower than if just one transformer was involved.

stason, if you are interested in going dip on this subject I would suggest to write a paper together. I could go back to the 90,s to find my results and share with you. What do you think?

Best regards,

Herivelto Bronzeado

 

[stason]

Thanks Bronzeado,

going back to the 90s sounds good.
But I am graduating in 2 weeks, so I will get back at you in a couple of weeks.

Best regards