"Zero offset" in tranformer's inrush current

[Bronzeado]

Hi Guys!

I have observed a "zero offset" in transformer inrush current waveforms.
Some researchers think this "offset" is due to saturation of the current transformer used in the measurements.
However, this "offset" has also appeared in transformer energization simulations.
Does anyone may explain this phenomenon?

Cheers,

Herivelto S. Bronzeado
Brasília, Brazil

https://www.linkedin.com/in/heriveltobronzeado/

https://www.researchgate.net/profile/Herivelto_Bronzeado

 

[davidbeach]

Just the right point on wave when it closed into that phase.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[waross]

First, magnetic forces are related to the square of the current, not the RMS or average current.
Thus the peak current is important when evaluating mechanical strengths of electrical equipment.
The peak current is also important for fast acting fuses.
When the point on wave of current initiation coincides with the wrong polarity of residual magnetism the transformer will easily saturate.
The current is then limited by winding resistance and some negligible, quadrature air core inductive reactance.
The peak of a sine wave is root 2 of the RMS value.
Thus the inrush of a transformer may approach 2 x 1.414 or 2.828 times the steady state short circuit current.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[prc]

The inrush current is maximum when the switching is done at zero of the voltage waveform. Simulations are done to achieve the maximum peak of inrush.

 

[Bronzeado]

Sorry guys, I didn't make myself understood.
Perhaps is due to my "native" Brazilian English.

I'm not referring to the peak inrush current but its "zero".
I know that the zero of the inrush current is not ZERO due to the asymmetry of the hysteresis cycle during energization. My question is: What causes this asymmetry?
Is it caused by the external flux that leaks the iron core?

Cheers,

Herivelto S. Bronzeado
Brasília, Brazil

https://www.linkedin.com/in/heriveltobronzeado/

https://www.researchgate.net/profile/Herivelto_Bronzeado

 

[sushilksk]

It is due to saturation of magnetic core. In steady state, Flux produced in one half cycle is cancelled out by MMF of next half cycle. During charging, we need to close CB at exact point on wave (taking residual magnetism in account) to acheive normal current levels without any inrush. Practically it is difficult and mostly not implemented. Energising at any random point on wave will cause core to saturate in either +ve or -ve half cycle, in next half cycle some of saturation will be cancelled out. It will take some more cycles to completly cancel the saturation. Till then we can see DC offset in currents.

 

[Bronzeado]

Thanks guys for your thoughts so far!

I think the attached waveform will clarify exactly what I'm asking about the "zero offset" of the inrush current.
Could this phenomenon be caused by the magnetic flux outside the iron core during transformer energization?

Cheers,

Herivelto S. Bronzeado
Brasília, Brazil

https://www.linkedin.com/in/heriveltobronzeado/

https://www.researchgate.net/profile/Herivelto_Bronzeado

 

[sushilksk]

What you mean by "magnetic flux outside the iron core"? I have used term "DC offset" instead of "zero offset", both are same.

 

[Bronzeado]

Hi sushilksk,

"Magnetic flux outside the core" is the flux that leaks from the core when it saturates.
The attached figure shows the magnetic flux leaving the upper part of the core and returning through the lower part.
In general, the path of this magnetic flux includes the transformer tank.

Cheers,

Herivelto S. Bronzeado
Brasília, Brazil

https://www.linkedin.com/in/heriveltobronzeado/

https://www.researchgate.net/profile/Herivelto_Bronzeado

 

[Bronzeado]

In this new format of Eng-Tips I could not find the figures I had posted.
Please, see attached the figures I had posted before in order to clarify my question.

 

[waross]

I suspect that your graph plots are misleading.
The DC offset of an energization curve is the center of of the curve.
I suspect that your plots show a situation with an externalDC bias as well as energization offset.
The lower part the curve should not cross the zero line unless an external DC bias is added.
The curve should go below the zero line by an amount determined by the DC resistance of the transformer winding.

 

[Bronzeado]

I suspect that your graph plots are misleading.
The DC offset of an energization curve is the center of of the curve.
I suspect that your plots show a situation with an externalDC bias as well as energization offset.
The lower part the curve should not cross the zero line unless an external DC bias is added.
The curve should go below the zero line by an amount determined by the DC resistance of the transformer winding.

Waross, the graph plots I posted are correct and there is no external DC bias.
Some researchers think this "zero offset" is due to saturation of the current transformer (CT) used in the measurements. Sometimes it happens.
However, this "phenomenon" can also be seen in transformer energization simulations (see paper attached) without any CT involved.
So, what do you think about?

 

[waross]

Sorry, but I can't help getting old.
Concepts that I once saw instantly now may take a few days of thought.
(Aside, I know from sad experience that both Trump and Biden are too damn old)
After much thought and going back to basics I have this suggestion.
Look at this as a filter circuit.
Initially the core saturates and the current is limited by the DC resistance and the air core inductive reactance.
The RC constant plays a part in this.The time constant of an RL circuit is calculated using the formula
Quote:
The time constant of an RL circuit is calculated using the formula

τ=L/R

L is the inductance in Henrys
and

R is the resistance in ohms.

The air core inductance in Henries is quite low, but the resistance of the circuit for a close in fault is also very low.
The time constant may be greater than 1/2 cycle.
So, we have a current through an inductor.
The voltage drops from a positive peak to a negative peak and the current drops at a rate determined by the time constant.
If the time constant is greater than about 1/2 cycle, the current will still be positive when the voltage again goes positive.

Actually, the time constant is the time to drop by 63%, so 1/2 cycle is not accurate but is used for a quick explanation. Feel free to work out the actual time constant required for the current to stay positive.

 

[waross]

If I understand your question, you are wondering why the current stays positive by a small amount.
The DC offset, as I understand it, is the offset of the mid point of the decaying sine wave above zero..

 

[Kiribanda]

Herivelto,
The non-zero happens due to high DC off set meaning very high X/R ratio. Depending on the transformer design meaning its X/R ratio,
it may happen.
If we are going to break the current during the short in-rush duration, then the non-existence of current zeros may be a problem for the bkr because the bkrs are supposed to extinguish the current during the current zeros.

 

[Bronzeado]

In my view this inrush current "zero offset" happens due to the assymetry of the iron hysteresis loop which do not returm to zero after core saturation during transformer energization.
What do you think about that?

 

[waross]

I have been moving and my texts are not readily available.
However, as I search the internet and review the basics, I more and more doubt that the inrush curve stays above the zero line.
What is the source of the curves that you posted?
I suspect a misunderstanding of the curves you posted.

Consider basics. Induced EMF and the resulting current are dependant on the rate of change of the excitation.
At the voltage peaks, there is a moment when the rate of change of excitation is zero, hence no induced EMF, nor current.
By this criteria, the inrush current must reach zero when the exciting voltage reaches the negative peak.
But is this the only source of current.
If the current does not drop to zero at the voltage negative peak, what other factor may be driving the current?
If this is happening, then I speculate that the cause may be a time constant of the inductive circuit that is long in relation to the duration of a half cycle.

 

[Bronzeado]

Waross, please read a paper I wrote 30 years ago, wich I have attached in this thread. I think this phenomenon is a "magnetic" not a "electric" phenomenon, and it is not easy to understand.
I know that there is a lot of experts over here that think "out the box" and I hope to get a good explanation.