"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