Transformer Magnetizing inruch, DC offset current soruce 2

[Pdave6]

We have 13kV-480 volt, 3300kVA customer Transformer. I have a question regarding Magnetizing Inrush current of the transformer. I am confused with DC offset current during switch on.
1. Is DC offset a real DC current which is diminishing in magnitude immediately after switch on ? 2. What is the cause of DC offset current? Transformer inductance only or residual magnetism in transformer or both ? May be both have combined effect on the magnitude of inrush current ? 3. what is the source of DC offset current.....Is utility source supplying DC offset current (say for 300 msec or so), to the customer transformer ? OR customer transformer is supplying DC offset current back to utility to neutralize the residual magnetism ?
Thanks.

 

[waross]

DC offset is a way to describe the asymmetrical transformer charging current.
The transformer core may present a different impedance to a positive half cycle than is presented to a negative half cycle.
Stated a different way, under certain conditions (residual magnetism and the point on wave of switching) The transformer may draw much more current on one half cycle than on the opposite half cycle.
This results in an asymmetrical AC waveform.
The difference between the positive half cycle and the negative half cycle may be considered as a DC current or a DC component.
If the DC component is accurately determined, it may be used to calculate the decay rate with an accuracy similar to the accuracy of the DC component determination.

Where does the DC come from?
The same could be asked about a simple half wave rectifier circuit.

Consider the following circuits.
1. A fixed resistor, Rf, that draws one Ampere from an AC source.
2. A variable resistor, Rv, in series with a diode that draws a half wave RMS current of three Amperes from the same AC source.
3. Put these two circuits in parallel and look at the waveform. You will have an offset AC wave form or an AC waveform with a DC offset.
Where does the DC come from? It comes from the source.
4. If the resistance of the variable resistor, Rv, is now increased, the offset or DC offset will be reduced until at Rv = infinity the DC offset will have been eliminated.
Call it an asymmetrical AC current or call it an AC current with a DC offset, The offset or the DC by whatever name comes from the supply or grid.
It is the result of a positive impedance that is different from the negative impedance. (Not to be confused with positive and negative sequences.)


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

 

[waross]

The reason for a DC offset:
An unfortunate combination of the point on wve that the transformer is energized may cause the core to become magnetically saturated on one half cycle polarity.
Normally, the magnetizing current is limited by both the resistance and by the inductive reactance of the transformer winding.
If the current is increased so that the core is pushed into saturation, further increases in current will be limited only by the resistance of the winding.
This happens on only one half of the cycle.
High current on one half of the cycle, and normal current in the other half of the cycle.
If these explanations help, then a Little Purple Star to Gunnar RIP, who helped to fine tune my understanding of this effect.

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

 

[Pdave6]

Bill,
Great Explanation ! Thanks much. (Gunnar has my salute ! )
I would certainly like to do fixed and variable resistor and diode experiment some day.
Prakash