Explanation of CT Saturation 1

[dunc2027]

I believe I understand the concept of saturation - that all the magnetic domains available in the core have been aligned, so flux density cannot increase, and therefore secondary emf and current collapses. However, I am trying to figure out why current increases so dramatically on a CT excitation curve past saturation.

Is it accurate to say that, at the point of saturation, the core can no longer do its job of transmitting (verb?) flux, and therefore:
1) the load-side winding is no longer coupled, leading the load seen by the supply-side winding to be simply the supply-side winding inductance, and
2) the supply-side winding essentially becomes an air-core inductor (with μ_air << μ_iron and therefore L_aircore << L_ironcore) and therefore have a very low Z_L

So the effect is a double whammy. Is that view correct?

 

[argotier]

My comments on a general point of view, not only CTs:

that all the magnetic domains available in the core have been aligned, so flux density cannot increase

Flux density can increase even with all the domains aligned, but just increases at the same rate of an air-core transformer with µ0.

and therefore secondary emf and current collapses

I don't know what you mean with that.

Is it accurate to say that, at the point of saturation, the core can no longer do its job of transmitting (verb?) flux

Not quite, the core still "transmits" the magnetic flux, it only has reached its "full capacity" and cannot longer add more to the magnetization. Saturation is a very fitting word for the phenomenon.

and therefore:
1) the load-side winding is no longer coupled, leading the load seen by the supply-side winding to be simply the supply-side winding inductance, and

No, the windings are still coupled magnetically.

2) the supply-side winding essentially becomes an air-core inductor (with μ_air << μ_iron and therefore L_aircore << L_ironcore) and therefore have a very low Z_L

Yes, this is the reason why the current increases abruptly when the core is saturated. As permeability drops, you need a lot more current to increase a little of flux. See below (the same curve that represents B/H can represent also V/I - or more properly emf/I0 - with the proper axis scale adjustments):

Take into account that, even when saturated, the iron core does not behaves exactly as an air-core. Granted you may have the same incremental permeability but you are already working on a much higher B (with much more flux and energy also) and the apparent permeability will remain pretty high for any practical working point.

This is all very theoretical, hope it helps.

 

[sushilksk]

After core gets saturated, How much time it needs to get de-saturated?

 

[argotier]

@sushilksk: this is a complex topic and cannot be explained in full in a forum. All I've said above was a simplified view for a stationary state (like DC or AC rms analysis).

For a stationary state the core will get saturated and stay that way as long as there is enough magnetic flux density (or applied voltage) or enough magnetic field strenght (or magnetizing current) applied. If then you de-energize the core, the magnetic flux will drop to a certain value (called remanent or residual flux density) and stay in that value for a relative long time and eventually will end dropping slowly to zero (depending on the material). If you need to demagnetize the core in less time, you will have to apply a "negative" magnetizing current to force the flux to reach zero (coercive force).

In AC, the curve posted above becomes a loop, called hysteresis loop, you can see that both flux and current follow each other with a certain "delay". Here the thing gets more complicated as the magnetizing current not only will change its rms value if the core is saturated but also its "shape".

@dunc2027: And now I get what you meant with:

and therefore secondary emf and current collapses

I didn't realize you were talking of instantaneus values.