CT Saturation

[mull982]

What happens when a CT saturates. I have heard the term "CT Saturation" being used when talking about CT's related to protection relaying.

My little amount of understanding about staturation, that when something saturates (a core for example) there is no more room for magnetic flux lines to pass through the core, therefore there will start to be a larger resistance or reluctance to additional current that is producing these magnetic flux lines?

I have heard that the current and magnetic flux are proportional up until the point of saturation at which point they then become very unproportional. After saturation the additional current is nothing but excess heat in wires and core and can cause damage.

Is saturatin of an object based off of a hysteresis curve?

 

[jim dungar]

I have heard that the current and magnetic flux are proportional up until the point of saturation at which point they then become very unproportional. After saturation the additional current is nothing but excess heat in wires and core and can cause damage.

Is saturatin of an object based off of a hysteresis curve?

Yes and yes.

After saturation the output is no longer directly proportional to the input. A hysteresis curve shows the relationship between the input and the output as the transformer moves toward saturation and then as it recovers from it. Saturation can occur in any transformer, and is part of the intentional design of a ferrosonant constant voltage unit.

 

[gar]

080624-0953 EST

mull982:

For magnetic materials saturation is not one fixed flux density but there is a gradual reduction in the increase of flux with increasing magnetic excitation. Some materials have a sharper corner than others.

A magnetic core tends to concentrate the magnetic flux in the core. As you go into saturation there is less change of flux relative to the change of magnetizing current and therefore less coupling to the secondary. The further you go into saturation the greater is the error between primary and the secondary current.

A rough way to look at this is:
As the flux density increases the percentage of total flux that couples to the secondary decreases, and that which does not couple can be classified as leakage flux.

"Electric and Magnetic Fields", Stephen S. Attwood, John Wiley, 1949 has a chapter #13 on Ferromagnetism. There are many magnetization curves shown for different materials.

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