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I have a current transformer from this page. I tried to measure the secondary voltage Vs when I apply Vp=230 V and I change the Ip, like this:

enter image description here

But at some point the voltage Vs is not changing anymore (saturated?) How can I calculate this?

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    \$\begingroup\$ I feel compelled to mention that having a CT secondary winding open-circuited, as you have shown, is a bad idea. Use the shorting links, Luke. \$\endgroup\$ – Li-aung Yip Jul 7 '14 at 15:46
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If you were really applying 230V there would be a fire - you are more likely to be applying a current through the CT that is sourced from an AC voltage and that voltage happens to be 230V.

So the real issue is how much current is the primary being fed. It's the primary current, and the core material that determine saturation. If you know the exact specification of the core materials and you had the appropriate manufacturer's data you can calculate this but where does this get you? If you do a test by injecting current (as you appear to be doing) then use this data.

I'll also point out that putting the recommended burden resistor on the secondary will significantly reduce the point at which the CT core saturates. You should have a burden resistor normally so I can't understand why you are testing without one. For instance, if the primary inductance (a single turn) is (say) 10 uH, this will have an impedance of about 3 milli ohms if the AC is 50 Hz. This impedance is "shunted" by the secondary burden resistor (referred to the primary) and reduces the current thru the primary magnetizing inductance by diverting current thru the secondary - if the burden resistance is (say) 100 ohms it will appear on the primary side as a resistance of: -

\$\dfrac{100\space\Omega}{(turns\space ratio)^2}\$ = 16 micro ohms

Clearly this is a lot smaller than the impedance of the primary single turn and duly reduces magnetization current which, in turn, reduces the point at which the core saturates.

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