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"EmonTX V3 can monitor only until 24 Kw"

Not true. The emonTx V3 can only monitor up to 100 A with our standard current transformer. With a different ct, it can work to any power you name, subject to recalibration inside the sketch of course, for which you will need a programmer.

The Magnelab range without an internal burden resistor, derated to 1/3 the rated current (because they are designed for 0.333 V rms output and we need about 1.1 V rms) are suitable. It will be necessary to change the burden resistor(s) inside the emonTx.

You don't say your system voltage, nor the number of phases, so power in kW doesn't mean much as far as specifying the ct is concerned.

My system voltage, after the voltage transformers, is 400 V 3 phase. Most of the switch that I would like to monitor are bigger than 100A. I have some 125, 160, 250, 400, 600, 800 A. 

To recalibrate the CT sensor is a complicate job?

No, not complicated at all. There are full instructions in "Building Blocks", but you need an accurate measurement of the current so that you can adjust the calibration constant to give you the same reading.

If you can disconnect the circuits temporarily, you can use solid core ring-type ct's, which are more accurate than the alternative split-core, which you need if you can't interrupt the circuit to thread the CT onto the cable.

The SCT3000 (split core)  is suitable for your 600 A & 800 A feeds, I don't have a note of the actual ratio so can't immediately tell you what value burden you need, but Magnelab are very helpful and I have a name there to contact. The SCT2000 would do the 400 A feed, and the SCT1250 would be good for the 250 A and below, or the solid core UCT1250 for 125 A and below. They don't list a solid core one above 400 A (though this would just do the 160 A feeds taking advantage of the 120% overload - but I'd like to check that with Magnelab).

I've just studied in building block section some theory, but i have not found an explanation on why we need an output 1,1 V. 

Can you explain me please?

Thanks

It's the maximum voltage that the input can handle with a 3.3 V supply. Any less and you lose resolution in the ADC.

Look at this page (which specifically mentions the 5 V supply used with the Arduino board) and do some simple maths, remembering to allow some headroom for if the wave shape is not a true sinusoid, supply voltage and component tolerances.