How would you connect a current transformer to a PLC analog input module?
You would almost certainly need some sort of signal conditioner, preferably a well-isolated one.
It would be helpful to know more about the specific application, but, absent that data, here's some things to get you started looking at the right kind of stuff to get it done.
Red Lion makes products that integrate a signal conditioner with a CT. Here are some links:
CTR True RMS AC Current Transducer
CTL Current Average Responding Transducer
CTD DC Current Transducer
{DISCLAIMER: No, I don't work for Red Lion, but I have found their stuff to be very useful in times past. With any controls application, the designer/implementer should always do their due diligence and assess for themselves the usefulness or fitness for a particular application of any piece of equipment.}
If I read you correctly, you already have a CT? You'd have to determine what its output is, and select a proper signal conditioner for it. Usually, CTs output in the millivolts, and you would need to convert that to a range acceptable to the PLC's analog inputs (usually either a current loop or voltage input, depending on the PLC, the input module, etc.)
There are many other vendors which make signal conditioning gadgets besides Red Lion. You might look at something from Moore Industries, Action Pak, or any of several others before you find what suits your application and price/performance point best.
Some examples of stand-alone signal conditioners that might suit your needs:
Action Pak Model AP6380
Moore Industries Signal Converter Selection Guide
Of course, if you are working outside the U.S., you may wish to consider other vendors that are more local, and I don't really have a clue there.
The tricky part is selecting a signal conditioner with enough input range to be able to calibrate it properly, and then having the prime standards (precise enough test equipment in this case) against which to calibrate it. It makes a good case for the more integrated CT/signal conditioner all in one transducer if the accuracy of the application is critical, since they are usually already well matched.
Here's wishing you all the best with your project.
Later On,
D