In Spanish "un captor" is the one that captures. Used in a technical enviroment, I think it´s better translated as a sensor.
So, in the phrase you are translating, it is ".. three capacitive tension (voltage) presence sensors for 24 kV"
PS: Notice that the author is speaking about "voltage presence" sensing. Its a simple device that transfers a bit of electric charge in a safety way to a display device that indicates wether is tension in the circuit or not. There is not necesarily any proportionality between the tension in the input and the one in the output.
Regards.