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.