Measurement of the Magnitude of Internal Corona in Cables

Consideration is given to corona tests on finished cable by means of general-purpose corona detectors working on detection frequencies below 1 MHz. It is shown that errors due to the superposition of direct and reflected corona transients can be substantial. Results of an experimental examination of superposition errors, and methods for their elimination or control are described. Circuit transformations by which a corona detector may be matched to a cable are outlined. A critierion is provided for the greatest length of cable that may be regarded as lumped capacitance and so tested in the normal way. A calibration procedure is outlined by which measurements may be corrected for the effects of cable attenuation and the test circuit configuration. The sensitivity to be expected is indicated.