Detection of AC corona discharges using an electro-optic E-field sensor

This paper present some preliminary results concerning the detection of partial corona or partial discharges in air using a new electro-optic (EO) E-field sensor. The E-field sensor used an ultra-compact single-ended EO probe allowing non-invasive, remote and unidirectional measurement of the E-field component. These characteristics were used to validate the sensitivity of the EO sensor to corona discharge activity. A series of experimental tests were performed using a rod-to-plane configuration with the EO probe position along the axis of the air gap. The EO probe orientation was kept constant to measure the vertical E-field component in the air-gap. The tests were carried out under AC applied voltage for different air gap lengths, EO probe positions and HV rod electrode radius. The experimental results obtained show that the output voltage of the EO sensor undergoes a sudden increase from at a given applied voltage threshold. It was demonstrated that this threshold is dependant of the air-gap length and HV electrode radius. The experimental threshold voltages were compared to corona onset voltage calculated using Peek´s empirical law. The good agreement between experimental and theoretical values demonstrated that the EO sensor was able to detect ignition of corona discharges.