CORONA

Electrical shielding of the line end (energized) connections and electrodes. The corona rings are energized at the same voltage and reduce the voltage gradient on the connections within the shielding envelope provided by the corona control rings. Corona rings may also be considered grading rings because they increase the coupled capacitance and expand the electrostatic field thereby altering the equipotential planes.

Briefly, outdoor electrical corona (or discharges) occur when the voltage gradient at the surface of a conducting material exceeds a critical value resulting in ionization of the surrounding air. In A.C. circuits there are essentially three types of corona. Glow corona has an inception gradient of about 20 kV rms/cm in air. It appears as a glow on sharp points. At a gradient of about 25 kV rms/cm, negative polarity brush corona discharges occur (so named because the ends of the discharges look like the bristles of a round bottle brush). At about 30 kV rms/cm positive polarity plume corona appears. Brush and plume corona can be distinguished both visually and audibly. Brush corona generates a continuous background type of hissing or frying sound. The sound associated with plume corona is a rather intense snapping and hissing sound. Glow discharge results in a very faint weak light that appears to hug the conductor surface and does not project from that surface.

Electrical corona also generates radio (RIV) and television (TVI) interference. Glow and brush corona do not normally result in objectionable RIV/TVI. However, plume corona generates interference of tens of thousands of microvolts.

The critical gradients mentioned above are fair weather values. Under wet weather conditions virtually all energized conductors are noisy.

In switch yards and substations, the bus sizes generally result in fair weather gradients on the order of 10 kV rms/cm with the other station equipment shielded accordingly.