This article briefly reviews the nature of corona discharges around high-voltage electric equipment in air. A previous article covered the theory of a self-sustained gas breakdown in a planar gap. Equation 9.47 gives the threshhold voltage for breakdown, Vs (volt) in a gap of width d (cm) at gas pressure p (torr). The gas parameters Vs,min and (pd)min are listed in the previous article. For dry air, the parameters are Vs,min = 327 V and (pd)min = 0.567 torr-cm.
Corona discharges in gases occur only in when electrodes have strong two- or three-dimensional variations (as in Figure 1). Corona (crown in Latin) is a pattern of bright sparks near a point or edge of an electrode. In such a region, the electric field is enhanced above the breakdown limit so that spark discharges occur. Taking the dimension of the corona region as dc, the approximate condition for breakdown is given by Eq. 9.49. In the equation, <E> is the average electric field in the corona region. A self-sustained breakdown cannot be maintained by the low electric field in the bulk of the gap. Current in the low-field region is conducted by a Townsend (or dark) discharge, with the corona providing the source current. The system is stabilized by the high resistivity of the dark discharge. A relatively constant current flows, even though the sparks of the corona fluctuate, extinguish, and reform rapidly. Inspection of Eq. (9.49) shows that for constant geometry, the size of the corona region grows with increasing electrode voltage. The voltage drop in the highly ionized corona is low; therefore, the voltage drop is concentrated in the dark discharge region. At some elevated voltage, bulk breakdown occurs and the electrodes are shorted.
Figure 1. Physical basis for a corona discharge at a pointed electrode.
Substituting air parameters in Eq. 9.49 at one atmosphere (p = 760 torr) and solving for the average electric field leads to the following relationship:
<E> ≅ 438/ln(1340 dc). [kV/cm]
The electric field depends weakly on the average size of the breakdown region. Very small electrode protrusions may not cause corona because the length of the resulting enhanced field is not be long enough to sustain a breakdown. For dc = 1.0 cm, the critical field is <E> ≅ 60 kV/cm. The number applies to dry air at sea level. The danger level for non-ideal conditions is generally taken as <E> ≅ 35 kV/cm.
LINKS
