Corona Discharge

Corona discharge is a form of electrical failure in gases. In some ways it is similar to partial discharges in solid insulating materials. There are main differences too. The similarities are:

• Corona discharge is a precursor of total breakdown.
• Corona discharge takes place in /ones where the voltage gradient is higher.
• Corona discharge current is very small in relation to the final breakdown current.

The differences are:

• Corona discharge takes place in the surrounding air and it causes no permanent dam­age. When the voltage is removed, the air becomes normal again. Partial discharge causes permanent damage.
• Corona discharge is visible and audible. Partial discharge is not.
• The quantity of electricity that Mows in corona discharges in gases is very high in com­parison with that flowing in partial discharges. The latter are of the order of a few picocoulombs (10~¹² coulombs) in solid insulating materials.

The significance of the reduced electrical strength of low pressure air gaps near the Paschen's minimum is considered in relation to the proper operation of electrical systems in high altitude and space vehicles. Experiments are described the occurrence and magnitude of partial discharges (corona) in air gaps between insulator and metal surfaces at low pressure and vacuum. The magnitude of the corona pulses observed with alternating voltages varied with pressure, being relatively small at atmospheric pressures and at pressures of about I0~¹ torr where glow discharges occurred; the corona pulses were con­siderably larger for intermediate pressures and for pressures less than I0~¹ torr. Although the pulse magnitude is low in the glow discharge region, considerable integrated charge may be transferred in this pressure region. Calculation of the pulse charge magnitudes indicated that the surface area discharged per pulse at low pressure is at least one third to one half of the total available area. The characteristics of corona dis­charges arising from direct voltages applied to low pressure gaps are also considered.

Questionnaire:

• What are corona discharges?