Corona discharge is a predominant failure driver in high-voltage equipment
since it causes degradation of the insulation system. Corona reduction
commences at the initial design phase and continues through manufacturing. A
key element involves controlling the internal geometry to maintain an
acceptable field strength throughout the assembly.
By definition, the terms Corona Discharge or Partial Discharge are
generally used to describe electrical discharges that involve only a portion
of the dielectric between two electrodes rather than bridging the electrodes.
In all of these discharges, gas molecules are ionized by the impact of
electrons. The liberated electrons gain speed in the electric field, ionizing
more atoms by impact, so that an avalanche of electrons is formed. The
electrons in the avalanche and the remaining ions move toward the electrodes,
thus forming a passage of current through the insulation system. Although the
energy dissipated with each discharge is small, these partial discharges can
cause deterioration and ultimate failure of the dielectric system.
Degradation caused by corona can be reduced if materials are chosen
carefully. A self-healing dielectric would be desirable but difficult to
achieve. Oil systems are somewhat tolerant to arcs as the degradation products
become dispersed. However, the degradation is still cumulative. Where the
system dielectric is a gas or vacuum, arcs or corona can cause damage between
interconnects and between components. Vacuum components such as traveling wave
tubes, vacuum relays, vacuum capacitors, vacuum triodes and diodes can
withstand a number of internal arcs without failure. Corona in solid
dielectrics, which usually occurs at dielectric interfaces or voids, is
cumulative and eventually results in destructive and catastrophic
The ability of a system to tolerate an occasional arc is an essential part
of a viable, high-voltage design. Circuit design and packaging techniques play
a key role in the ability to tolerate an arc. Most designs limit current
available to an arc through inductive di/dt or resistive limiting combined
with current sensors and rapid turnoff.
Conservative design practice dictates that the output
should be capable of surviving a shorted or open output, without degradation,
for an indefinite time.