||NAVSO P-3641A: More Power For The Dollar
Corona discharge is a predominant cause of failure in
high voltage power supplies since it results in degradation of the insulation
system. Successful corona reduction commences with initial power supply
design and continues through manufacturing. A key element involves
controlling the internal geometry to maintain acceptable field strength
throughout the power supply.
The terms Corona Discharge and 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 ultimately, failure of the
Degradation caused by corona can be reduced by careful
selection of materials. A self-healing dielectric would be desirable but
difficult to achieve. Oil systems are somewhat tolerant to arcs as the
degraded 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 dielectric, which usually occurs at dielectric
interfaces or voids, is cumulative and eventually results in catastrophic
High Voltage Field Control
Corona is generated by high concentrations of the high
voltage field, usually as a result of sharp points, small geometries and their
associated spacing. The high voltage field is characterized by the "E"
field gradient. The higher the number, the greater the risk. There
is an adjunct coefficient called the "utilization factor" that acts as a multiplier to the gradient number and this utilization
factor is dependent upon sharp edges and proximity. Optimum utilization
factors, based on the minimum voltage stress condition, are obtained
with a uniform voltage distribution across the insulating material.
Corona inception and associated problems can be avoided by the
- Specified geometries should be consistent with the voltages contained within the power supply. A high voltage "E" field gradient analysis should be performed and validated to ensure that the appropriate utilization factors were used with the specific geometries.
- Component case and conductor
shapes can create concentrated voltage fields and/or fracture planes. Component edges, comers and fasteners should have a radius or a fillet.
- Positioning of all components, connectors and cabling should be part of the design and verified throughout the assembly process.
- Conductors exiting from high voltage planes should not create concentrated voltage fields at the exit point.
- Where connections are made using soldering techniques, a minimum solder ball diameter should be
specified and controlled.
- Avoid the use of multiple insulating materials.
- Minimize interfaces.
- Establish and enforce cleanliness procedures to prevent contamination.
- Use vacuum impregnation and pressure curing techniques to minimize voids in encapsulation
- Prepare all surfaces for bonding using wet or dry plasma, or etching techniques and cleaning.
- Derate insulating materials based on maximum (not average) electrical stress.
- Pre-assign boundaries of high-field intensity and define voltage gradients.
- The voltage gradients within a resin system should be less than 50 VDC/mil. Across an
interface between insulating materials, the gradient should be less than 25 VDC/mil. It is recommended that AC voltage gradients should be less than one-half of the DC gradient,
o The use of laminated insulating barriers and printed wiring boards should be avoided.
- Where the use of printed wiring boards cannot be
avoided in high voltage fields, shielding or barriers should be added.
Corona testing is mandatory to demonstrate the amount of design
margin. The voltage gradient between conductors should be based on