Critical Disruptive Voltage & Visual Critical Voltage

Critical disruptive voltage is defined as the voltage at which complete disruption of the dielectric strength of air occurs. if the voltage increases above critical disruptive voltage corona occurs.

 𝛿: air density correction factor

 r: radius of conductor  d: distance of conductor separation   

 g₀: dielectric strength
= 30kv/cm

However, the existing temperature and pressure at the surface of the conductor are different from NTP. so at any other temperature and pressure critical disruptive voltage equation become

g’=dielectric strength at any temperature and pressure

g’=g₀𝛿
r: radius of the conductor
𝛿: air density factor

h: Atmospheric pressure in cm of Hg
t: temperature in C degree

If the surface of the conductor is irregular then the surface irregularity factor (m)will be introduced in the formula.

m= 1.0 for smooth conductor
m=0.93-0.98 for rough/stranded conductor
m=0.83-0.87 for seven stranded cable

Visual Critical Voltage

The visual critical voltage is defined as the minimum phase to the neutral voltage which glows and appears all along the line conductor.

It has been seen that in the case of parallel line conductors, the corona glow does not begin at the disruptive voltage but at a higher voltage called visual critical voltage.