Visual critical voltage is the minimum voltage at which a faint violet glow becomes visible along transmission line conductors due to corona. Factors like conductor size, spacing, and surface condition affect both the visual critical voltage and critical disruptive voltage. While corona causes power loss and can degrade conductors through ozone production, it also increases the effective diameter of conductors and reduces electrostatic stress and the effect of transients by partially dissipating surge energy. However, the non-sinusoidal current from corona can interfere with neighboring communication lines, and the noise generated can interfere with TV and radio signals.

14. • Visual Critical Voltage :
• In the case of transmission lines with parallel
conductors, the corona glow does not being at
the critical disruptive voltage Vc, but it begins at a
higher voltage called the Visual Critical Voltage. It
is denoted by Vv. Thus visual critical voltage is the
minimum phase-neutral voltage at which the
visual corona starts or appearance of the faint
luminous glow of violet color all along the
transmission line conductors.

16. Factors Affecting Critical Disruptive
Voltage and Visual Critical Voltage :
• Size of Conductors
• Spacing Between the Conductors
• Surface Condition of Conductors

17. Advantage of Corona
Following are some of the advantage of corona:
• The effective diameter of power conductor increases
due to Corona. How? Actually, due to corona, the air
surrounding the conductor becomes conductive and
hence effective diameter increases. Due to this
increase in effective diameter, the shunt capacitance of
line increases. Due to increased effective diameter,
electrostatic stress between the power conductors
reduces.
• Corona helps to reduce the effect of transient
produced by surge by dissipating the surge energy
partially. Due to corona, the steepness of surge is
reduced.

18. Disadvantage of Corona
• We know that, corona causes power loss. Due to this power loss,
the transmission efficiency reduces.
• Ozone is produced due to Corona. This Ozone may cause
degradation of conductor due to chemical action.
• Corona is basically discharge of current in air. This current is not
sinusoidal rather it is non-sinusoidal. Because of this no-sinusoidal
current drawn due to Corona, non-sinusoidal voltage drop occurs in
the transmission line. This non-sinusoidal voltage drop may cause
inductive interference with the neighboring communication line.
• Overhead transmission line do not interfere with TV or Radio
Signals. But the two major source for causing interference are
Corona and Gap Discharge. Due to corona, unwanted signals are
generated. The strength of such noise decrease with increase in
distance from the line and frequency. These noise may interfere
with the TV or Radio Signal if their quality is poor. Gap discharge is
entirely different from the Corona. Gap Discharge can take place at
any voltage. Basically it is current discharge in a gap in between the
two mechanical connected metallic parts. Such a discharge again
results in generation of unwanted signal or noise. These noise may
interfere with the Radio or TV signals.