Q3Power System - I
Question
Q.3. Define critical disruptive voltage and critical visual voltage.
Answer
Critical disruptive voltage is the minimum line voltage at which corona discharge begins on a conductor, while critical visual voltage is the minimum voltage at which the corona glow becomes visible to the naked eye, and is always slightly higher than the disruptive voltage.
Critical disruptive voltage (Vd) is the minimum phase-to-neutral voltage at which the electric field intensity at the conductor surface reaches the breakdown strength of air (about 30 kV/cm peak, or 21.1 kV/cm rms, at NTP), causing the air surrounding the conductor to ionize and corona to commence, given approximately by Vd = m0·g0·δ·r·ln(d/r), where g0 is breakdown strength of air, r is conductor radius, d is spacing, δ is air density factor and m0 is surface irregularity factor. Critical visual voltage (Vv) is the voltage at which corona becomes visible as a faint violet glow, and is somewhat higher than Vd (typically Vv = m_v·g0·δ·r·(1+0.3/√(δr))·ln(d/r)) because visible ionization/glow requires a slightly higher field intensity than the mere onset of ionization detectable electrically.