Question

Find the disruptive critical voltage for a transmission line having : conductor spacing = 1 m ; conductor (stranded) radius = 1 cm

barometric pressure = 76 cm of Hg ; temperature = 40ºC

Air break-down potential gradient (at 76 cm of Hg and at 25ºC) = 21.1 kV (r.m.s.)/cm.

Answer 1

V_C = 2.3 m₀ g₀ r log₁₀ D/r kV/phase

Here, g₀ = 21.1 kV (r.m.s.)/cm ; m₀ = 0.85 (assumed)

δ = 3.92 × 76/(273 + 40) = 0.952 ; log₁₀ D/r = log₁₀100/1 = 2 

∴  Vc = 2.3 × 0.85 × 21.1 × 0.952 × 1 × 2 = 78.54 kV (r.m.s.)/phase

Line value = 78.54 × √3 = 136 kV (r.m.s.)/phase