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Disruptive corona begins in┬а smooth cylindrical conductor in air at NTP if the electric field intensity at the conductor surface goes up to
21.1 kV_{rms}/cm
21.1 kV_{peak}/cm
21.1 kV_{average}/cm
21.1 kV_{rms}/m
21.1 kV_{rms}/cm
Disruptive corona is the phenomenon of electrical discharge that occurs when the potential gradient at a conductor surface exceeds the dielectric strength of the surrounding air. AtNormal Temperature and Pressure (NTP), this critical breakdown strength for smooth cylindrical conductors is 21.1┬аkVrmsтАЛ/cm.
Disruptive corona is the phenomenon of electrical discharge that occurs when the potential gradient at a conductor surface exceeds the dielectric strength of the surrounding air. AtNormal Temperature and Pressure (NTP), this critical breakdown strength for smooth cylindrical conductors is 21.1┬аkVrmsтАЛ/cm.
gcтАЛ=30m╬┤(1+╬┤rтАЛ0.301тАЛ) тАФ Peek's formula for critical disruptive gradient in kV/cm peak
EcriticalтАЛ=21.1┬аkVrmsтАЛ/cm тАФ Disruptive stress at NTP for air
When the voltage across a transmission line increases, the electric field intensity near the conductor surface rises. Once this intensity exceeds the breakdown strength of air (the dielectric strength), the air molecules become ionized. This ionization creates a conducting path, leading to a visible glow, hissing noise, and ozone production, known as the disruptive critical voltage condition.
Corona inception depends on air density, conductor radius, and surface condition.
The value 21.1 kV/cm refers to the root-mean-square (rms) value of the electric field intensity.
Roughness factor (m) reduces the disruptive voltage for stranded conductors.
Air density factor (╬┤)accounts for variations in temperature and pressure from NTP.
Acts as a surge discharge mechanism for transient overvoltages.
Helps in identifying faulty or damaged insulator strings through audible noise.
Causes power loss in transmission lines.
Produces ozone gas which is corrosive to nearby metallic components.
Generates radio and television interference.
High Voltage (HV) transmission line design.
Substation busbar spacing calculation.
NTP is defined as 25┬░C temperature and 76 cm of Hg pressure.
Option B (peak) is a common distractor because the breakdown of gas is a physical process related to peak voltage, but the standard engineering convention for this specific constant is given in rms values.
A is correct тАФ The disruptive corona for smooth cylindrical conductors occurs at a critical electric field intensity of 21.1 kVrms/cm at NTP.
Remember that while the breakdown of air is instantaneous and peak-dependent, power systems analysis standardizes to RMS values for convenience in calculation with line-to-neutral voltages.