Disruptive corona begins in smooth cylindrical conductors in air at NTP if the electric field intensity at the

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Disruptive corona begins in smooth cylindrical conductors in air at NTP if the electric field intensity at the conductor surface goes up to

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Option A: 21.1 kV_rms ∕ cm Explanation & Concepts: Disruptive corona in transmission lines occurs when the electrostatic stress on the conductor surface exceeds the dielectric strength of the surrounding air. At Normal Temperature and Pressure (NTP), the disruptive critical electric field strength for smooth cylindrical conductors is approximately 21.1 kVrms/cm. Working Principle: When the voltage of a conductor exceeds the disruptive critical voltage, the electric field intensity at the surface exceeds the breakdown strength of air (approx. 30 kVpeak/cm or 21.1 kVrms/cm). This leads to ionization of air molecules, turning the air surrounding the conductor into a conducting medium, resulting in the visible glow and audible noise known as corona discharge. Key Formulas: $E_{disruptive} \approx 21.1 ext{ kV}_{rms} ext{/cm}$ — Critical disruptive electric field strength; $V_c = m_0 g_0 \delta r \ln(\frac{d}{r})$ — Disruptive critical voltage formula Key Points: Corona onset depends on air density factor (\delta), surface roughness factor (m_0), and conductor radius (r).; The value 21.1 kVrms/cm is derived from the peak value of 30 kV/cm divided by \sqrt{2}.; Corona results in power loss, radio interference, and ozone production.; Higher altitude decreases air density, thereby reducing the corona onset voltage. Advantages: Corona acts as a natural discharge path for surges on lines.; Increases the effective diameter of the conductor, slightly reducing line reactance. Disadvantages: Causes significant power loss ($P \propto (V-V_c)^2$).; Causes radio and television interference due to high-frequency current pulses.; Ozone gas produced can lead to corrosion of nearby metal components. Applications: Used in high-voltage electrostatic precipitators.; Studied for design optimization of EHV and UHV transmission lines. Additional Info: NTP conditions are defined as 76 cm of Hg pressure and 25°C temperature.. Option B (21.1 kVpeak/cm) is incorrect as it confuses RMS with peak values; the peak dielectric strength is actually 30 kV/cm.. Option D is incorrect because the unit used is per meter instead of per centimeter. Answer: A is correct — Disruptive corona begins when the surface electric field intensity reaches the threshold value of 21.1 kVrms/cm at NTP. Exam Tip: Remember the relation $V_{peak} = \sqrt{2} imes V_{rms}$. Corona studies almost always use the RMS value 21.1 kV/cm for calculations, but the physical breakdown of air happens at a peak intensity of ~30 kV/cm.

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Option B: 21.1 kVpeak ∕ cm

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Option C: 21.1 kVaverage ∕ cm

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Option D: 21.1 kVrms ∕ m

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