When to use: Estimate the critical disruptive (corona onset) voltage of a transmission conductor using Peek's empirical formula, then compare it against the system's nominal phase voltage. Corona causes audible noise, radio/TV interference, and conductor surface power loss once the local electric field exceeds air's breakdown strength — this fair-weather estimate is a standard first check used in EHV/HV line conductor bundle selection.
Peek's formula gives the fair-weather disruptive critical voltage. Rain, fog, snow, and surface contamination lower the actual corona onset voltage substantially — utilities commonly apply an additional design margin for foul weather rather than relying on the fair-weather value alone. This tool does not model foul-weather conditions; treat E0 here as an upper-bound, best-case estimate.
This tool estimates the fair-weather critical disruptive (corona onset) voltage of an overhead transmission conductor using Peek's classic empirical formula, and compares it against the system's operating phase voltage. Corona discharge — a partial ionization of the air around a conductor once the local surface electric field exceeds air's breakdown strength — causes audible noise, radio/TV interference, ozone/NOx production, and real (though generally small) power loss, and is a key input to EHV/HV conductor bundle selection.
The critical disruptive voltage — the rms phase-to-neutral voltage at which corona begins under fair-weather conditions — is estimated as E0 = 21.2 × m × δ × r × ln(D/r) kV, where r is the conductor radius and D is the spacing (or geometric mean distance, GMD, for non-uniformly spaced phases) in the same units (cm), δ is the air density factor, and m is an empirical surface irregularity factor that accounts for stranded conductors and surface imperfections being less smooth than an idealized polished cylinder.
The air density factor δ = 3.92b/(273+t) corrects for barometric pressure b (cmHg) and ambient temperature t (°C) relative to standard atmospheric conditions (76 cmHg, 25°C, where δ = 1.0). Air breaks down at a lower field strength at higher altitude (lower b) and higher temperature, so δ falls below 1.0 and corona onset voltage drops in those conditions — an important reason EHV lines at high-altitude sites use larger conductor bundles than an equivalent sea-level line.
The irregularity factor m reflects how much rougher the real conductor surface is compared to an ideal smooth cylinder, since surface imperfections concentrate electric field locally and trigger corona at a lower average field than the smooth-cylinder theory predicts. Commonly cited typical ranges are approximately 1.0 for a smooth, polished single conductor; roughly 0.93–0.98 for a smooth stranded conductor in good condition; and roughly 0.80–0.90 for a stranded conductor that has weathered or has surface roughness/contamination. Bundle conductors (2, 3, or 4 sub-conductors per phase, standard on EHV lines above about 230 kV) raise the effective corona onset voltage compared to a single conductor of the same total cross-section, because the bundle spreads the surface charge over a larger effective radius — this tool models a single conductor, so treat bundle-conductor results as approximate unless you substitute an appropriate equivalent bundle radius.
Peek's formula and the related "visual corona" refinement describe fair, dry-weather conditions. In practice, rain, wet snow, fog, frost, and surface contamination (dust, salt, insects) lower the actual corona onset voltage — sometimes substantially — which is why utilities design with a margin below the fair-weather value rather than treating E0 as a hard operating limit. Final conductor bundle and spacing selection for a real EHV/HV line also weighs audible noise limits, radio-interference (RI) limits, and corona power loss under expected foul-weather frequency for the specific route, which this simplified educational calculator does not model.
Corona is a localized ionization of the air surrounding a conductor that occurs once the electric field at the conductor surface exceeds the breakdown strength of air. It appears as a faint bluish glow, audible hissing/crackling noise, and causes radio/TV interference and a small continuous power loss. It is most significant on EHV lines (230 kV and above) where surface gradients are highest.
Bundling 2-4 sub-conductors per phase spreads the charge over a larger effective surface, reducing the maximum surface electric field for a given phase voltage and total conductor cross-section. This raises the corona onset voltage and reduces corona loss, audible noise, and radio interference compared to a single conductor carrying the same current.
Air density factor δ = 3.92b/(273+t) falls as barometric pressure b drops with increasing altitude, and air breaks down at a lower electric field when it is less dense. Since corona onset voltage is directly proportional to δ in Peek's formula, a line at high altitude has a meaningfully lower corona onset voltage than the same line at sea level, all else equal — high-altitude EHV lines often need larger conductors or bundles to compensate.
No — treat it as a best-case upper bound. Rain, fog, snow, and surface contamination substantially lower the real corona onset voltage, so utilities apply a design margin below the fair-weather Peek value and separately check audible noise, radio interference, and corona-loss limits appropriate to the line's expected weather exposure. This calculator does not model foul-weather conditions.
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