What is Corona &
Why It Matters in
EHV Lines
There is a faint violet glow on every high-voltage transmission line in the world right now. Engineers have a name for it — corona — and a very long list of reasons to take it seriously.
Stand beneath a 132 kV or 220 kV transmission line on a damp night and you will hear it — a faint crackling, a low hiss. Look up in the dark and there may be a faint glow around the conductor. That is corona discharge. It is one of the most consequential electromagnetic phenomena in power transmission engineering, and yet outside specialist circles it is barely discussed. This guide explains what it is, why it happens, what it does to the transmission system, and what engineers do about it.
Photo: Unsplash — EHV transmission infrastructure
The word corona is Latin for crown. Astronomers use it for the glowing plasma envelope around the sun. Electrical engineers use it for something far smaller but equally interesting — the partial breakdown of air around a high-voltage conductor when the electric field intensity at the conductor surface exceeds the dielectric strength of the surrounding air. It is not a fault. It is not a flashover. It is a sustained, controllable, and somewhat inevitable consequence of trying to transmit large quantities of electrical power at very high voltages through open air. And in practical transmission engineering, it matters enormously.
Corona was first studied systematically in the early 20th century — Peek's Law, published in 1929, gave engineers the first quantitative tool for predicting when corona would occur on a conductor of a given diameter at a given voltage. Since then, corona has influenced the design of every high-voltage transmission line ever built: the diameter of the conductors, the spacing between phases, the use of bundled conductors (multiple sub-conductors per phase), the choice of corona rings on equipment terminals, and the acoustic and radio-interference clearances from residential areas. Understanding corona is foundational to understanding why EHV lines look the way they do.
// CORONA DISCHARGE — CONDUCTOR CROSS-SECTION (ILLUSTRATIVE)
Above: Cross-section of a single-phase EHV conductor showing the corona discharge zones. The electric field intensity is highest at the conductor surface and decreases radially. When surface field exceeds approximately 30 kV/cm (the dielectric strength of air at standard conditions), ionisation begins, creating the glowing partial-discharge zone visible as corona.
The Physics of Corona — Why Air Breaks Down
Dielectric strength, electric field geometry, and the ionisation cascade
Corona is a form of partial electrical discharge — the partial breakdown of the insulating medium (air) surrounding a conductor under high electric stress, without the complete path of ionised air that would constitute a full flashover or arc. To understand why it happens, you need to understand the relationship between conductor geometry and electric field intensity.
The electric field around a cylindrical conductor in free space is not uniform — it is highest at the conductor surface and decreases with distance. For a single conductor of radius r at voltage V above a distant ground plane or return conductor at separation D, the surface electric field is approximately: E = V / (r × ln(D/r)). This equation contains an important insight: for a fixed voltage and spacing, the surface field is inversely proportional to the conductor radius. A smaller conductor creates a higher surface field for the same voltage. This is why EHV conductors are made large — not primarily for current-carrying capacity, but to keep the surface field below the corona threshold.
Peek's critical electric field for air at standard conditions is approximately 30 kV/cm (peak). When Emax at the conductor surface exceeds this value, corona begins. At 220 kV, a conductor of 15mm radius in a typical three-phase configuration reaches this threshold — explaining why single conductors at EHV voltages cannot avoid corona, and why bundled conductors (increasing effective radius) are universally used above 220 kV.
Once the electric field exceeds the dielectric strength of air at the conductor surface, the air molecules in that zone begin to ionise. Free electrons — produced by cosmic ray ionisation of air, photoionisation, or field emission — are accelerated by the intense electric field and collide with neutral air molecules, knocking off additional electrons in a cascade process called an electron avalanche. The result is a localised region of partially ionised air around the conductor that forms the visible corona glow, emits photons (the bluish-violet light characteristic of ionised nitrogen), generates heat, produces acoustic noise, and radiates electromagnetic energy across a wide spectrum.
Critically, corona is a partial breakdown. The ionisation zone remains confined to a region close to the conductor where the field is high enough to sustain it. Beyond that zone, the field drops below the critical threshold and the air is non-conducting. The conductor is not shorted to ground — the corona zone is not a complete conducting path. But it is not harmless either: the ionisation process continuously draws energy from the transmission system, dissipating it as heat, light, and electromagnetic radiation.
Factors That Make Corona Worse
Humidity, altitude, surface condition, and voltage level
Corona onset and severity are not fixed quantities for a given transmission line — they vary significantly with atmospheric and surface conditions. This variability is important for maintenance engineers because it means that a line that operates without significant corona in dry fair weather may exhibit substantial corona under adverse conditions, with all the associated losses and interference effects.
| Factor | Effect on Corona | Severity Increase | Physical Reason |
|---|---|---|---|
| High humidity / rain | Significantly worsens corona | Water droplets on conductor surface increase local field concentration, reducing effective onset voltage | |
| Conductor surface roughness | Major factor — worsens corona | Surface protrusions create local field intensification (E ∝ 1/r). Nicks, scratches, and surface oxidation all lower critical gradient | |
| High altitude | Worsens corona — lower onset voltage | Air density decreases with altitude — fewer air molecules per unit volume means lower dielectric strength. Critical gradient ∝ air density | |
| Fog, snow, ice | Severe corona intensification | Ice crystals and supercooled water droplets on conductor surface dramatically reduce effective onset gradient | |
| Dust / pollution deposits | Moderate worsening | Conductive deposits lower surface resistivity; insulating deposits cause electric stress concentration at their edges | |
| Higher operating voltage | Proportional increase in surface field | Emax increases directly with voltage for fixed conductor geometry — unavoidable physics of EHV transmission | |
| Wind | Slightly reduces corona | Wind disperses ionised air, slightly improving the local ionisation conditions. Effect is secondary compared to humidity factors | |
| Smaller conductor radius | Higher surface field — worse corona | Emax ∝ 1/r. The key design parameter — conductor diameter is selected partly to keep surface gradient below critical value |
Consequences of Corona — What It Actually Does
Power loss, radio interference, acoustic noise, and chemical effects
Corona is not a single problem — it is a cluster of related problems that occur simultaneously whenever the discharge is active. Each consequence has a different severity, a different measurement method, and a different regulatory or engineering threshold that defines acceptability. Understanding all four main consequences is important for anyone involved in EHV line design, operation, or maintenance.
Corona Power Loss
What it is: Energy continuously dissipated as heat, light, and electromagnetic radiation by the ionisation process. For a transmission line in fair weather, corona losses may be modest. Under wet conditions — rain, fog, heavy dew — losses increase significantly, sometimes by several times the fair-weather value.
Why it matters: Power lost to corona never reaches the load. For long EHV transmission lines (hundreds of kilometres), even modest corona loss rates translate to substantial total energy loss. In wet weather, corona losses on a long 400 kV line can be significant enough to require operational attention.
Formula: Peek's formula (empirical): Pcorona ∝ f × (V − Vcritical)² / (log D/r)² · per phase per km
Radio Interference (RI)
What it is: Corona discharges generate broadband electromagnetic noise across a wide frequency range — principally affecting the AM radio band (0.5–1.6 MHz) and extending into VHF. This noise is radiated from the conductor and propagates for several hundred metres from the line.
Why it matters: Regulatory limits for radio interference from transmission lines are defined by national standards (IS 6267 in India, CISPR 18-2 internationally). Lines that exceed these limits affect AM radio reception in surrounding communities and violate telecommunications regulations. RI is a design constraint that influences conductor selection and minimum conductor diameter.
Measurement: Expressed as radio interference voltage (RIV) in dB above 1 µV at 1 MHz. Lateral profile measured at defined distances from line centre.
Acoustic Noise (AN)
What it is: The crackling, hissing, or buzzing sound audible beneath EHV lines — particularly in wet weather — is produced by the mechanical impulses of corona discharges acting on the surrounding air. Each discharge creates a small pressure wave; the combined effect of thousands of discharges per second produces the characteristic corona acoustic noise.
Why it matters: Acoustic noise from EHV lines is subject to environmental and planning regulations. Residential areas near transmission line routes must receive acceptable noise levels. CBIP Technical Report 20 and IEC standards define acceptable acoustic noise limits — typically 40–50 dB(A) at the nearest sensitive receptor. Acoustic noise is primarily a wet-weather phenomenon, since fair-weather corona on modern bundled conductors is generally much quieter.
Chemical Effects — Ozone & NOₓ
What it is: The ionisation of air in the corona zone generates ozone (O₃) and oxides of nitrogen (NO, NO₂ — collectively NOₓ). These chemical products are reactive — ozone is a strong oxidising agent, and NOₓ dissolved in water forms nitric acid.
Why it matters: Sustained ozone and NOₓ generation in the vicinity of EHV conductors and substation equipment causes accelerated degradation of rubber and polymer insulation, accelerated corrosion of metallic surfaces, and deterioration of conductor surface coatings. In enclosed substation environments (gas-insulated switchgear rooms, cable tunnels near EHV equipment), ozone accumulation can reach concentrations requiring ventilation management for occupational health reasons.
Illustrative Corona Loss — Relative Severity by Condition (Arbitrary Units, Fair-Weather Baseline = 1)
400 kV bundled
400 kV bundled
400 kV bundled
220 kV (single sub.)
Illustrative relative comparison only. Actual corona loss depends on conductor diameter, bundle spacing, phase spacing, atmospheric conditions, altitude, and line voltage. Figures based on qualitative trends described in CBIP Technical Report 20 and standard transmission line design literature.
How EHV Lines Are Designed to Limit Corona
Bundled conductors, conductor diameter, corona rings, and surface quality
The engineering response to corona is not to eliminate it — that is not generally possible at EHV voltage levels without impractically large conductors — but to manage it within acceptable limits. Several design measures are used, each targeting the electric field at the conductor surface or at equipment terminals where field concentration would otherwise cause severe corona.
// BUNDLED CONDUCTOR CONFIGURATIONS — VOLTAGE APPLICABILITY
Single
Up to ~132 kV
Corona-prone ≥110 kVTwin Bundle
220 kV
Acceptable 220 kVTriple Bundle
400 kV
Standard 400 kVQuad Bundle
765 kV / 800 kV
Required ≥765 kVBundle configurations used by Power Grid Corporation of India (PGCIL) for national transmission system. Sub-conductor spacing typically 300–450 mm. Effective bundle radius is much larger than individual sub-conductor radius, reducing surface field gradient significantly.
Bundled conductors are the primary design measure for corona control at 220 kV and above. Instead of a single large conductor per phase, two, three, or four sub-conductors are arranged in a geometric pattern held by spacer clamps. The effective radius of the bundle for the purpose of the field equation is much larger than the radius of any individual sub-conductor — which reduces the peak surface field proportionally. A twin bundle of two 300 mm² ACSR conductors spaced 400 mm apart has an effective bundle radius roughly 20 times the sub-conductor radius, reducing the surface gradient dramatically compared to a single conductor carrying the same current.
Conductor surface quality is critically important. Surface defects — scratches, nicks from installation damage, surface oxidation, embedded metallic particles — all create local field enhancement that initiates corona at lower voltages than the design predicts. Smooth conductors with minimal surface irregularities are essential, and installation procedures must specify that conductors are not dragged along the ground or pulled around tight bends that cause surface damage. Conductor joints and repair sleeves must be smooth and correctly installed — a poorly made conductor joint is a corona source that also represents a thermal weakness.
Corona rings — large toroidal metal rings fitted to transformer and reactor terminal bushings, insulator string hardware, and cable sealing ends at EHV substations — address the problem of field enhancement at physical features (conductor ends, hardware corners) where geometric concentration would otherwise cause severe corona localised to specific points rather than distributed along the conductor span. The large-radius torus of the corona ring distributes the electric field over a larger surface area, keeping the local field below the critical gradient despite the proximity of the EHV conductor.
Peek's Law and the Critical Disruptive Voltage
The quantitative foundation of corona design engineering
F.W. Peek published his empirical formula for the critical disruptive voltage of overhead line conductors in 1929, and despite nearly a century of subsequent research, Peek's Law remains the standard starting point for corona performance calculations in EHV line design. The formula gives the voltage at which corona begins (the critical disruptive voltage, Vd) and the voltage at which visual corona becomes established (the visual critical voltage, Vv).
Peek's critical disruptive voltage for a round conductor: Vd = m0 × g0 × δ × r × ln(D/r) — where m₀ is the surface irregularity factor (1.0 for smooth polished conductor, 0.93–0.98 for practical stranded conductors), g₀ is the critical field gradient of air (approximately 21.2 kV/cm RMS at sea level), δ is the relative air density (equals 1 at standard sea-level conditions, less at altitude), r is the conductor radius, and D is the conductor spacing. The formula gives the line-to-neutral RMS voltage at which corona onset occurs.
Peek also gave the power loss formula: P = (241 / δ) × (f + 25) × √(r/D) × (V − Vd)² × 10⁻⁵ kW per km per conductor — where f is the supply frequency and V > Vd. This formula captures the key characteristic of corona power loss: it is proportional to the square of the excess voltage above critical. A small increase in operating voltage above Vd produces a disproportionately large increase in corona loss — which explains why the design target is to keep operating voltage below Vd in fair weather for as much of the operating cycle as possible.
The surface irregularity factor m₀ in Peek's formula is the empirical acknowledgement that real conductors are not geometrically perfect. A smooth, polished round conductor has m₀ = 1.0 — corona onset at the theoretical value. A practical stranded ACSR conductor has m₀ in the range 0.83–0.87 (dry) to 0.72–0.82 (wet). This means a real stranded conductor in wet conditions begins to exhibit corona at a voltage roughly 20–28% lower than the theoretical critical disruptive voltage for its radius — a significant reduction that must be accounted for in line design. This is why the design criterion is not "operating voltage below theoretical Vd" but rather "operating voltage below m₀ × theoretical Vd under worst-case conditions."
Corona at the Steel Plant — Why It Matters at the Fence
132 kV receiving substations, equipment terminals, and HV maintenance
For the electrical engineer in a steel plant, corona is not an abstract grid-level concern — it appears at the boundaries of the facility where the EHV supply enters. The 132 kV or 220 kV incoming lines, the main receiving transformer bushings, the gantry structure insulators, and the outdoor switchgear of the main receiving substation are all subject to corona phenomena at the voltages at which large steel plants operate their supply connections.
The transformer bushing terminals — where the 132 kV conductors enter the main receiving transformer — are fitted with corona rings specifically because the junction between conductor, fitting, and insulator creates a geometric field concentration. Any maintenance activity that disturbs these corona rings (incorrect re-installation after bushing maintenance, missing corona ring, damaged ring surface) creates a localised corona source that accelerates insulator degradation, generates acoustic and RF noise, and indicates a potential reliability problem at a critical equipment point.
Outdoor 132 kV switchgear in the receiving substation — disconnectors, current transformers, potential transformers, and circuit breaker terminals — are all designed with corona geometry in mind. Terminal clamp surfaces must be smooth and correctly torqued; any loose connection or surface damage that creates a protrusion or sharp edge at 132 kV potential will exhibit intense local corona. In a steel plant substation, routine maintenance inspections should specifically include visual inspection of 132 kV terminal hardware surfaces, corona ring condition, and insulator contamination — particularly after adverse weather periods or nearby industrial activity that deposits conductive dust on insulator surfaces.
One of the simplest corona diagnostic methods is also the oldest: visual inspection of the 132 kV gantry after dark on a clear night. A healthy installation with clean insulators and correctly installed hardware shows no visible glow. Localised violet or bluish glow at specific points — hardware fittings, insulator caps, conductor clamps — indicates corona concentration at that location and warrants daylight inspection of the specific component. Widespread diffuse glow is more likely a wet-weather condition effect; localised bright glow suggests a hardware or installation defect. This inspection technique costs nothing and can reveal developing problems months before they manifest as failures or protection operations.
Steel plants located near residential areas should be aware that 132 kV incoming line corona is a regulated source of radio interference. IS 6267 and CISPR 18-2 define maximum permissible radio interference voltage from transmission lines. A new or modified 132 kV connection that causes AM radio interference in nearby residences is a regulatory issue, not just a technical nuisance. If corona-related RI complaints are received, the investigation should cover both the incoming line conductor condition (surface defects, contamination) and the substation terminal equipment (corona ring condition, hardware surface quality).
Living with Corona — Design, Operation, and Maintenance
The engineer's practical relationship with an unavoidable phenomenon
Corona is fundamentally unavoidable at the voltage levels required for efficient long-distance power transmission. The physics dictates that if you want to transmit 1,000 MW at 400 kV over several hundred kilometres, the electric field at the conductor surface will be above the fair-weather corona onset threshold during significant parts of the year when atmospheric conditions are adverse. The engineering task is not elimination but management — keeping corona within limits that are technically acceptable (losses, RI, acoustic) and not causing accelerated degradation of equipment.
The design measures — bundled conductors, large-radius hardware, smooth conductor surfaces, corona rings — address the field geometry. The operational measures — conductor cleanliness, hardware inspection and torque checking, insulator contamination monitoring, corona ring condition verification — address the surface and environmental factors that modulate corona severity for a given electrical configuration. Both are necessary; neither alone is sufficient.
From the perspective of an electrical maintenance engineer at a steel plant, the key practical points are three. First: the 132 kV supply connection represents EHV equipment where corona is a real operating condition, and the design features that manage it (corona rings, smooth hardware, clean insulators) must be maintained in their designed condition. Second: visual corona signs — glow, acoustic noise, ozone smell — are diagnostic indicators that should trigger investigation, not dismissal. Third: any work on EHV terminal equipment (bushing maintenance, hardware replacement, insulator cleaning) must restore the original corona-managed geometry of the terminal, not depart from it through incorrect re-installation.
Corona is one of those phenomena in electrical engineering that rewarded the engineers who understood it with better designs and avoided a generation of failures — and continues to reward the maintenance engineers who recognise its signs and take them seriously. The faint violet glow on the wire is not decoration. It is the system telling you something.
Sources & References
- Peek, F.W. (1929). Dielectric Phenomena in High Voltage Engineering. 3rd ed. McGraw-Hill. [Original formulation of Peek's Law — critical disruptive voltage, visual critical voltage, corona loss formula]
- https://industrialiq.blogspot.com/2026/03/the-3-am-breakdown-call-every.html
- Wadhwa, C.L. (2011). High Voltage Engineering. 3rd ed. New Age International. [Corona phenomenon, Peek's formula derivation, Indian EHV line practice, radio interference]
- Naidu, M.S. & Kamaraju, V. (2013). High Voltage Engineering. 5th ed. Tata McGraw-Hill. [Corona discharge physics, ionisation mechanism, bundled conductors, corona rings]
- Central Board of Irrigation and Power (CBIP). Technical Report No. 20 — Design of Overhead Transmission Lines. Government of India. [Indian EHV line design practice — corona, conductor selection, bundled configurations]
- Bureau of Indian Standards. IS 6267:1991 — High Voltage Alternating Current Circuit Breakers — Radio Interference Tests. BIS. [Radio interference from EHV equipment — measurement and limits]
- CISPR 18-2:2017. Radio Interference Characteristics of Overhead Power Lines and High-Voltage Equipment — Part 2: Methods of Measurement. IEC. [International standard for RI measurement from transmission lines]
- IEC 61284:1997. Overhead Lines — Requirements and Tests for Fittings. IEC. [Corona and RIV performance requirements for EHV line hardware and fittings]
- Power Grid Corporation of India Ltd (PGCIL). Technical Standards for Construction of Electrical Plants and Electric Lines. [India's national transmission utility — EHV line construction standards]
- Central Electricity Authority. Manual on Transmission Planning Criteria. Ministry of Power, India. [CEA standards for EHV transmission design including corona management]
- Glover, J.D., Sarma, M.S. & Overbye, T. (2011). Power Systems Analysis and Design. 5th ed. Cengage. [Transmission line corona — analysis, design criteria, loss calculation]
- Grigsby, L.L. (ed.) (2012). Electric Power Generation, Transmission and Distribution. 3rd ed. CRC Press. [EHV transmission engineering — corona, electromagnetic environment, conductor design]
- IEEE Standard 539-2022. IEEE Standard Definitions of Terms Relating to Corona and Field Effects of Overhead Power Lines. IEEE. [Standard corona terminology and definitions]
