What Is the Corona Effect in Transmission Lines?
The ionisation of air around high-voltage conductors — causes, consequences, Peek's Law, and why managing corona is central to every EHV line design decision.
There is a phenomenon that happens silently on every major high-voltage transmission line — invisible in daylight, visible as a faint violet glow at night, and audible near the towers as a low hiss or crackle during wet weather. It wastes energy continuously, degrades nearby electronics, produces ozone above the conductor, and slowly erodes the conductor surface itself. It is called the corona effect, and understanding it completely changes how you think about why EHV lines are designed the way they are.
The corona effect is not a failure or a defect. It is a physical phenomenon that occurs on any conductor operating above a certain voltage threshold — it simply cannot be avoided entirely on lines operating at 132 kV and above. What can be controlled is its severity, its power loss, and its interference effects. That control is precisely what drives decisions about conductor size, bundling configuration, stranding quality, tower height, and phase spacing on every high-voltage line ever designed.
This article works through the subject the way it deserves — not as a textbook bullet list, but as a connected narrative of physics, engineering consequences, and practical decisions. By the end, you should be able to look at an EHV transmission line and understand, from first principles, why every design element looks the way it does.
Defining Corona — What Is Actually Happening at the Conductor Surface
The corona effect is the localised ionisation of air in the immediate vicinity of a high-voltage conductor. To understand it precisely, start with the air itself. Air is normally an excellent insulator — at standard atmospheric conditions, it can withstand an electric field of approximately 30 kV per centimetre before it begins to break down. This figure is called the dielectric strength of air, and it is the boundary between insulation and conduction.
When a high-voltage conductor is energised, it creates a radially symmetric electric field around it. The strength of this field is highest at the conductor's surface and diminishes with distance according to the inverse of the radial distance from the conductor's centre. For a conductor of small radius, the surface electric field is concentrated and intense. For a large-radius conductor, the same voltage produces a lower surface field because the charge is distributed over a larger area.
When the surface electric field exceeds the ionisation threshold of air — typically around 21 kV/cm (peak) for a smooth conductor at sea level — the air molecules immediately surrounding the conductor begin to ionise. Free electrons are stripped from nitrogen and oxygen molecules, creating electron-ion pairs. These charged particles accelerate under the electric field, collide with other neutral molecules, and produce an avalanche of ionisation. The result is a thin layer of partially conducting plasma around the conductor — the corona envelope.
Beyond the corona envelope, the field drops below the ionisation threshold and the air remains insulating. The discharge is therefore localised and self-limiting — it does not propagate across the full insulation gap to the nearest earthed structure. If it did propagate completely, it would become a full flashover — a fundamentally different and far more serious event. Corona is a partial discharge, not a complete breakdown.
Why it is visible and audible
The ionised plasma in the corona envelope emits light as excited electrons return to lower energy states — predominantly in the blue-violet range of the visible spectrum, which is why corona glows purple-blue in darkness. The rapid oscillation of the ionisation process at power frequency (50 Hz) produces acoustic pressure waves — the familiar hissing and crackling noise near high-voltage lines. In dry weather, these sounds are subdued. In wet weather — when water droplets on the conductor surface locally intensify the electric field at their tips — corona activity intensifies significantly and the noise becomes more pronounced.
Peek's Law and the Critical Disruptive Voltage — Quantifying When Corona Begins
The first engineer to quantify corona onset systematically was F.W. Peek of General Electric, who developed his empirical formulas in the early 1910s based on extensive experimental work. Peek's contributions — now known as Peek's Law — gave transmission engineers a practical tool for predicting whether a given conductor at a given voltage would or would not exhibit corona under specified atmospheric conditions.
Two voltage thresholds from Peek's work are worth understanding clearly. The Critical Disruptive Voltage (Vc) is the voltage at which the electric field at the conductor surface just reaches the ionisation threshold of air — at this point, corona begins during the voltage peaks of each AC cycle. The Visual Critical Voltage (Vv) is the somewhat higher voltage at which the discharge becomes visually continuous and the glow is easily observed — this is typically about 10–15% above Vc.
For a transmission line to be corona-free in normal operation, the line voltage must be kept below the critical disruptive voltage. The key variable the designer can control is the conductor radius — larger radius means lower surface field at the same voltage, which means higher Vc. This is the direct physical reason why high-voltage lines use large conductors, and why above approximately 200 kV, single conductors of practical size can no longer meet the corona criterion, necessitating bundled conductors.
The factor δ (delta) in Peek's formula is the air density correction relative to standard conditions (25°C, 76 cm Hg). At higher altitudes, air is less dense, its dielectric strength is lower, and corona onset occurs at lower voltages. Lines crossing mountain terrain — including several Indian EHV corridors at altitudes above 1,500 m — are designed with the reduced air density factored into their conductor selection and spacing calculations.
Bundled conductors on a high-voltage tower. The bundle arrangement increases the effective conductor radius seen by the electric field — the single most effective design measure for suppressing corona on lines above 220 kV.
Factors That Control Corona Severity — What Makes It Worse or Better
Corona activity on a real transmission line is not constant. It varies with weather, with loading, with conductor age and condition, and with atmospheric chemistry. Understanding these factors helps make sense of both the design decisions embedded in EHV lines and the operational patterns of corona noise and loss that line managers observe.
The most fundamental design parameter. Larger radius means lower surface field, higher critical disruptive voltage, and less corona. Surface roughness and irregularities — nicks, scratches, bird deposits, contamination — create local field enhancements that can initiate corona at voltages well below the smooth-conductor Vc. The factor m₀ in Peek's formula accounts for this: stranded conductors have lower m₀ than hypothetical smooth conductors.
Corona loss from Peek's formula rises as the square of the excess voltage above critical (V − Vc)². Small increases in operating voltage above the critical threshold produce disproportionately large increases in corona loss. This is why voltage regulation — keeping system voltage close to nominal — has a direct effect on corona-related losses across the network.
Lower atmospheric pressure (higher altitude or low pressure weather systems) reduces air density, reducing its dielectric strength. The critical disruptive voltage decreases approximately in proportion to the air density factor δ. Lines designed for sea-level conditions will exhibit more corona activity at altitude, and new lines in high-altitude terrain must be designed to the local air density, not sea-level values.
Water on the conductor surface creates severe local field enhancement. A water droplet sitting on a conductor presents a sharp liquid tip that locally amplifies the electric field to several times the smooth-surface value. This is why corona loss and audible noise are substantially higher during rain, fog, and dew conditions than in dry weather. Falling rain also increases air ionisation, further lowering the breakdown threshold locally.
The distance between phase conductors affects the electric field distribution. Closer phase spacing intensifies the inter-phase field and can reduce the effective critical voltage. EHV line tower geometry is partly determined by the need to maintain adequate phase-to-phase clearance both for corona performance and for switching surge insulation requirements.
Conductor suspension clamps, spacer-dampers, vibration dampers, and tension hardware all create field enhancement points if their geometry is not carefully designed. High-voltage hardware is deliberately manufactured with smooth, rounded surfaces and large effective radii to avoid creating corona initiation points. A poorly designed clamp can cause localised corona at voltages well below what the conductor itself would produce.
The Five Consequences of Corona — What It Actually Costs
Corona is not just a curiosity — it carries real and quantifiable costs that affect grid economics, power quality, and infrastructure longevity. Each consequence has its own engineering literature, measurement methodology, and mitigation strategy.
Energy is continuously consumed in ionising and recombining air molecules. On long EHV lines without adequate corona management, this loss can be significant — particularly during adverse weather. Peek's formula shows that corona loss on a wet line can be 10 to 15 times higher than on the same line under dry conditions at the same voltage. Over the life of a 30–40 year transmission line, accumulated corona losses represent a substantial economic cost.
The ionisation process produces random electromagnetic pulses across a wide frequency spectrum — from AM broadcast frequencies (535–1605 kHz) up to several hundred MHz. These emissions can interfere with AM radio reception, communications systems, and sensitive electronic measurement equipment near transmission lines. Regulatory limits exist in most countries for Radio Interference Voltage (RIV) from EHV lines, and line designs must demonstrate compliance through analysis and testing.
The rapid pressure fluctuations from corona generate audible noise at 100 Hz and its harmonics, plus a broadband hissing component. Near heavily loaded EHV lines in wet weather, audible noise levels at the edge of the right-of-way can exceed 50–60 dB(A) — comparable to a busy office environment. Modern environmental regulations limit audible noise from transmission lines at specified receptor distances, and this limit influences conductor selection and bundling decisions.
The ionisation process produces ozone (O₃) from atmospheric oxygen and nitrogen oxides (NO and NO₂) from atmospheric nitrogen. While the concentrations generated by a typical EHV line are generally well below health concern levels at ground level, ozone is a strong oxidising agent. In concentrated form near the conductor surface, it accelerates the degradation of organic materials — insulator shed compounds, conductor greases, and cable insulation sheath materials — over the line's operating life.
Ozone and nitric acid formed from NOx react with moisture to attack the surface chemistry of glass and porcelain insulators. This creates micro-etching of the insulator surface, which increases its wettability and ultimately its susceptibility to flashover under pollution and wet conditions. The mechanism is slow but cumulative — insulators on heavily corona-affected lines show accelerated surface deterioration compared to those in cleaner environments.
The corona discharge and the ozone it produces cause slow oxidative erosion of the aluminium conductor surface at corona-active points. Over years of operation, this can create surface pitting and roughening that worsens the corona condition — a self-reinforcing degradation mechanism. At fittings and clamps where corona is most active, this can lead to accelerated mechanical fatigue at the contact points between the clamp and conductor.
Weather's Role — Why Corona Is a Seasonal and Daily Variable
Of all the factors that influence corona severity, weather is the most dynamic and operationally significant. The same transmission line can have dramatically different corona characteristics from one hour to the next depending on atmospheric conditions. Grid operators and line designers both need to understand this variability.
Water droplets on the conductor surface create intense local field enhancement at their tips. Corona onset effectively drops to significantly below the design Vc. Line noise and corona loss increase substantially. Measurements of corona loss under rainfall can be 10 to 15 times the fair-weather value.
A thin film of moisture on the conductor surface changes the surface condition from rough-dry to effectively smoother-but-wet. The transition from dry to dew-covered condition is often when the most intense corona bursts are observed — before moisture accumulation reaches the steady-rain state.
Snow accumulating on the conductor can mask the metallic surface and change its effective radius. Partial snow deposits that leave some conductor exposed create sharp boundaries where field enhancement occurs. Icing can be followed by dramatic corona increase as ice surface melts into water drops.
The best corona condition. Surface field enhancement from contamination is lower, air dielectric strength is at its standard value, and corona loss on well-designed lines may be negligible. This is the "fair-weather" condition used as the baseline in corona loss comparisons.
Lower atmospheric pressure reduces air density and therefore its dielectric strength. A line segment crossing a mountain pass at 2,000 m altitude experiences a corona environment equivalent to a line operating at higher voltage at sea level. Altitude correction is mandatory in Indian EHV line design for routes in Himalayan foothills and western mountain ranges.
Near steel plants, cement works, and chemical facilities, airborne particles contaminate conductor surfaces and create multiple field-enhancement points. Saline contamination (coastal areas) is particularly aggressive. Contaminated conductors can exhibit significant corona activity at voltages well below the clean-conductor critical disruptive voltage.
A transmission engineer once told me: design your line for dry weather first, then check it for rain. If it fails the rain check, no amount of operational management will compensate — you need to go back to the conductor selection.
Field Engineering Perspective — EHV Line Design
Wet weather conditions dramatically amplify corona activity on EHV lines. Water droplets on conductor surfaces create intense local electric field enhancement, raising corona loss by an order of magnitude compared to dry-weather operation.
Positive and Negative Corona — The Asymmetry of AC Discharge
In an AC system, the conductor alternates between positive and negative polarity during each cycle. The physics of corona discharge differs meaningfully between the positive half-cycle and the negative half-cycle — a distinction that matters for both loss calculation and interference characterisation.
Negative corona (negative half-cycle)
When the conductor is at negative polarity relative to earth, free electrons at the conductor surface accelerate outward, ionising air molecules they encounter and creating a relatively uniform, diffuse glow around the conductor — called a Trichel pulse discharge. The ionisation zone is more symmetric and the light emission more uniform. Negative corona produces more regular, lower-amplitude electromagnetic pulses.
Positive corona (positive half-cycle)
During the positive half-cycle, the conductor attracts electrons from the surrounding air. The mechanism of ionisation is different — positive ion streamers form and propagate outward from the conductor surface in a less uniform pattern. These "streamer" discharges are more intense, more variable, and produce larger amplitude electromagnetic pulses. Positive corona is the primary contributor to Radio Interference — the asymmetric, burst-like nature of streamer discharge generates stronger high-frequency emissions than the more regular negative corona pulses.
For HVDC transmission lines, where the polarity is fixed, this distinction becomes particularly significant. The positive polarity conductor on an HVDC line generates significantly more radio interference than the negative polarity conductor, and this asymmetry must be accounted for in the RFI assessment of HVDC line designs.
Voltage Level Comparison — How Corona Performance Changes With EHV Rating
The relationship between voltage level and corona severity explains the progression of transmission conductor design from single conductors at 132 kV through to quad and six-bundle arrangements at 765 kV. The following table illustrates the critical disruptive voltage margin for representative conductor configurations at each major voltage level.
| Voltage (kV) | Config. | Typical Conductor | Phase-to-Neutral V (kV) | Approx. Vc (kV) | Margin V/Vc | Corona Performance |
|---|---|---|---|---|---|---|
| 66 kV | Single | ACSR Panther | 38 kV | ~65 kV | ~0.58 | Well below onset — corona-free |
| 132 kV | Single | ACSR Wolf/Moose | 76 kV | ~105 kV | ~0.72 | Acceptable — minor wet-weather corona |
| 220 kV | Single | ACSR Moose (single) | 127 kV | ~120 kV | ~1.06 | Marginal — corona in all weather |
| 220 kV | Twin bundle | 2×ACSR Moose | 127 kV | ~175 kV | ~0.73 | Good — corona controlled |
| 400 kV | Single | Any practical ACSR | 231 kV | <180 kV | >1.28 | Unacceptable — severe corona |
| 400 kV | Triple bundle | 3×ACSR Moose/Bersfort | 231 kV | ~310 kV | ~0.74 | Good — standard Indian EHV design |
| 765 kV | Twin/Triple | Any 2–3 bundle | 441 kV | <380 kV | >1.16 | Unacceptable — quad bundle minimum |
| 765 kV | Quad bundle | 4×ACSR Bersfort | 441 kV | ~580 kV | ~0.76 | Acceptable — PGCIL standard design |
The table makes the pattern unambiguous. As voltage level rises, single conductors cross the corona threshold at progressively lower voltage-to-Vc ratios. At 220 kV, a single ACSR Moose already operates above its critical disruptive voltage. At 400 kV, no practical single overhead conductor can be sized to remain below critical disruptive voltage — bundling is physically mandatory, not merely desirable. At 765 kV, quad bundles are the minimum configuration that achieves acceptable corona performance.
The conductor itself is not the only source of corona on a high-voltage line. All hardware connected to the energised conductor — suspension clamps, spacer-dampers, vibration dampers, jumper connectors at towers, and strain clamp hardware — operates at or near line voltage. If any of these items has sharp edges, poor surface finish, or inadequate effective radius, it will generate corona at voltages where the conductor itself is performing acceptably. This is why high-voltage hardware is designed to specific corona-free certification standards (IEEE Std 1829 for test methods) and why quality control in hardware manufacturing is so critical.
High-voltage hardware on an EHV tower — suspension clamps, spacer-dampers, and conductor fittings are all manufactured with smooth, rounded profiles and large effective radii to avoid creating corona initiation points that would otherwise negate the conductor's own corona management.
Corona Rings and Grading — How Designers Manage Field Enhancement at Terminals
At the ends of transmission lines — at the substation connection points, at transformer bushings, and at the terminal towers where the overhead line transitions to underground cable or substation gantry — the conductor geometry changes abruptly. These transition points concentrate the electric field and are prime corona initiation sites even when the transmission line itself is performing acceptably.
The solution is the corona ring — also called a grading ring or anti-corona ring. A corona ring is a toroidal aluminium structure mounted at the end of an insulator string or at a bushing terminal. Its large effective diameter distributes the electric field over a larger surface area, reducing the peak field gradient below the corona onset threshold. The ring's geometry is carefully calculated using numerical electric field analysis (finite element methods) to ensure that no point on its surface, or on the insulator fittings near it, exceeds the critical gradient.
On 400 kV and 765 kV substations in India, corona rings are visible at nearly every significant connection point — transformer bushings, circuit breaker terminals, current transformer heads, and the ends of strain insulator assemblies at tension towers. Their presence indicates the care that EHV substation design must take with field management, even at points that look straightforward from a current-carrying perspective.
Measuring and Testing Corona — Methods Used by Grid Engineers
Quantifying corona performance is not just a design exercise — it is a quality assurance requirement for both line hardware and completed transmission lines. Several established measurement techniques are used at different stages of a transmission project.
Radio Interference Voltage (RIV) testing
Hardware items destined for high-voltage service — insulator strings, fittings, bushings — undergo RIV testing to verify they are corona-free at specified test voltages. The test measures the electromagnetic interference generated at 0.5 MHz (the CISPR 18 standard frequency for transmission line interference measurements) while the test object is energised at specified voltage levels. Acceptance criteria are typically set at a maximum of 50–500 μV depending on the application and voltage class.
Acoustic emission monitoring
Portable acoustic detectors and ultrasonic probes allow field personnel to detect corona activity on energised lines without de-energising them. The characteristic frequency signatures of corona discharge are detectable even in the presence of ambient wind and traffic noise using directional microphones and signal processing. This technique is used for condition monitoring of insulators, conductor hardware, and substation equipment.
UV corona cameras
Daylight-visible UV cameras (solar-blind UV cameras) detect the ultraviolet emissions from corona discharge and display them as a visible overlay on a normal daylight image. These instruments are now widely used for corona patrol of EHV lines — either from the ground or from helicopters — allowing maintenance teams to identify corona-active locations without approaching the energised line. They can detect degraded insulators, damaged conductor surfaces, and hardware problems at distances of tens to hundreds of metres.
In industrial facilities near 132 kV or 220 kV incoming substations, corona-related radio interference from the incoming transmission infrastructure can interfere with AM radio communications, wireless sensors, and certain industrial communication systems. If your control room experiences unexplained interference on communication systems that worsens during rainy weather and correlates with wind direction toward the substation, corona-generated RFI from the transmission line hardware is a plausible cause worth investigating with an RF spectrum analyser.
Managing and Mitigating Corona — The Full Engineering Toolkit
Complete elimination of corona on an EHV transmission line is neither achievable nor necessary. The practical goal is to reduce corona loss, interference, and degradation to levels that are economically justified and compliant with applicable regulations. The tools available to achieve this span from conductor selection at the design stage to operational practices throughout the line's life.
- Bundled conductors: The primary design measure. Using two, three, or four sub-conductors per phase dramatically increases the effective conductor radius and reduces surface electric field below the corona onset threshold. This is the standard solution for all lines above 220 kV and is discussed in detail in the companion article on bundled conductors.
- Large conductor diameter: For lines below 220 kV where single conductors are practical, selecting the largest conductor size that meets the current-carrying requirement also provides the best corona performance — larger radius means lower surface field gradient at the same voltage.
- Conductor surface quality: Using conductors with smoother outer strand surfaces reduces the surface irregularity factor m₀ in Peek's formula. Some modern AAAC (All Aluminium Alloy Conductor) types are manufactured with trapezoidal wire strands that produce a smoother outer surface than traditional round-wire ACSR, improving corona performance.
- Hardware design standards: All high-voltage hardware must be designed to meet corona-free standards at specified test voltages. Rounded edges, smooth surfaces, and adequate radii at all conductor contact points are requirements, not preferences. Corona-free hardware certification is a contract requirement for any EHV line procurement.
- Corona rings at terminals: Installed at all substation terminal points, bushing ends, and hardware-to-conductor transitions where the geometry creates concentrated field enhancement zones.
- Adequate conductor-to-earth clearance: Sufficient sag at midspan ensures that the ground plane below the conductor does not intensify the conductor's electric field. Minimum clearance calculations at maximum sag temperature are a standard part of line design.
- Voltage regulation: Since corona loss rises as (V − Vc)², keeping operating voltage close to nominal — rather than at the upper end of the permissible range — has a directly beneficial effect on corona performance. Voltage regulation is partly a corona management measure, in addition to its equipment protection role.
- Regular corona inspection: UV corona camera surveys and acoustic emission monitoring during maintenance patrols identify developing problems — damaged conductors, failing hardware, contaminated insulators — before they cause significant loss or interference. Condition-based maintenance informed by corona survey results is increasingly standard practice on major EHV lines.
Closing Perspective — Corona as a Design Boundary Condition
The corona effect is one of those phenomena that reveals how deeply interconnected physics and economics are in electrical engineering. A simple physical fact — that air ionises above a certain electric field threshold — drives an intricate chain of design decisions: conductor size, bundling configuration, hardware geometry, tower dimensions, right-of-way width, insulator selection, and substation terminal design. Every EHV transmission line that has ever been built carries the imprint of corona management decisions, visible in the size of its conductors, the number of wires per bundle, and the grading rings at its terminals.
For engineers working in heavy industry — where the incoming supply arrives via EHV lines, where substation management is a daily responsibility, and where power quality and supply reliability are critical — understanding corona connects the visible physical infrastructure to the operational behaviour of the grid. The line that feeds your plant's main transformers was designed, in significant part, to manage the physics of ionised air. That is worth understanding.
Sources & References
- Peek, F.W. (1929). Dielectric Phenomena in High Voltage Engineering, 3rd Edition. McGraw-Hill. — The foundational empirical work on corona onset, critical disruptive voltage, and corona loss formulas that are still the starting point for line design today.
- Glover, J.D., Sarma, M.S., & Overbye, T.J. (2012). Power Systems Analysis and Design, 5th Edition. Cengage Learning. — Detailed treatment of corona loss calculation, Peek's formula derivation, and bundled conductor corona performance.
- Wadhwa, C.L. (2012). Electrical Power Systems, 6th Edition. New Age International. — Comprehensive coverage of the corona effect including Peek's formula, visual critical voltage, and weather correction factors in the context of Indian EHV systems.
- EPRI. (1982). Transmission Line Reference Book: 345 kV and Above, 2nd Edition. Electric Power Research Institute, Palo Alto. — The definitive reference for EHV corona performance, radio interference, audible noise assessment, and design guidelines.
- IEEE Std 1829-2017. IEEE Guide for Conducting Corona Tests on Hardware for Overhead Transmission Lines and Substations. IEEE Power and Energy Society. — Test methodology for corona-free hardware certification.
- CISPR 18-1:2010. Radio Interference Characteristics of Overhead Power Lines and High-Voltage Equipment. International Electrotechnical Commission / CISPR. — International standard for RIV measurement and limit assessment for transmission lines.
- Central Board of Irrigation and Power (CBIP). Manual on Transmission Line Design. CBIP Publication No. 268, New Delhi. — Indian standard reference covering corona performance criteria, conductor selection for corona, and hardware specifications.
- Bureau of Indian Standards. IS 5613 (Part 2/Section 2):1989 — Code of Practice for Design, Installation and Maintenance of Overhead Power Lines. BIS, New Delhi.
- Power Grid Corporation of India Limited (PGCIL). Technical Standard for 765 kV Transmission System. Available at: powergrid.in
- Rakosh Das Begamudre. (2000). Extra High Voltage AC Transmission Engineering, 2nd Edition. New Age International. — Dedicated treatment of EHV corona phenomena, loss formulas, altitude correction, and bundle conductor corona analysis.
