Corona discharge is one of the most important phenomena in high-voltage power transmission. It causes power loss, radio interference, and conductor corrosion — yet it also provides a natural protection against voltage surges. Understanding corona is essential for designing efficient transmission lines.
In this article, you will learn what corona discharge is, how it forms, the critical disruptive voltage formula, factors affecting corona, power loss calculation, and methods to reduce corona in transmission lines.
Table of Contents
What is Corona Discharge?
Corona discharge is an electrical discharge that occurs due to the ionization of air surrounding an electrically charged conductor. It happens when the electric field intensity (voltage gradient) at the conductor surface exceeds the breakdown strength of air.
In overhead transmission lines, when the applied voltage is low, the air around the conductors behaves as a perfect insulator. But when the voltage exceeds a certain limit — called the critical disruptive voltage — the air molecules near the conductor surface get ionized.
This ionization produces:
- A faint violet or bluish glow around the conductor
- A hissing or crackling sound
- Production of ozone (O₃) gas
- Power loss
- Radio and TV interference
Definition: The phenomenon of violet glow, hissing noise, and production of ozone gas in an overhead transmission line due to the ionization of air surrounding the conductor is called corona.
Theory of Corona Formation
Under normal conditions, air always contains some free ions and electrons due to cosmic rays, ultraviolet radiation, and natural radioactivity. These particles are normally too few to conduct electricity.
When a potential difference is applied between two conductors, the electric field accelerates these free ions. The process of corona formation follows these steps:
- Step 1: Existing free ions gain kinetic energy from the electric field
- Step 2: As voltage increases, ions accelerate faster
- Step 3: When the voltage gradient reaches approximately 30 kV/cm (at standard atmospheric conditions), the ions have enough energy to knock electrons out of neutral air molecules
- Step 4: Each collision produces new ions, which in turn ionize more molecules
- Step 5: This cumulative ionization (avalanche effect) creates a conducting region around the conductor
If the electric field is strong enough only near the conductor surface (not across the entire gap), the ionization remains localized — this is corona. If the field is strong enough across the entire gap, a complete breakdown occurs — this is a spark or arc.
Critical Disruptive Voltage
The critical disruptive voltage is the minimum phase-to-neutral voltage at which corona begins. Below this voltage, no ionization occurs. Above it, corona starts.
Where:
- m₀ = Irregularity factor (1.0 for smooth polished conductors, 0.98–0.92 for stranded conductors, 0.80–0.87 for dirty/rough conductors)
- g₀ = Breakdown strength of air at standard conditions = 21.2 kV/cm (rms) or 30 kV/cm (peak)
- δ = Air density factor = 3.92b / (273 + t), where b = barometric pressure in cm Hg, t = temperature in °C
- r = Radius of conductor (cm)
- d = Spacing between conductors (cm)
- ln = Natural logarithm (loge)
At standard conditions (76 cm Hg, 25°C), δ = 1.0.
Visual Critical Voltage
The visual critical voltage is the voltage at which corona glow becomes visible to the naked eye. It is slightly higher than the critical disruptive voltage because a certain minimum ionization intensity is needed for visible glow.
Where mv is the roughness factor for visual corona (1.0 for smooth conductors, 0.72 for stranded conductors in rough weather).
The visual critical voltage is always greater than the critical disruptive voltage (Vv > Vc).
Factors Affecting Corona
Power Loss Due to Corona
Corona causes continuous power loss in transmission lines. This loss is given by Peek's formula:
Where:
- f = Supply frequency (Hz)
- δ = Air density factor
- r = Radius of conductor (cm)
- d = Spacing between conductors (cm)
- V = Phase-to-neutral voltage (kV, rms)
- Vc = Critical disruptive voltage (kV, rms)
Key observations from the formula:
- Corona loss is zero when V ≤ Vc
- Loss increases with the square of (V − Vc) — so even a small increase above critical voltage causes significant loss
- Higher frequency increases corona loss
- Loss is greater in bad weather (lower δ, lower Vc)
Advantages of Corona
Although corona is generally undesirable, it does provide some benefits:
- Surge protection: Corona acts as a safety valve during voltage surges (lightning or switching). The ionized air absorbs energy from the surge, reducing the steepness of the voltage wave and protecting equipment
- Reduced electrostatic stress: Corona increases the effective (virtual) diameter of the conductor, which reduces the voltage gradient between conductors and decreases the chance of a complete flashover
Disadvantages of Corona
- Power loss: Continuous energy dissipation reduces transmission efficiency, especially in bad weather
- Conductor corrosion: Ozone (O₃) produced by corona is chemically active and corrodes conductor surfaces over time
- Radio and TV interference: Corona produces electromagnetic noise that interferes with communication lines and broadcast signals
- Audible noise: The hissing and crackling sound can be a nuisance near populated areas
- Vibration: Non-uniform corona can cause conductor vibration (corona-induced vibration)
Methods of Reducing Corona
Since corona causes power loss and other problems, engineers use several methods to minimize it:
1. Increase Conductor Diameter
A larger conductor has a lower surface voltage gradient for the same applied voltage. This raises the critical disruptive voltage. ACSR conductors (Aluminium Conductor Steel Reinforced) with larger cross-sectional area are preferred for high-voltage lines.
2. Use Bundled Conductors
Instead of one large conductor per phase, two or more smaller conductors are used in a bundle (spaced 30–45 cm apart). This effectively increases the equivalent radius, reducing the surface gradient. Bundled conductors are standard for 220 kV and above.
3. Increase Conductor Spacing
Greater spacing between phase conductors reduces the electric field at the conductor surface. However, this increases tower width and cost, so there is a practical limit.
4. Use Smooth Conductors
Smooth, polished conductor surfaces have fewer local high-field points. In practice, stranded conductors are used but with tight stranding to minimize surface irregularity.
5. Use Corona Rings
At line terminations and insulators, corona rings (grading rings) are used to distribute the electric field more uniformly, preventing localized corona at hardware fittings.
FAQs
At what voltage does corona start in transmission lines?
Corona typically starts at voltages above 100 kV in fair weather for standard conductor sizes and spacings. The exact voltage depends on conductor diameter, spacing, altitude, and weather conditions. Lines operating at 66 kV and below rarely experience corona.
Why is corona loss higher in bad weather?
In rain, fog, or snow, the air contains more free ions and water droplets on the conductor surface create local high-field points. Both effects reduce the critical disruptive voltage, causing corona to start at a lower voltage and increasing the (V − Vc) term in Peek's formula.
What is the difference between corona and arc?
Corona is a partial, localized ionization near the conductor surface — the air between conductors remains non-conducting. An arc is a complete breakdown of the air gap between conductors, creating a conducting channel. Corona is a low-energy continuous phenomenon; an arc is a high-energy fault condition.
Why are bundled conductors used for EHV lines?
Bundled conductors increase the effective radius of the conductor system, which reduces the surface voltage gradient. This raises the critical disruptive voltage and significantly reduces corona loss. They also reduce line inductance and increase capacitance, improving power transfer capability.
Does corona occur in DC transmission lines?
Yes, but the behavior is different. In DC lines, corona occurs mainly on the positive conductor (positive corona). DC corona loss is generally lower than AC corona loss for the same voltage level because there is no alternating field to continuously re-ionize the air. This is one advantage of DC transmission over AC transmission.