- Corona Discharge Definition: Corona discharge is defined as an electrical phenomenon where a high-voltage conductor ionizes the surrounding air, visible as a violet glow and audible as a hissing sound.
- Critical Disruptive Voltage: The voltage at which air around a conductor breaks down and becomes ionized, initiating the corona effect, is typically around 30 kV.
- Major Influences: Factors like atmospheric conditions, conductor condition, and spacing between conductors significantly impact the occurrence and intensity of the corona effect.
- Reduction Strategies: To reduce the corona effect, strategies include using larger or bundled conductors, spacing adjustments, and the implementation of corona rings.
- Corona Effect on Energy Loss: The corona effect leads to power losses manifested as light, heat, sound, and ozone production, impacting the efficiency of high-voltage power systems.
Corona Discharge, or the corona effect, is a localised gas discharge around an energised conductor or fitting. It begins when the surface voltage gradient makes the surrounding air ionise, but the field is not yet sufficient to form a complete arc across the insulation gap. Designers of AC and high-voltage systems control conductor geometry and hardware to limit the peak electric field.
Corona can dissipate power and produce audible noise, radio interference, ozone and local surface stress. Line and substation designs set corona-performance criteria for the required voltage, weather conditions and applicable environmental limits.
Corona can create hissing, crackling or broadband noise as repeated discharges ionise air near highly stressed surfaces. On high-voltage electric power transmission lines, it can also produce a visible glow in low light, radio noise, ozone and power loss. Weather and surface condition strongly affect what can be observed or measured.
What is the Corona Effect?
Air contains a small population of charged particles created by natural ionising processes. An electric field accelerates positive and negative charge carriers in opposite directions. Near a conductor with a high surface gradient, some carriers gain enough energy between collisions to ionise additional gas molecules.
Ionising collisions create an electron avalanche and a local conductive region. Corona remains confined to the high-field area because the field falls with distance from the conductor. If the applied stress and gap conditions support a continuous discharge path, the event can develop into streamer, leader or spark breakdown, but corona does not automatically become an arc.
Electric power transmission moves bulk energy between generation and load centres. Higher transmission voltage reduces current for a given power transfer, but it also raises electric-field stress around conductors, insulators and fittings. Corona performance is therefore one of several constraints in overhead-line design.
Corona loss contributes to total line loss, especially at extra-high voltage and during adverse weather. Audible noise and radio interference can be more restrictive design criteria than energy loss, so engineers assess each effect against project requirements.
The protected list below is an incomplete legacy summary. Corona can occur under AC or DC voltage, and onset depends on local surface field rather than alternating potential or one spacing-to-diameter rule.
- Alternating electrical potential differences must be supplied across the line.
- The spacing of the conductors must be large enough compared to the line diameter.
When an AC conductor is energised, the applied alternating current system voltage establishes an alternating electric field around each conductor of the transmission line. Conductor radius, phase spacing, bundle arrangement, surface condition and nearby hardware determine the peak surface gradient.
Below corona onset, the local field does not sustain a detectable discharge. As voltage rises, the surface gradient can reach the critical disruptive voltage condition and ionisation begins. There is no universal 30 kV threshold: inception voltage depends on conductor and electrode geometry, air density, weather, surface condition, polarity and the measurement criterion.
Charge movement in the ionised region produces current pulses and energy transfer. Depending on field strength, polarity and observation conditions, the discharge may produce a faint glow, audible noise, radio-frequency emissions and ozone.
This local discharge on high-voltage line conductors or fittings is known as the corona effect. Increasing the surface gradient above onset generally increases corona activity, but audible noise, radio interference and loss vary with weather, conductor condition and polarity rather than voltage alone.
Factors Affecting Corona Loss
Line voltage contributes to conductor surface gradient, but it is not the only determining factor. Below the relevant inception criterion, the local field does not sustain observable corona. Above it, discharge activity depends on the full electric-field geometry and atmospheric conditions.
Corona begins at locations where the electric-field stress is highest. For an overhead line, conductor size and bundle geometry, hardware shape, surface contamination, rain, air density and phase arrangement all affect performance. The protected list groups some of these factors.
- Atmospheric Conditions
- Condition of Conductors
- Spacing Between Conductors
Each factor changes the local field or the air conditions at which ionisation starts.
Atmospheric Conditions
Air density affects corona inception, so altitude, pressure and temperature matter. Rain, fog, snow and water droplets can create local increases in field strength on a conductor surface. Wet or foul weather therefore changes audible-noise and radio-interference performance, often more than wind-driven ion concentration.
Line and hardware tests must state the weather condition, air-density correction and acceptance criterion. The design should meet the applicable limits in both fair and specified foul-weather conditions.
Condition of Conductors
For a given line geometry and voltage, increasing conductor radius generally reduces the surface gradient. Bundled subconductors can create a similar effective-diameter benefit, although bundle count and spacing must be optimised for the complete line design.
Strands, scratches, damaged fittings, contamination and attached water droplets raise the local field strength. These surface irregularities can lower corona inception and increase noise or radio interference, so clean profiles and correctly installed hardware matter.
Spacing Between Conductors
Phase and subconductor spacing affect the electric-field distribution. Corona does not require spacing to be much larger than conductor diameter; it requires the local surface field to exceed the relevant inception level.
Increasing phase spacing can reduce the field contribution from adjacent phases, but the benefit is not unlimited and cannot be considered apart from tower size, clearances, inductance and capacitance. Bundle spacing also has an optimum range because uneven charge distribution can raise the maximum subconductor gradient.
Strategies to Reduce Corona Discharge
Corona control aims to keep surface gradients within the project’s loss, audible-noise, radio-interference and hardware-test criteria. The most effective measures reduce sharp local fields at conductors, clamps, fittings and equipment terminals.
Common design measures are listed below. Their dimensions and spacing must be established by calculation, testing and the applicable line or equipment specification.
- Increasing the conductor size: A larger conductor diameter results in a decrease in the corona effect.
- Increasing the distance between conductors: Increasing conductor spacing decreases the corona effect.
- Using bundled conductors: Bundled conductors increase the effective diameter of the conductor – hence reducing the corona effect.
- Using corona rings: Electric fields are stronger at points of sharp conductor curvature, hence corona discharge first occurs at sharp points, edges, and corners. Corona rings, which are electrically connected to the high-voltage conductor, encircle the points where the corona effect is most likely to occur. They effectively ‘round out’ the conductors, reducing the sharpness of the conductor surface, and distributing the charge over a wider area, thereby reducing corona discharge. Corona rings are used at the terminals of very high-voltage equipment (such as at the bushings of high-voltage transformers).
Corona Discharge and Current
Corona is driven mainly by electric-field stress from conductor voltage. The line load current primarily sets the magnetic field and conductor temperature, so it should not be confused with corona discharge current.
The Role of Current in Corona Discharge
Voltage establishes the electric field around a transmission conductor. Once corona begins, moving ions and electrons form a much smaller discharge current in the surrounding air.
Ionisation occurs where the local voltage gradient is high enough to sustain avalanches. The field is strongest at small radii, protrusions, water droplets and poorly graded hardware.
Corona severity is not proportional to load current. A lightly loaded but fully energised line can still produce corona because its conductor voltage remains high. Load current may have secondary effects through conductor temperature or sag, but it is not the main cause.
Current and Corona Losses
Corona loss is the real power dissipated by discharge activity around energised conductors and hardware. It increases with surface gradient above onset and varies with weather, polarity and geometry. It is separate from resistive I²R loss caused by line load current.
Current Management to Mitigate Corona Discharge
Reducing load current is not a normal corona-control method. Engineers instead manage surface electric field through conductor size, bundle geometry, phase spacing and well-graded hardware.
Current-limiting components are selected for fault duty or power-flow control, not for routine corona mitigation. Inspection and maintenance can still reduce corona by finding damaged strands, loose fittings, contamination and sharp protrusions.
Direct Current (DC) Corona Discharge
Direct Current corona has polarity-dependent ion flow and space-charge effects that differ from Alternating Current corona. Both can produce audible noise, radio interference and power loss.
Fundamentals of DC Corona Discharge
Laboratory studies often use an observed experimental phenomenon such as a needle-to-plane gap to create a controlled high-field region in air. Positive and negative DC polarity produce different ionisation, pulse and space-charge behaviour. Results from that geometry cannot be transferred directly to an overhead-line conductor.
Differences Between Positive and Negative Corona Discharge
Positive corona can appear as glow, burst pulses or streamers, depending on voltage, geometry, gap and gas conditions. A smooth faint glow is one possible regime, not a universal appearance.
Negative corona can show Trichel pulses, glow or streamer behaviour. Its brightness and filament structure depend on the operating regime, so polarity alone does not guarantee a brighter filamentary discharge.
DC Corona Discharge Characteristics
DC corona has a nonlinear current-voltage characteristic. Once applied voltage exceeds the onset criterion for the electrode system, average discharge current generally rises, but transitions between pulse, glow and streamer regimes can change the curve.
DC onset voltage depends on electrode shape and size, gap distance, polarity, pressure, temperature, humidity, surface condition and gas composition. A measured onset must also state the optical, current or radio-interference detection criterion.
