A corona discharge is an electrical discharge in the air around a live conductor. It starts when the electric field strength near the conductor surface exceeds a critical threshold, but it is not strong enough to cause a full breakdown to the opposite electrode. Under standard conditions air ionises at around 30 kV/cm, and at sharp points voltages of 2 to 6 kV are enough (Wikipedia, Corona discharge). Corona shows up as a bluish glow, a crackling sound and a smell of ozone. This article explains where corona forms, how it differs from partial discharge, what it does to power lines and electric motors, and which insulation materials withstand it for years.
Corona discharge starts at the smallest radius
What decides corona is the field strength right at the surface, and that depends mainly on curvature. A thin wire, a point or a burr concentrates the field lines in a small space, while a large sphere or a smooth tube spreads them out. That is why corona glows first at edges, wire ends and screw heads, while the smooth surface next to them stays quiet. The basics of electric field strength and its formula are covered in a separate article.
Above the threshold, free electrons accelerate so strongly that they knock further electrons out of air molecules on impact. An electron avalanche forms, but it only grows in the thin zone of high field strength around the conductor. Further out the field drops, the avalanche dies, and the current stays in the microampere range (chemie-schule.de). If the highly curved conductor is the positive pole, it is called positive corona: an even, bluish white sheath. Negative corona sits in tufts at individual points and produces considerably more ozone.
Since 1929 the inception voltage has been described by Peek's law. It takes the breakdown field of air as 30 to 32 kV/cm and corrects it for conductor radius, air density and a surface factor: 1 for a polished wire, 0.93 to 0.98 for a rough or dirty one and 0.83 to 0.87 for a stranded cable (Wikipedia, Peek's law). This gives the five variables that decide between corona and no corona in practice:
- the voltage, above all its peak value and the steepness of its edges
- the radius of curvature of the conductor or the edge
- the surface, because scratches, dust, insects and water droplets act like small points
- the air density, which falls with altitude and temperature
- the distance to the opposite electrode, which can turn corona into a spark
Air density is often underestimated. The German electrical industry association ZVEI shows that an air gap with a breakdown voltage of 1,000 V at sea level holds only about 630 V at an altitude of 4,000 m. DIN EN 60664-1 therefore requires larger clearances above the standard altitude of 2,000 m, by 29 % at 4,000 m and by 48 % at 5,000 m (ZVEI, partial discharge measurement guide, 2024).
Corona discharge and partial discharge: where the line is
Corona is a form of partial discharge, namely the external one. It burns in the gas at a highly curved conductor. Internal partial discharge, by contrast, ignites in a void inside the solid or liquid insulation, and surface discharge travels along a surface. The ZVEI groups corona, glow and surface discharges together as external and surface partial discharges and sets them against the internal discharges, which are decisive in most equipment.
| Discharge | Location | Perceptible | Typical consequence |
|---|---|---|---|
| Corona discharge | gas around a point, wire or edge | glow, crackling, smell of ozone | losses, radio interference, ozone, erosion of nearby insulation |
| Internal partial discharge | void or bubble in the insulation | only by measurement | erosion from inside, treeing, later breakdown |
| Surface discharge | along an insulating surface | sometimes glowing tracks | tracking, especially with dirt and moisture |
| Spark, arc | the whole gap between the electrodes | bang, flash | breakdown, short circuit |
For design work the key point is that corona can be seen and heard, and internal partial discharge cannot. A UV camera finds corona on an overhead line in the 240 to 280 nm range (elektronebel), but it will never find a void in a slot liner. How partial discharges are measured to IEC 60270 and what inception and extinction voltage mean is covered in the article on partial discharge. The surface variant with tracking is covered in the article on surface leakage current.
Corona discharge on overhead lines: crackling, losses and hum
Corona is best known from high-voltage lines. The German trade journal Elektropraktiker gives a critical surface field strength of about 20 kV/cm for stranded conductors, less than the 30 kV/cm of a smooth electrode because the strand surface is rough. On lines, corona mainly occurs from 220 kV rated voltage upwards, and mainly in rain, fog and snow. Every water droplet on the conductor is a new point.
- Corona losses: for a 380 kV overhead line, an approximate value of 3 kW per kilometre applies, and under the most unfavourable conditions losses can reach up to 20 % of the transmitted power (Elektropraktiker).
- Noise: the crackling is broadband, plus a hum at 100 Hz, twice the grid frequency (ingenieur.de). For new lines from 220 kV, the pre-standard DIN VDE V 0210-30 makes the noise predictable (DKE).
- Weather: more than 80 % of the hours in a year are dry, and an AC line stays largely quiet then. Noise forecasts are based on a rain rate of around 3.6 mm/h (DAGA 2016, DIN SPEC 8987).
- Radio interference from the current pulses of the many small discharges.
Grid operators work on curvature. From 220 kV upwards, lines normally use bundle conductors with two to four sub-conductors per phase, which act like one conductor with a large equivalent radius (fgeu, diploma thesis 2001). Corona rings on insulator strings and fittings do the same at the ends. Building a line completely free of corona would be uneconomic, so corona is limited to an acceptable level. The insulators themselves are covered in the article on electrical insulators.
What corona does to insulating materials
The energy of a single corona is small. It becomes dangerous through duration and chemistry. Every discharge generates UV light, ions, ozone and nitrogen oxides. This hits every organic insulating material nearby: films, papers, resins, varnishes and seals. The ZVEI explicitly lists UV exposure and ozone formation from partial discharge as ageing factors and gives the rule of thumb that 10 K more temperature halves the life of an insulation system.
Manufacturer data show how quickly a material gives up. DuPont tested Kapton HN and the corona-resistant grade Kapton CR together with ABB and Siemens to IEC 343: at 20 kV/mm and 50 Hz a 25 µm Kapton HN film fails after about 200 hours, while Kapton CR lasts more than 100,000 hours. For aramid paper, DuPont itself states that Nomex, like any organic insulating material, is gradually eroded under corona attack. In closed housings, ozone cracks in elastomer seals add to this, and film capacitors gradually lose capacitance under corona (Wikipedia, Corona discharge).
Corona in the electric motor: from around 300 V air becomes a risk
For a long time corona was considered a high-voltage topic. That has changed. According to Paschen's law, discharges in air are theoretically possible from around 300 V peak, and in practice rare below 500 V (ZVEI). DuPont states a corona inception voltage of 425 V for a 25 µm Kapton film. Every inverter-fed drive reaches these values today.
The reason is switching. The inverter applies the DC-link voltage to the motor as steep square pulses, and at the winding the incoming and reflected waves overlap. The ZVEI describes how enamelled wire insulation is stressed with up to twice the DC-link voltage. In an 800 V traction drive that means up to 1,600 V at points where only micrometres of enamel and an air gap separate adjacent turns. Fast-switching SiC and GaN semiconductors make the edges even steeper, which is why the ZVEI calls for insulating materials that are free of or resistant to partial discharge in these drives. Testing such windings with voltage impulses is covered by IEC 60034-18-41 (RM Prüftechnik).
In the motor, corona sits at a few well-known points: in the end winding between phases, at the slot exit, at the welds of hairpins and at every transition between wire, air and flexible insulation. This is where insulating parts such as phase separators, slot liners and slot closures sit, which GOBA manufactures. How motor insulation changes for high-voltage insulation is shown in the article on stator insulation.
Which insulating materials withstand corona
Organic materials erode under corona, inorganic ones hardly at all. That is why mica has been the standard in high-voltage machines for decades: its platelets form a barrier the discharge cannot get past. Because mica paper alone lacks strength, it is bonded with resin to carriers of glass fabric, fleece or film. The table shows the materials GOBA processes with their corona behaviour according to the manufacturer.
| Material | Corona behaviour | Typical use |
|---|---|---|
| Mica tape, Micanite | inorganic, retards erosion from partial discharge permanently (ZVEI) | high-voltage machines above 6 kV, generators, heating elements |
| Nomex 418 (aramid with 50 % mica) | much higher voltage endurance under corona than Nomex 410 (DuPont) | stator coils up to 13.6 kV class, inverter and traction motors |
| Kapton CR | more than 100,000 h at 20 kV/mm and 50 Hz (DuPont, IEC 343) | inverter drives, traction motors, wind generators |
| Kapton HN | about 200 h at 20 kV/mm and 50 Hz | thermal class above 180 °C without corona |
| Nomex 410 | gradually eroded under corona (DuPont) | slot and phase insulation below the inception voltage |
Our recommendation: first check whether your application reaches the inception voltage at all. If the peak value stays safely below 300 V, there is no corona, and a corona-resistant special material only costs money. Above that it pays off quickly. A factor of 500 in service life, as between Kapton HN and CR, decides years of running time in a traction motor. The HN, CR and MT grades are described in the article on Kapton, the aramid papers in the article on Nomex.
Preventing corona discharge: geometry first, then material
Every countermeasure either lowers the field strength at the surface or removes the air in which the discharge burns. This order is worth following:
- Increase radii: round edges, fit corona rings, use bundle conductors instead of single conductors, and avoid points, burrs or protruding corners inside equipment.
- Limit field strength: for Nomex 418 in transformers, DuPont recommends continuous stresses below 3.2 kV/mm to avoid partial discharge. More insulation thickness lowers the field in the air gap.
- Displace air: avoid voids during impregnation and potting, ideally under vacuum, as the ZVEI describes.
- Account for the environment: extend clearances for sites above 2,000 m to DIN EN 60664-1 and allow for humidity and contamination.
- Use corona-resistant materials where the first four steps are not enough: mica, Nomex 418, Kapton CR.
The order is deliberate. A corona-resistant material on a sharp edge lasts longer, but the edge remains the weak point. If you enlarge the radius first, you often need no special material at all.
Where GOBA comes in: at the edge of the insulating part
Insulating parts follow the same rule as overhead conductors, only on a scale of tenths of a millimetre. A frayed cut edge, a burr or delamination at the cut of a laminate becomes the point in service where the field strength rises locally. That is where corona starts, and that is where erosion starts. The quality of the edge is therefore an electrical property.
GOBA die-cuts slot liners, phase separators, slot closures and covers from Nomex, Kapton, laminates and mica materials in thicknesses from 0.023 to 3.0 mm and up to 1,000 x 2,000 mm, using steel rule dies or solid steel tools. In stamping and forming we pre-emboss fold lines in the same stroke so that the Nomex does not break when bent. For drives in electromobility we match material and edge quality to the voltage class. Send us your drawing and the voltage at the inverter, and we will recommend the right material from our range of insulation and molded parts.
Corona treatment: the same discharge as a tool
Anyone searching for corona discharge often ends up with films. Film producers use corona on purpose to activate the surface of polyethylene, polypropylene and polyester. A generator applies 10 to 20 kV at 10 to 60 kHz to an electrode above the moving web, the discharge introduces oxygen groups into the top molecular layer and raises the surface tension to 38 to 44 mN/m (Bleher, film glossary). Only then do adhesives, printing inks and laminations bond.
The effect does not last forever. After four weeks the surface tension is about 10 % below its initial value (Bleher), and depending on the polymer it decays over more than 140 days (Fraunhofer, cited in Farbe und Lack). For adhesive and laminated composites this means: process freshly treated film promptly. Examples are given in the articles on laminating film and polypropylene films.
