What Is Corona Discharge? Physics, Real-World Examples, and Prevention

Corona discharge is a localized electrical discharge that occurs when the electric field gradient around a conductor exceeds the dielectric breakdown strength of the surrounding air, ionizing it into a conductive plasma without forming a complete arc. You hear it as a distinct hiss, smell it as ozone, and see it as a faint violet glow in the dark. But on a workbench or inside a switchgear cabinet, it is a destructive parasite that eats insulation, corrodes metals, and generates broadband electromagnetic noise.

The Physics and the Math: Calculating the Onset

At the atomic level, air is normally an excellent insulator. However, no air is perfectly pure; there are always a few free electrons floating around from cosmic rays or background radiation. When the electric field (E-field) near a conductor becomes intense enough, it accelerates these stray electrons to high velocities. When they smash into neutral air molecules (like O2 and N2), they knock off additional electrons. This creates an 'electron avalanche'—a localized chain reaction that turns the surrounding gas into a conductive plasma.

Because the E-field strength is inversely proportional to the radius of the conductor, the field is always strongest right at the surface of the wire or terminal. If the field drops below the ionization threshold just a few millimeters away from the surface, the avalanche stops. You get a localized glow (corona) rather than a full bridge to ground (an arc).

A Worked Numeric Example: The 15kV Cable Termination

To see why geometry matters, let us look at a common medium-voltage (MV) failure point: terminating a 15 kV underground distribution cable. The cable features a copper conductor, 4.0 mm of XLPE (cross-linked polyethylene) insulation, and an outer semiconducting shield.

When you strip the shield back to install a termination lug, you create a sharp geometric cutoff. Let us calculate the electric field stress at that exact point.

• System Voltage: 15 kV phase-to-phase.
• Phase-to-Ground Voltage: 15,000 / √3 ≈ 8,660 V.
• Insulation Thickness: 4.0 mm.
• Average E-field: 8,660 V / 4.0 mm = 2.16 kV/mm.

Both air and XLPE typically break down around 3.0 kV/mm. At first glance, an average field of 2.16 kV/mm seems safe. But average field is not what matters at a sharp edge.

This peak field vastly exceeds the 3.0 kV/mm ionization threshold of the air trapped in the termination void. The air instantly ionizes, initiating corona. Over months of continuous 60 Hz cycling, this corona etches the XLPE, creating conductive carbon 'water trees' until the cable faults to ground. This exact mathematical reality is why MV terminations require geometric stress cones or high-K (High Dielectric Constant) stress relief tape to smoothly grade the field and keep the surface gradient below 3.0 kV/mm.

What Corona Changes in a Real Installation

Corona is not just a visual quirk; it fundamentally alters the operating environment of your circuit or installation. If you ignore it, three things will change:

1. Power Loss and Heating: Corona represents a continuous leakage current. On high-voltage transmission lines, this 'corona loss' can amount to several kilowatts per mile during humid or foggy conditions, slightly reducing transmission efficiency and locally heating the conductor.
2. Chemical Warfare (Ozone and Nitric Acid): The ionization process splits O2 molecules, which recombine into ozone (O3). Ozone aggressively attacks and cracks natural rubber and silicone insulation. Worse, ionized nitrogen combines with ambient humidity to form trace nitric acid. This corrosive soup eats copper lugs, degrades aluminum busbars, and turns solder joints brittle.
3. RFI and EMI Generation: The rapid, repetitive ionization and de-ionization of the air acts as a broadband radio frequency transmitter. A corroded 11kV insulator on a nearby pole can completely wipe out the AM radio band and inject high-frequency noise into sensitive unshielded control wiring, causing PLCs to throw phantom faults.

Where You Meet This in Practice

You will encounter corona discharge across a wide spectrum of electrical work, from massive utility infrastructure to benchtop power supplies.

• High Voltage Transmission Lines: If you look up at a 345 kV or 500 kV transmission tower, you will notice the conductors are not single wires. They are 'bundled'—two, three, or four sub-conductors held apart by spacers. This bundling increases the effective radius of the phase, lowering the surface E-field gradient below the corona onset threshold (Electrical Engineering Portal).
• Flyback Transformers and CRTs: The 20-30 kV anode supplies in older CRT monitors or modern high-frequency switching power supplies are notorious for corona. If the silicone potting compound has a microscopic internal void, corona starts inside the void, eventually carbonizing the potting and destroying the transformer.
• Electrostatic Precipitators and Ion Generators: Here, corona is intentional. A sharp needle or thin wire energized to -10kV intentionally generates a massive corona cloud to charge dust particles or smoke, which are then attracted to a grounded collection plate.
• High-Frequency PCB Traces: In modern SiC (Silicon Carbide) and GaN (Gallium Nitride) motor drives, the extreme dV/dt (voltage change over time) can induce partial discharge and corona in the microscopic air gaps between tightly routed high-voltage PCB traces.

Common Confusions: Corona vs. Arcing vs. Tracking

People often use 'corona', 'arcing', and 'tracking' interchangeably. They are distinct failure modes with different physical signatures.