What Is Corona?

Corona is a form of electrical discharge that occurs when the electric field around a high-voltage conductor becomes strong enough to ionize the surrounding air without bridging fully to an adjacent conductor. The discharge produces a faint glow, a characteristic hissing or crackling sound, and the generation of ozone and other reactive byproducts. Corona is a well-studied phenomenon in high-voltage power engineering, and its presence in electrical infrastructure is treated as both a diagnostic signal and a source of degradation.

The discharge takes its name from the luminous crown-like appearance that surrounds a conductor under ionizing conditions. This visual effect results from electron avalanches in the air immediately adjacent to the conductor surface, driven by field intensities that typically exceed 30 kV/cm at sea level. Corona onset voltage depends on conductor geometry, surface roughness, atmospheric pressure, humidity, and the presence of contamination on insulator surfaces.

Relationship to Partial Discharge

Corona is classified as a subset of partial discharge, a broader category covering all localized dielectric breakdowns within an electrical insulation system that do not form a complete conductive path. The IEEE Guide for Making Corona (Partial Discharge) Measurements on Electronic Transformers establishes standardized electrical methods for detecting and quantifying corona activity, including acceptable discharge pulse energy limits and test apparatus calibration requirements. What distinguishes corona from other partial discharge types is its occurrence in a gaseous medium, most commonly air, at an electrode surface rather than within a solid dielectric material.

Understanding the relationship between corona and partial discharge is essential for interpreting condition-monitoring data from power transformers, cables, and switchgear. In high-voltage equipment, sustained corona activity leads to progressive insulation degradation, surface erosion of conductors, and interference with nearby electronic systems through radiated electromagnetic noise.

Detection and Measurement

Corona detection relies on several physical phenomena that accompany the discharge. Electrical methods measure the charge pulses associated with each ionization event, and this approach forms the basis of the IEC 60270 standard for partial discharge measurement. Acoustic detection uses ultrasonic transducers or directional microphones to locate discharge activity from the sound it produces. Optical methods, including UV-sensitive cameras, can visualize corona on energized outdoor equipment in daylight, making them useful for inspection of transmission lines and substations.

Phase-resolved partial discharge (PRPD) analysis is among the most widely used diagnostic tools in power engineering, capturing the statistical relationship between discharge activity and the phase of the applied AC voltage. Research published on IEEE Xplore describes how PRPD patterns differentiate between corona discharge modes and other partial discharge types, enabling engineers to assess the severity and location of insulation defects.

Mitigation and Design Considerations

In transmission-line design, corona losses are reduced by increasing conductor diameter or by bundling multiple sub-conductors, both strategies that reduce surface field strength. High-voltage equipment is designed with grading rings and shielded electrodes to redistribute the electric field away from points where corona would initiate. For indoor high-voltage apparatus, enclosure in sulfur hexafluoride (SF6) gas eliminates corona entirely because SF6 has a much higher dielectric strength than air.

The formation of ozone and nitrogen oxides during corona activity introduces additional concerns: these reactive compounds attack polymer insulation, produce acid deposits on insulator surfaces, and contribute to degradation of transformer oil when corona occurs within enclosed apparatus. Work on characterization of corona under variable frequency and pressure conditions has extended understanding of how operating conditions shift corona onset thresholds in non-standard environments.

Applications

Corona has applications in a range of fields, including:

• High-voltage power transmission line design and loss estimation
• Condition monitoring of transformers, cables, and switchgear
• Electrostatic precipitators for industrial air filtration
• Ozone generation for water treatment and sterilization
• Photocopier and laser printer drum charging systems