If you've ever stood near a high-voltage transmission line on a damp night and heard a faint hissing or crackling sound, you've witnessed corona discharge. It's the same phenomenon that sailors centuries ago called "St. Elmo's fire"—a bluish glow that appeared on ship masts during storms. But while it may look dramatic, for power engineers, corona is an invisible drain on efficiency, a source of interference, and a design challenge that must be carefully managed.
As transmission voltages continue to rise to meet growing demand, understanding corona discharge—its causes, its effects, and how to mitigate it—has become essential knowledge for anyone working in the power utility industry. Let's break down what corona is, why it matters, and how design choices can keep it under control.
What Is Corona Discharge?
At its core, corona is a localized ionization of air. It occurs when the electric field intensity around a conductor exceeds the dielectric strength of air—approximately 30 kV/cm under standard temperature and pressure. When that threshold is crossed, the air molecules around the conductor become ionized, creating a self-sustaining plasma that produces visible light, audible noise, and small leakage currents.
Importantly, corona is not a full electrical breakdown or arc between conductors—it's a partial discharge that happens in the air immediately surrounding the conductor. But despite being localized, its effects ripple throughout the entire transmission system.
The Hidden Costs of Corona
Corona may seem like a minor nuisance, but its impacts are significant and multifaceted:
Power Loss. The ionized air around a conductor becomes conductive, allowing small leakage currents to flow. While these losses are typically less than 1% for well-designed lines, they are continuous and increase under adverse weather conditions. Over the lifetime of a transmission line, that adds up to substantial wasted energy.
Audible Noise. The rapid ionization and deionization cycles produce a hissing or crackling sound in the range of 1 to 20 kHz. Near substations or heavily loaded lines, this can be a nuisance to nearby residents.
Radio and Television Interference. Corona generates broadband electromagnetic interference (EMI) that can disrupt communication systems, particularly in the medium-frequency and high-frequency bands.
Ozone and Chemical Effects. Corona breaks down air molecules, producing ozone and other reactive gases. These can accelerate the aging of insulation and other equipment.
Traveling Wave Attenuation. Corona is the dominating effect in attenuating and distorting traveling waves or surges on a transmission line, which affects system protection and transient performance.
What Makes Corona Worse?
Corona doesn't happen uniformly—it's influenced by a range of factors that design engineers must consider:
Conductor Surface Condition. Rough, dirty, or weathered conductors initiate corona at lower voltages than polished, clean conductors. Even small irregularities on the conductor surface can create localized high-field regions.
Conductor Diameter. Smaller conductors have higher surface electric field gradients for the same voltage, making them more prone to corona. Larger conductors distribute the electric field more evenly.
Voltage Level. Lines operating above 220 kV almost invariably require special consideration for corona control. At EHV and UHV levels, corona becomes a primary design driver.
Weather Conditions. Humidity, rain, fog, and snow all exacerbate corona formation. Water droplets on conductor surfaces distort the local electric field, creating transient high-field regions that promote ionization.
Air Density. Higher altitudes with lower air density reduce the dielectric strength of air, making corona more likely at lower voltages.
How Design Choices Mitigate Corona
The good news is that corona can be managed through thoughtful design. Here are the most effective strategies:
Increase Conductor Diameter
Using larger conductors is the most direct way to reduce surface electric field gradient. For a given voltage, a larger diameter means the electric field is spread over a greater surface area, raising the corona onset voltage.Use Bundled Conductors
For EHV and UHV lines, single conductors become impractical—they would need to be impractically large. Instead, engineers use bundled conductors: two or more sub-conductors per phase, separated by spacers.
Bundling increases the effective radius of the conductor bundle, which lowers the electric field intensity at the surface of each sub-conductor. Common configurations include:
220–400 kV: 2 or 3 sub-conductors
500–765 kV: 4 or 6 sub-conductors
1000+ kV (UHV): 6 or 8 sub-conductors
Each sub-conductor is typically separated by 30 to 45 cm. Twin-bundle conductors have been shown to effectively reduce the maximum surface electric field and improve field uniformity, increasing the corona inception margin. Beyond corona mitigation, bundling also reduces inductive reactance and improves voltage regulation.
Optimize Phase Arrangement
In double-circuit lines, changing the arrangement of phases between circuits can reduce corona losses at no additional cost during the design stage. Different phasing arrangements affect conductor surface gradients and ground-level electric fields in different ways.Use Corona Rings
Corona rings—also called grading rings—are toroidal metal rings installed at the ends of insulator strings. They smooth the electric field distribution and prevent localized high-field concentrations. Studies have confirmed that increased conductor spacing and the use of corona rings are among the most effective mitigation strategies.Maintain Conductor Surfaces
Keeping conductors clean and smooth reduces the likelihood of corona initiation at lower voltages. This is why routine inspection and maintenance are critical parts of line management.
Corona in the Context of Line Design
Corona management doesn't happen in isolation—it's one of many factors considered in the broader transmission line design process. As noted in the critical path steps for EHV line design, audible and radio noise analysis is a distinct step that must be addressed before final tower design.
There are also design trade-offs to consider. For example, in double-circuit lines, phasing arrangements that improve corona performance may increase ground-level electric fields, and vice versa. The use of underbuilt auxiliary conductors can help decouple these relationships, but adds cost and complexity.
Why This Matters for Your Career
Corona may be invisible to the naked eye, but its effects are anything but. Understanding how to predict, measure, and mitigate corona is a core competency for transmission line engineers, and it's exactly the kind of practical, industry-specific knowledge that isn't typically taught in university classrooms.
The power utility industry is poised for massive growth over the next two decades, and it needs professionals who understand real-world design challenges—not just textbook theory. Whether you're an engineer, a technician, or a project manager, mastering topics like corona management will set you apart.
Ready to Build Your Foundation?
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About the Instructor
Mike has been working for many years in the power utility industry, experiencing various roles and teaching engineering concepts to the public, fellow engineers, and power line professionals. After graduation, he discovered that much of the practical knowledge from the power utility world wasn't being taught in university courses—and he's made it his mission to change that. These courses teach real-life skills that are applicable to the industry and help students land their dream jobs. Mike promises you that there are no other courses out there as comprehensive and as well explained catering specifically to the power utility industry.
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