Audible and Radio Noise (RN) Analysis in Transmission Line Design
When you stand near a high-voltage transmission line on a damp morning, you might hear a faint crackling or hissing sound. On your car radio, you might notice a slight static interference as you drive underneath. These aren't signs of a faulty line—they're the audible and radio frequency (RF) noise generated by a natural electrical phenomenon called corona discharge.
As transmission voltages have increased to meet growing demand for electricity, managing these corona effects has become a critical design consideration. For EHV (extra-high voltage) and UHV (ultra-high voltage) lines, radio interference (RI) and audible noise (AN) are often ruling design factors. Understanding how to analyze and control these environmental impacts is essential knowledge for anyone working in transmission line engineering.
Let's break down what causes this noise, how engineers analyze it, and the design strategies used to keep it within acceptable limits.
What Causes Audible and Radio Noise?
The Corona Discharge
The root cause of both audible and radio noise is corona discharge. This occurs when the electric field intensity around a conductor exceeds a critical threshold—typically around 30 kV/cm in dry air—causing the localized ionization of surrounding air molecules.
Unlike a full arc or short circuit, corona is a partial breakdown of air. It doesn't cause a complete fault, but it does create a self-sustaining plasma region around the conductor. This manifests as:
A visible bluish glow in darkness
A hissing or crackling sound
Electromagnetic emissions that interfere with radio signals
Audible Noise
The audible noise you hear from transmission lines is primarily caused by the rapid ionization and deionization cycles of the air surrounding the conductor. These cycles occur at twice the power frequency (e.g., 100 Hz or 120 Hz), producing a characteristic buzzing or crackling sound in the range of 1 to 20 kHz.
Radio Interference
Radio interference (RI) is caused by electromagnetic emissions from ionized particles accelerating in the electric field. These emissions can interfere with AM radio, television signals, and other communication systems in the medium-frequency (MF) and high-frequency (HF) bands. Streamer corona, in particular, produces RI in the 0.5 to 1.6 MHz range.
Factors That Influence Noise Levels
The intensity of corona discharge—and thus the level of audible and radio noise—is affected by several variables:
Conductor Surface Condition
Rough, dirty, or weathered conductors initiate corona at lower voltages than polished, clean conductors. Surface irregularities create high local electric field concentrations that trigger ionization sooner. Even long-term operation can roughen conductor surfaces, increasing noise over time.
Conductor Size
Larger conductors distribute the electric field more evenly and thus raise the corona onset voltage. This is why conductor diameter is a critical design parameter.
Weather Conditions
Humidity, rain, fog, and snow significantly exacerbate corona formation. Water droplets on the conductor surface distort the local electric field and create transient high-field regions that are conducive to ionization. This is why audible noise is often more noticeable during wet weather.
Voltage Level
Voltages above 220 kV almost invariably require special consideration for corona control. As voltages increase, the electric field strength at the conductor surface increases, making corona more likely.
Air Density and Altitude
At higher altitudes, the lower air density reduces the corona inception voltage, meaning corona discharge and its associated noise become more serious.
Design Strategies for Noise Control
Engineers have developed several strategies to analyze and mitigate audible and radio noise in transmission line design. The goal is to balance performance, cost, and environmental impact.
1. Bundled Conductors
One of the most effective techniques for reducing corona effects is the use of bundled conductors—using multiple conductors per phase instead of a single large conductor.
A bundle of smaller conductors acts similarly to one large conductor in terms of electric field distribution but offers several advantages:
Reduces the electrical field on each subconductor
Lowers corona losses and noise
Improves thermal performance due to enhanced surface area
For EHV lines (typically above 300 kV), multiple conductors are almost always used in a bundle configuration.
2. Increasing Conductor Size
Simply using larger conductors is another way to reduce corona effects. A larger diameter distributes the electric field more evenly, raising the corona onset voltage.
3. Optimizing Phase Spacing and Height
Moving conductors further apart or increasing the height of the line can reduce corona effects. However, these measures come with significant cost implications. For example, raising a 735 kV tower cross-arm by just 1 meter has been known to increase the cost of a line by as much as 10%.
4. Special Conductor Coatings and Surface Treatments
Research has investigated the use of coatings and surface treatments to reduce corona discharge, particularly under rainy and foggy conditions when water droplets on conductors maximize corona activity. These treatments aim to make conductor surfaces more resistant to water accumulation and ionization.
5. Hardware and Insulator Selection
Insulators and hardware must also be designed to minimize corona effects. They are tested for Radio Influence Voltage (RIV) in laboratory environments, with limitations as low as 250 or 500 µV. Tension is maintained on all insulator assemblies to assure positive contact and avoid sparking.
Standards and Measurement
To ensure consistent evaluation and comparison of audible noise performance, the industry relies on established standards:
IEEE 656-2018 provides uniform procedures for the measurement of audible noise from overhead transmission lines
IEC TR 62681:2022 provides guidance on electromagnetic environment issues for HVDC overhead transmission lines, including radio interference and audible noise
These standards allow engineers to quantify noise levels and verify that designs meet regulatory requirements.
Why This Matters for Your Career
Corona discharge and its associated noise effects are not just academic concerns—they are practical design constraints that directly impact project costs, public acceptance, and regulatory compliance. For transmission line engineers, understanding how to analyze and mitigate audible and radio noise is essential.
The industry is experiencing rapid growth, with EHV and UHV transmission lines increasingly used to meet growing demand for electrical transmission capacity. This growth creates tremendous opportunities for professionals who understand these fundamental design principles.
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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.
