Acoustic Imaging for Power Line Corona Detection: A Practical Guide for Grid Maintenance

What Is Corona Discharge and Why Detect It with Sound?

Corona discharge is a partial electrical breakdown of air surrounding a high-voltage conductor. It occurs when the electric field strength at the conductor surface exceeds the dielectric strength of the surrounding air (approximately 30 kV/cm at sea level). Corona manifests as a faint blue-violet glow accompanied by a characteristic hissing or crackling sound, ozone production, and radio frequency interference. Left unchecked, corona can cause: insulation surface erosion, accelerated aging of polymer insulators, radio and TV interference, audible noise complaints, and power losses (typically 1–5 kW per kilometer on 230 kV lines).

Crews have traditionally used ultraviolet (UV) cameras or nighttime visual inspection to detect corona. However, UV cameras are expensive (typically $50,000–80,000+), and night inspections are logistically challenging on an energized line. Acoustic cameras offer a cost-effective alternative: the GSW Series Acoustic Camera from Unitech Tools costs a fraction of a UV camera and detects corona by its ultrasonic signature in real time, day or night.

Corona discharge produces strong ultrasonic emissions in the 20–100 kHz range. An acoustic camera with a multi-microphone array can detect these emissions from distances of 50 meters or more, providing a safe and effective corona inspection method for transmission lines.

The GSW Series provides the best balance of detection range, cost, and ease of use for wide-area transmission line surveys. Unlike a point ultrasonic detector that must be aimed precisely, the GSW’s 136-microphone array captures sound from a wide field of view simultaneously, making it dramatically faster for surveying long spans of line.

How Acoustic Cameras Detect Corona

Acoustic cameras work by detecting the ultrasonic sound waves produced by corona discharge. When a high-voltage conductor ionizes the surrounding air, the resulting micro-sparks and air breakdown generate broadband acoustic emissions spanning from audible hissing (2–20 kHz) to ultrasonic frequencies (20–100 kHz). The ultrasonic components are particularly useful because they attenuate less rapidly in open air and are less susceptible to background industrial noise.

The GSW Series Acoustic Camera uses a 136-microphone spiral array arranged in a precisely calibrated pattern. The camera applies beamforming algorithms to calculate the direction of arrival of sound waves at each microphone. By measuring the microsecond-level time differences across the array, the camera computes the exact azimuth and elevation of the sound source and overlays a colormap onto the visual image from the integrated optical camera. The result: the operator sees a “sound map” superimposed on the live video feed, with corona hot spots shown in red and orange.

Key specifications for corona detection include a frequency range of 2–65 kHz (capturing both audible and ultrasonic components), a detection distance of up to 50 meters for active corona sources, and real-time sound source localization at 25 frames per second. For particularly faint corona, the operator can adjust gain and narrow the frequency band to isolate the corona signature from ambient noise. The FA611S Acoustic Imaging Detector offers a compact form factor ideal for bucket-truck and drone inspections where weight and size are constraints.

Field Inspection Workflow for Transmission Lines

A typical transmission line corona inspection using an acoustic camera follows this workflow:

1. Pre-survey preparation: Review line drawings to identify known problem areas, weather conditions (ideally >70% humidity or light rain, which intensifies corona), and safety approach distances per NESC/IEC 61936.
2. Ground-level survey: Walk or drive the right-of-way with the GSW Series camera aimed at the conductor. The 136-microphone array captures corona from all conductors in the field of view simultaneously. Typical survey speed: 5–10 km of line per hour depending on terrain.
3. Detailed inspection: For suspect locations identified in the ground survey, perform a closer inspection from a bucket truck (minimum approach distance maintained). Focus on insulators, conductor splices, dead-ends, jumper loops, and shield wire hardware.
4. Documentation: Capture still images and video clips with acoustic overlay. The GSW Series records both acoustic and visual data simultaneously, including GPS coordinates, decibel levels, and frequency spectrum.
5. Reporting: Generate a corona report with severity ratings based on acoustic intensity. Typical severity: <20 dB above background (low/observe), 20–40 dB (moderate/schedule repair), >40 dB (severe/urgent repair).

For utilities managing large transmission networks, the GSW Series can survey up to 30 km of line per day depending on access conditions — a significant improvement over the 5–10 km achievable with point ultrasonic detectors.

One South American utility reported finding 47 active corona sources on a single 50 km, 230 kV line section during a two-day acoustic camera survey. All were confirmed during subsequent UV camera follow-up, demonstrating the reliability of acoustic detection for grid maintenance.

GSW Series vs FA611S: Choosing the Right Tool for Corona Detection

Unitech Tools offers two acoustic imaging solutions suitable for corona detection, each optimized for different inspection scenarios:

For most transmission line corona detection programs, the GSW Series is the recommended choice due to its wider frequency range (capturing more corona harmonics), greater detection distance, and larger field of view. The FA611S is an excellent complement for confined-space or aerial inspection use cases where weight matters.

Case Study: Corona Detection on 230 kV Lines in Colombia

A Colombian regional transmission utility managing 1,200 km of 230 kV and 500 kV lines across mountainous terrain implemented an acoustic corona inspection program in early 2026. Using the GSW Series acoustic camera, the utility’s maintenance team surveyed 400 km of 230 kV line over four weeks. The results: 183 active corona sources were identified — 92 on conductor splices (50%), 47 on insulator strings (26%), 29 on jumper loops (16%), and 15 on shield wire hardware (8%). Severity assessment: 21 sources were classified as urgent (acoustic intensity >40 dB above background), 87 as moderate (20–40 dB), and 75 as low.

The urgent items were repaired within two weeks. Post-repair acoustic surveys confirmed a 95% reduction in corona activity at repaired locations. The utility estimated that proactive corona management would extend insulator replacement intervals by 3–5 years and reduce annual power losses by approximately 1.2 GWh. The acoustic camera program cost was recovered within 8 months compared to the alternative of blanket insulator replacement.

This case study demonstrates that acoustic corona detection is not a theoretical concept — it is a field-proven method delivering measurable ROI for power utilities in developing and emerging markets where transmission infrastructure spans difficult terrain.

Integrating Acoustic Inspection into Grid Maintenance Programs

For maximum benefit, acoustic corona inspection should be integrated into a broader condition-based maintenance (CBM) program. Best practices include: establishing baseline acoustic signatures for each line section during commissioning or after major maintenance; conducting an annual acoustic survey of critical lines (those serving substations supplying industrial customers or densely populated areas); re-inspecting lines after storms, wildfires, or other events that may have damaged hardware; and correlating acoustic data with thermal imaging data from Hammer II thermal cameras to detect both corona-related heating and resistive heating from loose connections. Combining acoustic and thermal inspection in a single patrol yields a more complete picture of transmission line health. The Hammer II’s dual FOV (25° and 12°) allows operators to inspect detailed hardware and scan wide areas in a single patrol, while the GSW Series handles the acoustic side of the inspection.

For utilities in Latin America, IEC 60826 (Design criteria for overhead transmission lines) and local grid codes increasingly recommend or require corona surveys for lines above 138 kV. Integrating acoustic camera inspection into your maintenance schedule ensures compliance while reducing unplanned outages.

Limitations and Best Practices

While acoustic cameras are powerful tools for corona detection, they have limitations: detection range depends on wind speed (above 5 m/s, corona acoustic signatures scatter and become harder to localize); heavy rain generates its own ultrasonic noise that can mask corona signals; conductor grading rings and corona rings may partially shield acoustic emissions; and ambient noise from nearby traffic, industrial facilities, or transformers may raise the noise floor. Best practices to mitigate these factors include performing acoustic surveys in calm weather (wind <3 m/s preferred), avoiding times of heavy precipitation, using the frequency filter on the GSW Series to isolate the corona band (typically 20–60 kHz), familiarizing operators with typical corona acoustic signatures through training, and re-scheduling surveys when ambient acoustic noise exceeds acceptable thresholds.

Despite these limitations, acoustic cameras remain the most cost-effective and practical tool for routine transmission line corona detection — particularly for utilities in developing markets where UV cameras are prohibitively expensive and maintenance budgets are constrained.

The GSW Series acoustic camera from Unitech Tools makes corona detection accessible to any utility, regardless of budget. At a fraction of the cost of UV corona cameras, it provides reliable, field-proven corona detection for transmission line maintenance programs worldwide.