Acoustic Camera for Power Line Corona Detection

What Is Corona Discharge on a Power Line?

Corona is the electrical breakdown of air around a conductor or hardware that is carrying high voltage. When the electric field at a sharp point — a chipped insulator edge, a burr on a conductor, a damaged grading ring, or a loose damper — exceeds the breakdown strength of air, the surrounding gas ionizes and produces a faint discharge. On a 69 kV line it is usually harmless. On a 230 kV or 500 kV line, sustained corona is a real operating problem.

Three things make corona worth finding. First, it wastes energy: every corona point is a small but constant power loss that adds up across a long circuit. Second, it attacks the asset: corona produces ozone and nitric acid compounds that erode polymer insulators and accelerate tracking on porcelain. Third, it interferes with communications — corona is one of the main sources of radio-frequency interference (RFI) reported by utilities, and it can also trigger false readings in nearby monitoring systems. A transmission team that can locate corona early and schedule maintenance avoids all three costs.

"We used to find corona by night patrols with UV cameras — slow, weather-dependent, and only after the problem was already loud on the radio spectrum. An acoustic camera finds the same discharge in daylight, in minutes, from the ground." — Line Maintenance Engineer, 500 kV corridor, Brazil

Why Thermal Cameras Miss Corona — and Acoustic Cameras Find It

Thermal imaging is the workhorse of substation inspection, but it is the wrong tool for corona on open transmission lines. The discharge itself releases very little heat, and the conductor surface is often hotter than the corona point — in sunlight, a loaded conductor can run 30–40 °C above ambient, completely masking any corona heating. That is why corona patrols historically needed ultraviolet (UV) cameras or night-time visual inspection, both of which are expensive, limited in range, and highly sensitive to weather and daylight.

Corona is, however, an excellent ultrasonic source. Every corona pulse produces a sharp acoustic transient with energy spread across the 20–60 kHz band, well above the audible range of a human inspector and above most industrial noise. A modern acoustic camera with a large microphone array hears this signature, localizes it to a specific insulator or fitting, and overlays the source on a live optical image. The result is a daytime, ground-based patrol that finds corona points you can photograph, measure, and prioritize.

The GSW Series and SW136 from Unitech Tools combine a 136-microphone acoustic array with real-time spectrum display, which is exactly what corona detection needs: the array locates the source, and the spectrum confirms the 50/60 Hz-synchronized crackle pattern that distinguishes corona from a mechanical vibration or a leak.

The Acoustic Signature of Corona: What to Listen For

Not every ultrasonic hotspot on a transmission line is corona. The acoustic image is the starting point; the spectrum is the verdict. Corona has three distinguishing features:

• Bursts synchronized with the power frequency. Corona pulses occur near the voltage peaks of the 50 or 60 Hz cycle, so the acoustic energy arrives in a rapid, repetitive crackle — typically in a 100 Hz (or 120 Hz) envelope. Partial discharge inside substation equipment shares this rhythm, which is why the same camera models serve both applications.
• Energy concentrated in the high-frequency band. Corona radiates strongly above 20 kHz, often peaking between 30 and 60 kHz. A camera that only listens in the low ultrasonic band will miss it or confuse it with mechanical noise.
• Stability with position. Move the camera across the conductor and the hotspot stays locked to the physical defect. Mechanical noise moves with the whole structure; corona stays pinned to the point where the field concentrates.

Wind is the main nuisance factor. Strong wind excites the conductors and produces broadband noise that can bury a weak corona signature. Early morning, after rain, and in still air give the cleanest readings — which conveniently matches the conditions when corona is strongest, since wet and contaminated surfaces discharge more readily.

Where Corona Forms: Insulators, Conductors and Hardware

Corona is not random. It concentrates wherever the electric field is locally intensified, and experienced patrol teams know exactly where to aim:

• Insulator strings. Chipped, cracked, or contaminated glass and porcelain insulators, and aged polymer insulators with tracking, are the most common corona sources on transmission lines. The discharge usually appears at the energized end of the string.
• Conductor surfaces. Burrs, nicks, and damage from handling or lightning create sharp points on the conductor. Corona forms at the damaged spot, not along the healthy run.
• Splices and compression fittings. A poorly installed splice leaves a sharp edge or a gap in the field grading — a classic corona point that also risks becoming a hot joint.
• Hardware: dampers, spacers, clamps, grading rings. Loose or corroded fittings, and rings that are damaged or misaligned, generate corona at the air gaps they create.
• Transition points. Where a line connects to a substation bus, a cable termination, or an arrester lead, the field concentration is highest and corona is most likely.

An efficient patrol scans these known risk points first, then sweeps the spans between them. On a 20 km corridor, a two-person ground team with an acoustic camera can typically survey the critical hardware in a day; the same survey by UV helicopter patrol costs an order of magnitude more.

Corona Defects and Their Acoustic Signatures

The pattern to remember: corona lives in the high-frequency band and pulses with the line frequency. The SW136 displays the live spectrum, so the operator sees the 120 Hz envelope and the 30–60 kHz energy before classifying the source — no guesswork, no UV night patrol required.

Choosing an Acoustic Camera for Transmission Inspection

For transmission corridors where the camera must see conductors tens of meters away, the SW136 is the natural choice: its 0.3–130 m detection range and high-frequency bandwidth are built for distance work. Teams that also patrol substations on the same route prefer the GSW Series, which combines the acoustic array with a 640×512 thermal imager so a single survey finds corona, hot joints, and partial discharge in one pass.

A Field Workflow for Corona Surveys

1. Pick the conditions. Survey in still, dry air when possible. Fog and rain increase corona activity (good for detection) but wind buries the signature in mechanical noise.
2. Scan the risk points first. Insulator strings at the energized end, splices, dampers, spacers, and grading rings — in that order.
3. Localize with the array. Sweep the camera along the span until the hotspot appears on the optical image, then hold position to let the source stabilize.
4. Confirm with the spectrum. Check that the energy sits above 20 kHz and pulses with the line frequency. If it is broadband and low-frequency, suspect wind or mechanical noise instead.
5. Record distance and level. Note the inspection distance and the displayed level. Keep the distance constant when comparing multiple points on the same circuit.
6. Compare healthy twins. Read a neighboring healthy insulator or fitting of the same type. The delta between healthy and suspect is the most reliable severity indicator.
7. Prioritize repairs. Combine acoustic level, defect location, and line criticality to schedule maintenance — corona on a critical 500 kV tie-line outranks a mild point on a 69 kV lateral.

Corona in Latin America: Altitude, Salt and 60 Hz

Latin American utilities face two factors that make corona more common and more damaging than in temperate regions. The first is altitude: transmission corridors in the Andes run at 2,500–4,500 meters, where air density is low and the breakdown voltage of air drops — corona starts at lower voltages and burns hotter at the same service voltage. Utilities in Colombia, Peru, and Bolivia routinely fight corona on lines that behave normally at sea level. The second is coastal salt contamination: salt-laden fog on the coasts of Brazil, Mexico, and Chile deposits conductive film on insulators, intensifying surface discharge and accelerating tracking.

Both conditions make periodic corona surveys a genuine operational need, not a luxury. An acoustic-camera patrol program gives line maintenance teams the same early-warning capability that PD testing gives substation crews — performed in daylight, from the ground, by the maintenance staff they already have.

Field rule: if you operate transmission lines above 2,000 meters or within 10 km of a saltwater coast, schedule a corona survey every 12 months — more often if you already have RFI complaints from the local community.

The Bottom Line

Corona is one of the few high-voltage defects that thermal cameras cannot see reliably — but it is perfectly visible to a 136-microphone acoustic camera. The GSW Series, SW136, and FA611S from Unitech Tools turn a daylight, ground-based patrol into a corona survey: the array localizes the discharge, the spectrum confirms it, and the GPS-tagged image gives your maintenance planner everything needed to schedule the repair. Add a corona program to your line inspection routine, and you stop losing energy, radio spectrum, and insulator life to a discharge you never saw.