9.3 Ultrasonic, Corona, and Vibration Survey Techniques

Ultrasonic, Corona, and Vibration Survey Techniques

Quick Answer: Infrared finds resistive heating. It is largely blind to partial discharge, corona, and surface tracking, which release very little energy as heat but a great deal as high-frequency sound and ultraviolet light. A survey program that relies on infrared alone will miss the entire class of insulation defects that kills medium-voltage switchgear.

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1. Why infrared is not enough

A loose connection converts electrical energy into heat, and heat is what a camera sees. A partial discharge inside a switchgear compartment converts electrical energy into acoustic energy, light, ozone, and localized erosion of the insulation — with a thermal signature so small it is generally undetectable through an open door, let alone a viewing window.

The failure progression is real and fast: a small void or a contaminated surface begins discharging, the discharge erodes a track, the track grows, and the eventual result is a phase-to-ground or phase-to-phase flashover inside energized switchgear. The complementary technologies exist because that progression is invisible to a thermal camera until the moment it is not.

2. Airborne ultrasound

What it detects. Electrical discharge produces broadband sound extending well above the audible range. An ultrasonic detector listens in a narrow band, typically centred around 40 kHz, and heterodynes it down into the audible range so the technician can hear it in headphones.

Listening at 40 kHz rather than in the audible band is the key design choice: it sits above almost all mechanical and environmental background noise, so the signal-to-noise ratio for a small discharge is enormously better than it would be at audible frequencies.

What it finds:

Distinguishing corona from tracking and arcing is the interpretive skill. Corona is steady and periodic because it re-ignites at the same point on each half cycle. Tracking and arcing are erratic because they follow a physical path that changes as it erodes. Instruments that display the signal against a 60 Hz phase reference make this distinction visible rather than merely audible, and a discharge that is locked to the power frequency and shows a characteristic double-hump pattern per cycle is a classic corona signature.

Field technique:

• Use the contact (structure-borne) probe for enclosed equipment where sound cannot escape into the air, and the airborne dish or open probe for radiated sound.
• Scan the seams — door gaps, ventilation louvres, cable entries, bushing boots, viewing windows. Ultrasound leaks out of openings, so the loudest point at a seam localizes the source inside.
• Compare across phases and across identical cubicles, exactly as with infrared.
• Ultrasound works through a closed door where infrared cannot, because sound escapes through gaps. This makes it a genuinely lower-risk survey tool: much of the scan can be performed without opening an enclosure.

3. Corona and ultraviolet imaging

Corona is partial ionization of the air adjacent to a conductor where the local electric field exceeds the breakdown strength of air, without a complete breakdown across the gap. It occurs at sharp points, damaged strands, contaminated insulators, poor terminations, and hardware with inadequate corona shielding.

Why it matters:

• It erodes. Corona chemically attacks polymer insulation and hardware over time.
• It produces ozone (O₃) and, with moisture, nitric acid, both of which are corrosive to insulation, metals, and equipment.
• It generates radio interference and audible noise.
• It is an early indicator — corona often precedes tracking, which precedes flashover.

The smell test is a genuine field technique. The sharp, sweet, chlorine-like smell of ozone at a switchgear lineup is a discharge indicator. It is also a warning to investigate before opening the enclosure — an ozone smell means active discharge inside energized equipment, and opening the door casually is exactly the wrong response. Also look for white or grey powder deposits (nitric acid reaction products), surface tracking marks, and pitting on hardware.

Solar-blind UV cameras. Corona emits ultraviolet light, but sunlight swamps most of the UV band. The solar-blind window is a narrow band around 240-280 nm in which atmospheric ozone absorbs virtually all solar UV before it reaches ground level. A camera filtered to that band sees essentially no background from the sun, so a corona discharge appears as a bright, unambiguous point even in full daylight. Modern instruments overlay the UV image on a visible image so the discharge is located precisely on the hardware.

This is the standard tool for outdoor substation and transmission hardware inspection, where the equipment is inaccessible and the defect produces no heat.

4. Where each technology belongs

The programme point: these are complements, not alternatives. A defect that one technology cannot see is often the one the other detects first.

5. Vibration analysis on rotating machinery

Electrical testing establishes the condition of a motor's insulation and windings. It says almost nothing about its mechanical condition, and most motors are removed from service for mechanical reasons.

The diagnostic principle: different faults produce vibration at different, identifiable frequencies expressed as multiples of running speed (1×, 2×, and so on).

Two signatures worth committing to memory because they distinguish electrical from mechanical causes:

• 2× line frequency (120 Hz) vibration points to an electrical cause — stator eccentricity, unbalanced supply, shorted laminations. The give-away test: remove power and watch. Electrically induced vibration disappears instantly; mechanically induced vibration coasts down with the rotor.
• Pole-pass sidebands around the running speed peak indicate broken or cracked rotor bars, which is a fault that no insulation test detects at all and which shows up as gradual loss of torque and eventual failure to start under load.

Bearing current damage is an increasingly common finding on VFD-driven motors: common-mode voltage from the inverter drives current through the bearing, producing electrical discharge machining that leaves a characteristic fluting pattern on the race. Detection combines vibration analysis with shaft voltage measurement; the remedies are insulated bearings, shaft grounding rings, and output filters. This is a genuinely modern failure mode and a good Level IV analysis scenario, because the symptom is mechanical and the cause is electrical.

Exam trap: A question describes a switchgear compartment where an infrared survey through the viewing window found nothing, yet operators report a sharp ozone smell. Infrared is largely blind to partial discharge because it releases very little heat. The indicated tools are ultrasonic detection and, where the hardware is visible, UV corona imaging — and the enclosure should not be opened casually while active discharge is suspected.