Visualization of partial discharges with ultrasound

ByCarlos Aroeira

This article describes how to visualize partial discharges with ultrasound within the scope of a predictive maintenance.

WHAT ARE PARTIAL DISCHARGE

Partial discharges (DP) are small internal electrical faults (or superficial) in insulation, and ultrasound equipment are one of the most practical ways to detect, measuring and locating these failures — especially in the field.

PHYSICAL PRINCIPLE

When a Partial Discharge Occurs, she generates:

  • rapid electrical impulses
  • mechanical vibrations in the insulating material
  • high frequency acoustic waves (typically 20 a 100 kHz, above human hearing)

Ultrasound sensors capture these waves and convert them into analyzable electrical signals.

Ultrasounds measure:

  • Sound pressure level
    • dBµV, dB SPL or relative units from the manufacturer
  • Acoustic energy
  • Signal temporal pattern
  • Dominant frequency

That is: is an indirect measurement, excellent for trending and comparison, not for normalized absolute value.

VISUALIZATION OF PARTIAL DISCHARGE WITH ULTRASOUND – HOW TO DO IT

Visualization can be done in several ways:

Numeric value

  • Ultrasonic signal intensity (the bigger, greater PD activity)

Graph over time

  • Repetitive pulses synchronized with the mains indicate PD
  • Random noises are often mechanical interference

Frequency spectrum

  • Partial discharges have typical “signatures”
  • Crown, tracking and arc have different patterns

Spatial location

  • Moving the sensor and comparing levels with simple ultrasound meters
  • Not SONASCREEN: acoustic triangulation (BEAMFORMING)

THE SONASCREEN ACOUSTIC CAMERA TO VISUALIZATION OF PARTIAL DISCHARGE WITH ULTRASOUND

Visualization of partial discharges with ultrasound

The portable acoustic camera SONASCREEN 2 is the tool for visualizing partial discharges with ultrasound.

The portable acoustic camera SONASCREEN 2 generates acoustic images from the audible and ultrasound frequency range. The device locates sound sources in real time and displays the results immediately on the screen. Besides that, the camera provides audible feedback through industrial headphones. We make ultrasounds audible and visible.

  • Reliable detection of leaks and partial discharges, even in noisy industrial environments and with larger sound sources present
  • Matrix optimized with 176 microphones and acoustic imaging 100 fps for accurate results in real time

  • Thermal Imaging Module (AND) integrated for more comprehensive diagnosis and analysis of recorded events
  • Integrated data acquisition and analysis software
  • Real-time acoustic results 100 fps
  • Wide range of frequencies (2 kHz – 100 kHz) for detecting audible sounds and ultrasound
  • Acoustic feedback through industrial headphones
  • Portable device with IP54 protection (splash proof)
  • 8 configurable buttons for quick control
  • Low weight and small size
  • Detection of fast-moving sound sources thanks to high resolution 100 fps and global shutter
  • In-depth recording and analysis: the only camera that allows further processing of recorded raw data
  • Portable device: low weight, excellent ergonomics and compact size
  • 8 configurable buttons for an intuitive workflow
  • 4 Different modes with predefined settings for intuitive operation without the need for extensive training
  • IP 54 / protection against water and dust: encapsulated microphones suitable for industrial use
  • HD multi-touch screen 7 inches

HOW TO USE SONASCREEN TO VISUALIZATION OF PARTIAL DISCHARGE WITH ULTRASOUND

In the video below you can see how this equipment is used to visualize partial discharges with ultrasound.

WHERE ULTRASOUND MEASUREMENT TECHNIQUE IS MOST EFFECTIVE

This technique is most effective on the following equipment:

  • Energized substations
  • Medium and high voltage panels
  • Insulators, bushings and disconnectors
  • GIS (in conjunction with UHF)
  • Transformers (with contact sensors)

limitations:

  • Sensitive to mechanical noise (vento, engines, vibration)
  • Does not replace electrical testing in the laboratory
  • It largely depends on the operator's experience

Typical workflow:

  1. Energized equipment
  2. Visualization with ultrasound sensor
  3. Identification of points with greater intensity
  4. Comparison with history (trend)

Confirmation with other methods (UHF, electric, thermography)

HOW TO DIFFERENTIATE CROWN, TRACKING AND ARCHING WITH ULTRASOUNDS

Sound is one of the quickest ways to distinguish corona, tracking and arc when using ultrasound. Each has a typical “acoustic signature”.

CROWN

It consists of the ionization of the air around energized conductors (usually spikes, edges, screws).

Som characteristic

  • Continuous wheezing
  • Reminds me of a gas leak or “ssssss”
  • Very stable over time

Ultrasound signature

  • Frequency: 30–60 kHz
  • Amplitude: low to average
  • Constant signal, little pulsating
  • Normally synchronized with voltage

Where does it appear

  • Buses
  • Connectors
  • Metal tips
  • Insulators in humid environments

When the camera moves away and the sound gently decreases, it's usually corona.

TRACKING

It consists of superficial discharges that “walk” over the contaminated insulation (dust, sal, humidity).

Som characteristic

  • Crepitus
  • Frying type, "I believe-I believe-I believe"
  • Intermittent

Ultrasound signature

  • Frequency: 20–80 kHz
  • Amplitude: variable
  • Irregular pulses
  • Evolves over time (gets more intense)

Where does it appear

  • Dirty insulators
  • Bushings
  • Cable terminations
  • Surfaces with moisture + Contamination

In practice, if the sound level “jumps” and changes intensity → tracking.

ELECTRIC ARC

The electric arc consists of a complete discharge between two potentials (serious failure).

The characteristic sound is:

  • Strong pops
  • "ON! YES! CRACK!”
  • It can be scary

Ultrasound signature

  • Frequency: long (wideband)
  • Amplitude: very high
  • Violent and random pulses
  • Does not need to be synchronized with the network

Where does it appear

  • Loose connections
  • Defective contacts
  • Switches and circuit breakers
  • Advanced tracking failures

In practice, if the signal saturates the equipment → imminent or active arc. Attention to security.

Quick Summary

TypeSeamBehaviorGravity
CrownContinuous wheezingStable⚠ Low
TrackingCrepitusIrregular⚠⚠ Average
ArcoDry cracklesExplosive🚨 Alta

It is very important to never use just your ear or just the numerical value. It must also be combined:

  • Sound in headphones
  • Graph over time
  • Frequency spectrum
    Visual inspection and thermography

HOW THE PATTERN CHANGES OVER TIME (CROWN → TRACKING → ARCO)

Behavior trend is very important for predictive diagnosis. In practice, crown, tracking and arcing are not isolated events, but stages of insulation degradation, as can be seen in the diagram below.

Next, it is shown how the acoustic pattern (ultrasound) evolves over time and what to observe in the field.

Internship 1 - Crown (start of degradation)

Insulation condition: still intact, but with a concentrated electric field.

Seam

  • Gentle, continuous hissing
  • Very repetitive

Pattern on ultrasound

  • Dominant frequency: 30–60 kHz
  • Amplitude: low and stable
  • Little variation over days/weeks

What is happening physically

  • Air ionization
  • Ozone and NOx production
  • Slow chemical attack on insulation

Risk

  • Low in the short term
  • Alto, in the long term, be ignored

Classic sign: the value in dB increases slowly month by month.

Internship 2 – Tracking (active degradation)

Insulation condition: already contaminated and charred surface.

Seam

  • Irregular crackling
  • “Electric frying”

Pattern on ultrasound

  • Widest band (20–80 kHz)
  • Floating amplitude
  • Intermittent pulses, non-uniform

What is happening physically

  • Formation of conductive paths
  • Repeated surface discharges
  • Localized heating

Risk

  • Moderate to high
  • Can evolve quickly (days or weeks)

Classic sign: “edgy” graphic, with peaks that appear and disappear.

Internship 3 – Arco (imminent or active failure)

Insulation condition: ruptured or almost ruptured.

Seam

  • Dry and strong cracking
  • Impulsive sounds

Pattern on ultrasound

  • Espectro wideband
  • Very high amplitude
  • Sensor saturation
  • Chaotic pulses

What is happening physically

  • Permanent ionized channel
  • High current
  • Thermal and mechanical erosion

Risk

  • Critical
  • May cause explosion, fire or immediate shutdown

Classic sign: abrupt increase in ultrasonic level in a short time.

 The typical evolution over time is as follows:

Corona ────────► Tracking ───► Arco

(years/months)        (weeks)       (hours/days)

This appears in the measurement history as follows:

  • Crown: smooth curve, slow growth
  • Tracking: irregular curve, with heels
  • Arco: almost vertical jump

This is why trend is more important than absolute value when measuring ultrasounds.

The recommended action at each stage is as follows:

InternshipRecommended Action
CrownMonitor + geometric correction
TrackingCleaning, drying, planned replacement
ArcoImmediate intervention

The field rule of thumb is as follows:

Continuous sound = time to plan
Intermittent sound = act soon
Explosive sound = stop everything

REAL EXAMPLES OF TRENDS IN DB

Below are examples of how the trend in dB appears in practice, as seen in ultrasound inspections according to IEC 62478.


Important: values ​​are real/typical of the field, but always relative to the equipment and gain used.

Example 1 — Corona stable (controlled situation)

History (same point, same setting)

MonthUltrasonic level
January18 dB
February19 dB
March20 dB
April20 dB

What is heard

  • Continuous wheezing
  • Clean sound, no pops

Interpretation

  • Slow and predictable growth
  • IEC framework: initial PARTIAL DISCHARGE activity
  • Action: monitor

Classic corona signature
Almost straight curve, no spikes.

Example 2 — Corona → Tracking (ongoing degradation)

History

WeekLevel
S122 dB
S224 dB
S329 dB
S427 dB
S533 dB

What is heard

  • Wheezing + occasional crackling
  • Intermittent sounds

Interpretation

  • Non-linear increase
  • Fluctuations → contamination / humidity
  • IEC framework: Partial surface discharge (tracking)

Yellow alert
Here a lot of people make the mistake of thinking it is “normal” because the value sometimes drops.

Example 3 — Advanced tracking (pre-failure)

History

DMC will be present at theLevel
D130 dB
D236 dB
D334 dB
D441 dB
D545 dB

What is heard

  • Strong frying
  • Frequent pulses

Interpretation

  • Accelerated trend
  • Great variability
  • IEC framework: active insulation degradation

Short intervention window
Plan stop now, not in the next cycle.

Example 4 — Abrupt jump → Arc imminent

History

HourLevel
08:0038 dB
10:0040 dB
12:0058 dB
12:05Saturation

What is heard

  • Dry crackles
  • Explosive sounds

Interpretation

  • Almost vertical increase
  • Outside the partial discharge regime (IEC 60270)
  • Active or imminent electrical arc

Immediate action

How to mentally draw curves

Crown:        ────────╮

Tracking:      ──╮─╯╮─╯╮

Arco:          ────│││⬆

Practical criteria (used in the field)

✔ +2 a 3 slow dB → normal aging
⚠ +5 a 10 dB in weeks → tracking
🚨 +10 dB in hours/days → imminent arc

(as long as regulation, sensor and distance are constant)

Rule of thumb for IEC reporting

Never write:

“the value reached 35 dB, therefore it is critical”

Always write:

“an increase in 13 dB relative to reference measurement, indicating progression of partial discharge activity in accordance with IEC 62478”

ULTRASOUNDS IN IEC STANDARDS

Overview of Where Ultrasound Enters IEC

IEC standards do not replace the classical electrical method, but recognize ultrasound as a complementary method, especially for:

  • online inspections
  • energized equipment
  • qualitative diagnosis + trend

The main ones are:

  • IEC 60270
  • IEC 62478

IEC 60270 — a base (electrical method)

What does she say:

  • Defines partial discharge as pulses measured in pC
  • Electric conventional method
  • Controlled environment (laboratory)

Connection with corona / tracking / arco

  • A IEC 60270 does not classify by “sound”
  • It provides the physical reference for partial discharge
  • Corona and tracking appear as patterns of electrical pulses
  • Arc usually already exceeds the scope of partial discharge

The translation in practice is as follows:
Use a IEC 60270 as a theoretical reference, but don't try to force pC into ultrasonic measurements.

IEC 62478 — where ultrasound officially comes in

This is the key norm for what was described.

What she recognizes:

  • Ultrasound as an unconventional method
  • Use in detection, location and trend analysis
  • Qualitative assessment of partial discharge activity

Direct connection with internships

Crown

  • IEC 62478: Low energy partial discharge
  • Ultrasound:
    • continuous signal
    • narrow band
  • Classification: initial partial discharge activity

Described as:

“incipient Partial Discharge activity detected by acoustic method”

Tracking

  • IEC 62478: Partial surface discharge
  • Associated with:
    • Contamination
    • insulation aging
  • Ultrasound:
    • irregular pulses
    • wider band

Described as:

“Surface Partial Discharge activity with progressive insulation degradation”

Arco

  • Here it is crucial
    The IEC makes it clear:

The electric arc is no longer a partial discharge

  • Ultrasound:
    • saturation
    • chaotic impulses
  • Normative classification:
    • dielectric failure
    • abnormal operating condition

The correct language to use is as follows:

“acoustic emission compatible with electric arc (beyond the Partial Discharge phase)

How to write this correctly in an IEC report

Example text 100% aligned with standards:

“Acoustic activity compatible with partial discharges was detected, according to non-conventional methods recognized by the IEC 62478.

The evolutionary pattern of the signal indicates progression of gaseous ionization (crown) for surface discharges (tracking), at risk of transition to electric arc, condition that exceeds the PARTIAL DISCHARGE regime defined in IEC 60270.”

Summary Table (in accordance with IEC)

InternshipIEC frameworkSituation
Crowngaseous partial dischargeMonitorable
TrackingPartial surface dischargeActive degradation
ArcoDielectric failureOutside the PARTIAL DISCHARGE regime

Common mistakes to avoid

PARTIAL DISCHARGE measured in dB”
Absolute value of ultrasound discharge”

Correct:

  • relative level
  • trend
  • correlation with IEC 60270

Normative rule of thumb

IEC 60270 defines what PARTIAL DISCHARGE is
IEC 62478 defines how to detect outside the laboratory

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