A partial discharge event produces several measurable effects that engineers can use to detect and diagnose insulation defects. Depending on the asset and operating environment, engineers may monitor electrical current pulses, electromagnetic emissions, or acoustic energy to assess insulation condition. Because these signals propagate differently through different types of equipment, manufacturers and researchers have developed multiple detection technologies to capture them effectively.
This is where partial discharge detection methods play a significant role. Each technique detects a different physical effect associated with partial discharge, making it suitable for specific assets and diagnostic objectives.
The choice between online and offline PD testing determines when and under what operating conditions engineers assess an asset.
The detection method determines how engineers capture, analyse, and interpret partial discharge activity.
That distinction is important because a single partial discharge event can generate multiple measurable signals, each requiring a different detection approach.
One Partial Discharge Event, Multiple Detectable Signals
Although partial discharge is a single event, it leaves behind multiple detectable clues. It generates electrical, electromagnetic, and acoustic effects that can be captured using different monitoring technologies.
Partial discharge event → electrical current pulse → electromagnetic emission → acoustic emission
This is why UHF, TEV, HFCT, and ultrasonic methods all exist. Rather than detecting the same thing in the same way, each method measures a different signature of the discharge and is therefore suited to different assets and diagnostic objectives.
How UHF, TEV, HFCT and Ultrasonic Detection Differ
Although all four methods are used to detect partial discharge, they measure different discharge signatures. This influences where each technology is most effective and how it is applied in practice.
The following sections examine what each method detects, how it works, and where it fits within a partial discharge monitoring strategy.
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UHF — Electromagnetic Detection for GIS and Enclosed HV Assets
What does UHF detect?
UHF partial discharge detection captures the ultra-high-frequency electromagnetic energy produced by rapid PD pulses. Instead of measuring the discharge current directly, the sensor detects electromagnetic emissions generated by the event.
How does it work?
When PD occurs, electromagnetic energy propagates away from the source. In enclosed equipment such as gas-insulated switchgear (GIS), part of this energy can propagate through the metallic enclosure and reach appropriately positioned UHF sensors. The measured signal can then be analysed to identify and trend PD activity.
Detection performance depends on the complete signal path. Enclosure geometry, sensor position, attenuation, internal barriers, and the location of the discharge can all influence how much electromagnetic energy reaches the sensor.
Where does UHF fit best?
UHF is particularly well established for GIS, where the enclosed construction provides an environment suited to high-frequency electromagnetic detection. It can be used for diagnostic measurements as well as permanently installed online monitoring of critical GIS assets.
For example, UHF sensors installed across critical GIS bays can provide ongoing visibility into PD activity without requiring direct access to energized internal components. Changes in the detected activity can then be investigated as part of the asset’s condition-monitoring strategy.
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TEV — Non-Intrusive Detection for Metal-Enclosed Switchgear
What does TEV detect?
TEV partial discharge detection measures transient voltages that appear on the external surface of grounded metal enclosures because of high-frequency PD activity inside the equipment.
How does it work?
When partial discharge occurs inside metal-enclosed switchgear, the resulting electromagnetic energy can propagate through the enclosure. At seams, joints, openings, and other discontinuities, part of that energy can couple onto the external metal surface as a transient voltage.
A capacitively coupled TEV sensor placed against the enclosure detects these fast transients. Because the measurement can be taken externally, the technique allows engineers to assess energized equipment without opening the switchgear.
The measured TEV signal is influenced by how effectively energy propagates from the partial discharge source to the measurement location. External electrical interference can also create transient signals, making repeatability, location, comparison with adjacent panels, and supporting diagnostic evidence important during interpretation.
Where does TEV fit best?
TEV is particularly useful for non-intrusive screening of metal-enclosed MV switchgear. Maintenance teams can survey multiple panels and identify equipment showing unusual or repeatable activity that warrants closer investigation.
For example, consistently elevated TEV activity on one panel compared with neighbouring panels can help engineers narrow the investigation before deciding whether additional partial discharge measurements or maintenance are required.
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HFCT — Current-Pulse Detection for Power Cables
What does HFCT detect?
HFCT partial discharge detection captures the high-frequency current pulses produced by partial discharge as they propagate through suitable conductive paths.
How does it work?
A high-frequency current transformer (HFCT) is installed around an accessible conductor carrying the partial discharge current, such as a cable screen earth or grounding connection. The sensor couples to the high-frequency component of the current without requiring direct electrical connection to the energized conductor.
The current path is fundamental to detection. If the partial discharge pulse does not propagate through the conductor enclosed by the HFCT, or if substantial attenuation occurs before it reaches the measurement point, the detected signal may be weak even when partial discharge is present.
Sensor location therefore forms part of the measurement design rather than being simply an installation consideration.
Where does HFCT fit best?
HFCT is particularly useful for power cables and cable accessories because cable screens and grounding connections can provide accessible paths for propagating PD currents. It can support online measurements as well as permanently installed monitoring where continuous condition visibility is required.
For example, HFCT sensors installed at suitable cable grounding points can monitor critical cable circuits over time, allowing maintenance teams to identify changes in PD activity without taking the circuit out of service.
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Ultrasonic — Acoustic Detection for Accessible PD Sources
What does ultrasonic detection measure?
Ultrasonic partial discharge detection captures the acoustic energy generated by a discharge event. Unlike UHF, TEV, and HFCT measurements, it provides a non-electrical measurement of partial discharge activity.
How does it work?
A discharge creates a rapid localized release of energy that produces pressure waves. Depending on the asset, these waves can propagate through air, insulating gas, liquid, or solid structures before reaching an acoustic sensor.
The acoustic path determines how effectively the discharge can be detected. Distance, structural interfaces, equipment geometry, and the material through which the wave travels can attenuate or reflect the signal. Mechanical activity can also produce competing acoustic signals that need to be distinguished from genuine PD.
Because it measures a different physical effect, ultrasonic detection can also complement electrical or electromagnetic techniques.
Where does ultrasonic detection fit best?
Ultrasonic techniques can be useful for switchgear, terminations, and other equipment where the discharge has a viable acoustic path to an accessible measurement point. They can also assist with confirming or localizing PD detected using another method.
For example, TEV measurements may identify suspicious activity around an MV switchgear compartment while ultrasonic measurements detect acoustic activity in the same region. Agreement between two independent measurement principles provides stronger evidence for further investigation than either measurement considered alone.
The choice between UHF, TEV, HFCT, and ultrasonic detection therefore starts with the asset rather than the sensor. Engineers need to understand what signal the discharge is likely to produce, how that signal will propagate through the equipment, and whether it can reach the selected measurement point with enough integrity to be useful. Only then can sensor capability, sensitivity, and monitoring configuration be evaluated meaningfully.
Why There Is No Single Best Partial Discharge Detection Method
The effectiveness of a partial discharge detection method depends on more than sensor sensitivity. Asset construction, probable defect location, signal propagation, electrical interference, and the purpose of the measurement all influence whether a partial discharge event can be detected reliably.
For example, UHF detection is well suited to GIS because electromagnetic signals can propagate within the enclosed structure and reach appropriately positioned sensors. An HFCT may be more practical for a power cable where high-frequency partial discharge currents can be measured through an accessible cable screen or grounding connection. Neither technology is inherently better; each solves a different measurement problem.
When selecting a partial discharge detection method, engineers should consider:
- Asset construction:
The physical design of the equipment determines where sensors can be installed and how PD signals can propagate. A method suitable for GIS may not provide the same visibility on a cable circuit or metal-enclosed switchgear. - Probable discharge location:
The likely position of the insulation defect influences which physical signal can reach an accessible measurement point. A deeply enclosed source may require a different detection approach from activity occurring near a termination or external surface. - Signal propagation path:
A sensitive sensor provides limited value if the electrical, electromagnetic, or acoustic signal is heavily attenuated before reaching it. The complete path from the PD source to the sensor must therefore be considered. - Interference environment:
Electrical noise can complicate TEV, UHF, or HFCT measurements, while mechanical and background acoustic activity can affect ultrasonic detection. The selected method must provide sufficient discrimination between genuine PD and interference. - Monitoring objective:
Screening, source localization, detailed diagnosis, and continuous condition monitoring do not necessarily require the same measurement approach. The method should match the decision engineers need the data to support. - Need for complementary measurements:
In some cases, a second detection principle can reduce diagnostic uncertainty. For example, TEV may identify abnormal activity on an MV switchgear enclosure while ultrasonic detection provides independent acoustic evidence from the same region.
The objective is therefore not to identify the most sensitive or sophisticated partial discharge sensor. It is to select a detection approach that provides reliable visibility into the relevant discharge activity and enough diagnostic confidence to support the next maintenance decision.
Using Partial Discharge Monitoring to Assess Asset Health
Reliable detection is only the first step in understanding insulation condition. Detecting partial discharge answers an important question: is abnormal discharge activity present? For maintenance teams, however, that is only the beginning. A single measurement does not establish whether the condition is stable, deteriorating, or becoming more significant under operating conditions.
Continuous measurements provide that missing context.
By trending partial discharge activity over time, engineers can evaluate changes in magnitude, repetition rate, pattern, and correlation with factors such as loading or environmental conditions. A developing trend may justify closer investigation even when individual measurements have not crossed a predefined alarm threshold.
Continuous monitoring is particularly valuable for critical assets because deterioration does not necessarily align with inspection schedules. Maintaining visibility between periodic tests allows changes in partial discharge behaviour to be identified earlier and evaluated before the next planned maintenance window.
The maintenance decision should then combine partial discharge information with asset context. Source location, discharge characteristics, rate of change, equipment criticality, maintenance history, and the consequence of failure all influence whether an asset should continue to be monitored, undergo further diagnostic testing, or be prioritized for intervention.
This is where Rugged Monitoring’s solutions contribute to the wider condition-monitoring strategy. By matching sensing technologies to the electrical asset and continuously capturing relevant partial discharge activity, the monitoring system can provide the trend information needed to understand how insulation condition is changing over time.
