Infrared Thermography,UHF,TEV,HFCT,Ultrasonic sensors: Which test tells you more in PD Testing?

Infrared Thermography (IRT) and Partial Discharge (PD) testing are two widely adopted condition monitoring techniques in modern power systems, each with distinct applications and technical strengths. IRT detects surface temperature anomalies to identify potential faults caused by increased electrical resistance, such as loose connections, overloads, or deteriorated contacts. Based on passive thermal radiation measurement, IRT is simple to operate, allows for non-intrusive and live inspections, and is particularly effective for components like switchgear, busbars, and cable terminations. However, it has notable limitations: it can only detect faults that produce significant heat and cannot identify early-stage insulation degradation or internal defects. The results are also influenced by ambient temperature, surface emissivity settings, and operator experience. In contrast, PD testing detects small electrical discharges occurring within or on the surface of insulation systems. These discharges often indicate the onset of insulation breakdown, such as voids, cracks, surface contamination, or moisture ingress, allowing for much earlier detection of critical insulation failures.

PD testing employs various sensor technologies, including Ultra High Frequency (UHF), Transient Earth Voltage (TEV), High-Frequency Current Transformers (HFCT), and ultrasonic sensors. These systems analyze signal amplitude, phase, repetition rate, and waveform characteristics to assess discharge severity and type. PD signals typically exhibit short-duration, high-frequency pulses, and can propagate through metallic enclosures, making them suitable for metal-clad medium-voltage equipment. TEV measurements are effective for detecting internal discharges, while ultrasonic methods are better suited for surface or corona discharges. Unlike IRT, PD testing not only identifies defects before any heat is generated but also supports long-term condition-based monitoring and trending analysis. It is particularly applicable for critical equipment such as cable terminations, ring main units, GIS, and transformer windings. However, PD testing requires more sophisticated instrumentation and trained personnel to distinguish real signals from background noise and to minimize false positives.

According to international standards and industry best practices, IRT and PD testing should be used as complementary techniques. For instance, NFPA 70B in the United States recommends regular infrared and PD inspections for high-voltage equipment (>1000V). Similar guidelines are found in the UK and Australia for substations and critical assets. For metal-enclosed equipment, TEV and ultrasonic PD testing can be performed externally through the panel, while IRT is ideal for detecting heat-related issues in cable connectors, bus joints, and breaker contacts. In older systems or environments with significant electromagnetic interference, combined techniques (e.g., simultaneous TEV and ultrasonic monitoring) enhance diagnostic accuracy and confidence in results.

In summary, IRT is well-suited for identifying resistive faults and surface heating, while PD testing is more effective at detecting internal insulation degradation at an early stage. Integrating both methods enables a comprehensive asset monitoring strategy, combining surface thermal detection with internal dielectric diagnostics. Industry standards recommend joint application, and the reliability of the results depends heavily on the competency of the operators. Certified training (e.g., FLIR Level I/II for thermography or PD specialist training from EA Technology) is strongly recommended. By selecting the appropriate method based on asset type, voltage level, and environmental conditions, utilities and asset managers can significantly improve system reliability and reduce the risk of unplanned outages and catastrophic failures.

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