Understanding TEV and Ultrasonic PD Detection Methods for Distribution Equipment

Introduction

Partial discharge (PD) is one of the most significant early warning indicators of insulation degradation in medium-voltage (MV) and high-voltage (HV) distribution equipment. Left undetected, PD activity can progressively erode insulation materials, eventually leading to catastrophic equipment failure, unplanned outages, and substantial financial losses. Among the various PD detection technologies available today, two methods have emerged as particularly effective for field applications: Transient Earth Voltage (TEV) detection and ultrasonic (acoustic emission) detection. This article provides a comprehensive technical comparison of these two methods, examining their underlying physics, practical deployment considerations, sensitivity profiles, and optimal application scenarios for distribution switchgear, ring main units (RMUs), and related assets.

Understanding Partial Discharge in Distribution Equipment

Partial discharge refers to localized electrical discharges that only partially bridge the insulation between conductors. In MV/HV distribution equipment—including air-insulated switchgear (AIS), gas-insulated switchgear (GIS), ring main units, cable terminations, and busbar systems—PD can originate from multiple sources: voids in solid insulation, surface tracking across contaminated insulators, floating metal particles, protrusions on high-voltage conductors, and delamination within epoxy resin components.

The physics of PD generation involves the rapid acceleration of charge carriers within a high-electric-field region. When the local electric field strength exceeds the dielectric breakdown strength of the insulating medium (gas, liquid, or solid), an avalanche breakdown occurs across the defect. This event generates several measurable physical phenomena simultaneously:

• Electromagnetic transient signals propagating along the metallic enclosure surfaces
• Acoustic emissions in the ultrasonic frequency range (typically 20 kHz to 300 kHz)
• Optical emissions (ultraviolet and visible light flashes)
• Chemical byproducts such as ozone and nitrogen oxides in air-insulated equipment
• High-frequency current pulses measurable on grounding conductors

TEV sensors capture the electromagnetic transients while ultrasonic sensors capture the acoustic emissions. Each detection method has distinct advantages and limitations that make them complementary rather than competitive technologies when properly deployed in the field.

TEV Detection: Principles and Technology

Physical Mechanism

When a partial discharge occurs within a metal-clad switchgear enclosure, the electromagnetic wave generated by the discharge propagates outward along the internal surface of the metallic housing. At any discontinuity in the metal enclosure—such as gasketed joints, viewing windows, or cable entry points—a transient voltage pulse appears on the external surface of the enclosure. This phenomenon, known as the Transient Earth Voltage effect, was first characterized by researchers at the UK Electricity Council Research Centre in the 1970s.

The TEV signal typically has a very short rise time (on the order of nanoseconds) and contains frequency components ranging from approximately 3 MHz to 100 MHz. The pulse amplitude, measured in mV or dBmV, correlates with the severity of the discharge activity. Because the signal propagates along the entire metallic enclosure, a single TEV sensor mounted on the external surface of the switchgear can detect PD activity occurring anywhere within the internal volume of that panel section.

Sensor Design and Signal Processing

Modern TEV sensors employ capacitive coupling plates that are pressed against the outer surface of the switchgear enclosure. The sensor detects the transient voltage difference between the enclosure surface and earth. Advanced signal processing techniques including pulse shape analysis, phase-resolved partial discharge (PRPD) pattern recognition, and time-of-flight measurements enable practitioners to not only detect PD but also characterize its type (internal discharge, surface discharge, or corona) and estimate its location.

TEV measurements are typically expressed in dBmV, with values below 20 dBmV generally considered normal background noise, values between 20-30 dBmV indicating elevated PD activity requiring monitoring, and values above 30 dBmV representing critical conditions warranting immediate investigation.

Key Advantages of TEV Detection

• Non-intrusive measurement: TEV sensors are applied externally to the switchgear enclosure, requiring no shutdown or access to energized components
• Whole-panel coverage: A single measurement point can detect PD anywhere within the metallic compartment
• Quantitative trending: TEV readings can be trended over time to assess insulation degradation rates
• Online monitoring capability: TEV sensors can be permanently installed for continuous condition monitoring, as demonstrated by products like the Integrated Online Monitoring Device for PD, Temperature, and Leakage Current which combines TEV and ultrasonic dual detection for comprehensive switchgear monitoring
• Rapid screening: Entire substations can be surveyed quickly using handheld TEV instruments

Limitations of TEV Detection

• Susceptibility to electromagnetic interference (EMI): External RF sources including radio transmitters, variable frequency drives, and arcing contacts can generate false TEV readings
• Attenuation across gaskets: The TEV signal attenuates at each gasket joint, making it difficult to detect PD in panels that are not electrically continuous with the sensor location
• Limited spatial resolution: TEV can identify which panel contains PD but cannot pinpoint the exact component or location within the panel
• Sensitivity to external noise: In electrically noisy environments such as industrial plants, distinguishing PD from background EMI requires skilled interpretation

Ultrasonic PD Detection: Principles and Technology

Physical Mechanism

When partial discharge occurs in air-insulated equipment, the rapid expansion and contraction of the ionized discharge channel generates pressure waves that propagate through the surrounding medium. These acoustic emissions span a broad frequency spectrum, but the most diagnostically useful frequency range lies in the ultrasonic band from approximately 20 kHz to 300 kHz—well above the audible range of human hearing but below the frequencies where atmospheric absorption becomes excessive.

Different types of PD produce characteristic acoustic signatures. Surface discharge and corona activity generate relatively low-amplitude, broadband acoustic signals, while internal discharges within solid insulation produce sharper, more impulsive acoustic transients. Tracking discharge across contaminated insulation surfaces typically produces a distinctive crackling pattern in the ultrasonic domain.

Sensor Design and Signal Processing

Ultrasonic PD detectors employ piezoelectric transducers optimized for sensitivity in the 40 kHz range, which represents a practical compromise between detection sensitivity and immunity to audible-frequency background noise. Advanced instruments incorporate parabolic reflectors or acoustic horns to provide directional sensitivity, enabling operators to scan switchgear panels and localize PD sources with considerable precision.

Modern ultrasonic detection systems convert the high-frequency acoustic signals into audible sounds through heterodyne down-conversion, allowing experienced technicians to audibly identify different PD types based on their characteristic sonic patterns. Quantitative measurements are typically displayed in dB V, and phase-resolved analysis can further characterize discharge behavior.

Key Advantages of Ultrasonic Detection

• Immunity to electromagnetic interference: Ultrasonic detection is fundamentally immune to EMI, making it the preferred method in electrically noisy environments
• Precise source localization: The directional nature of ultrasonic sensors allows technicians to pinpoint the exact switchgear compartment, bushing, or component experiencing PD
• PD type discrimination: Experienced operators can distinguish between corona, surface discharge, and internal discharge based on acoustic characteristics
• Detection in non-metallic enclosures: Unlike TEV which requires a conductive surface, ultrasonic detection works on any enclosure material including plastic and composite housings
• Versatile deployment: As demonstrated by the SCYC-CW30 Passive Wireless Temperature Monitoring System, ultrasonic sensors can be integrated into comprehensive condition monitoring platforms alongside other sensor modalities

Limitations of Ultrasonic Detection

• Limited penetration through solid barriers: Ultrasonic signals cannot penetrate metal switchgear walls effectively, requiring open panel doors or dedicated inspection ports for internal PD detection
• Attenuation with distance: Acoustic signals attenuate rapidly in air, limiting effective detection range to within a few meters of the source
• Background acoustic noise: Compressed air leaks, cooling fans, and mechanical vibration can mask PD ultrasonic signatures in industrial settings
• Lower sensitivity for internal solid insulation defects: Discharges occurring deep within epoxy or XLPE insulation produce weaker acoustic signals at the external surface compared to surface discharge

Comparative Analysis: TEV vs. Ultrasonic Detection

Sensitivity Comparison

TEV detection generally offers higher sensitivity for internal discharges within metal-clad switchgear because the electromagnetic signal couples efficiently to the enclosure. For a typical 10 pC discharge inside an 11 kV switchgear panel, TEV sensors can detect signals of approximately 2-5 mV at the panel surface. Ultrasonic detection, by contrast, typically requires discharge magnitudes of 50-100 pC or greater in the same configuration to produce a detectable signal above background noise when measured through closed panel doors.

However, for surface discharge and corona activity in open-air configurations, ultrasonic detection often provides superior sensitivity because the acoustic emissions propagate directly to the sensor without attenuation through metal barriers.

Environmental Considerations

Factor	TEV Performance	Ultrasonic Performance
Electrically noisy environments	Compromised – requires noise gating	Excellent – immune to EMI
Acoustically noisy environments	Excellent – immune to noise	Compromised – requires filtering
Outdoor substations	Good, but affected by corona	Good but affected by wind/rain
GIS installations	Excellent – ideal coupling	Limited – signal blocked by SF6
Underground vaults	Good	Excellent – quiet environment

Application Scenarios

TEV is preferred for:

• Rapid screening surveys of large substation switchgear populations
• Continuous online monitoring of metal-clad switchgear panels, as implemented in the SCYC-HLJC2304 Integrated Online Monitoring Solution for high-voltage cable sheath and PD monitoring
• Detection of internal PD in GIS and metal-enclosed busbar systems
• Trending PD activity over months and years for condition-based maintenance planning

Ultrasonic detection is preferred for:

• Pinpointing the exact location of PD sources within accessible equipment
• Surveying non-metallic or partially open equipment
• Applications in electrically noisy industrial environments
• Verifying and localizing PD sources initially detected by TEV screening

Best Practices for Field Testing

Field experience has demonstrated that the most effective PD detection strategy employs both TEV and ultrasonic methods in a complementary fashion. The recommended workflow for distribution equipment PD surveys is as follows:

1. Initial TEV screening: Conduct a rapid TEV survey of all switchgear panels to identify those exhibiting elevated PD activity (typically above 20 dBmV). Document baseline readings for future trending.
2. Ultrasonic localization: For panels identified in the TEV screening, perform detailed ultrasonic scanning to locate the specific component or compartment generating the PD activity.
3. PD type classification: Use phase-resolved analysis patterns and acoustic signatures to classify the PD type (internal, surface, or corona), which informs the urgency and nature of required corrective actions.
4. Severity assessment: Correlate TEV amplitude, ultrasonic intensity, and PD pattern characteristics to assess the severity of insulation degradation and prioritize maintenance activities.
5. Trending and monitoring: For critical assets, deploy permanent online monitoring systems such as the DT801 Online Monitoring Device which provides continuous surveillance of key parameters including partial discharge activity, enabling predictive maintenance strategies.

The Role of Integrated Monitoring Systems

The evolution of distribution equipment monitoring has moved beyond single-parameter detection toward integrated, multi-sensor platforms. Modern systems combine TEV and ultrasonic PD detection with temperature monitoring, leakage current measurement, and environmental sensing to provide a holistic view of equipment health. This approach enables utilities and industrial facilities to transition from reactive maintenance to true condition-based and predictive maintenance strategies.

The SCYC-HLJC2304 represents this integrated approach, combining sheath circulating current monitoring with partial discharge detection for high-voltage cable systems. Similarly, the SCYC-CW30 provides passive wireless temperature monitoring that complements PD detection by identifying thermal anomalies that may indicate developing faults. When deployed together, these systems create a comprehensive condition monitoring ecosystem that significantly reduces the risk of unexpected equipment failure.

Conclusion

TEV and ultrasonic partial discharge detection methods each offer distinct advantages for distribution equipment condition assessment. TEV provides rapid, non-intrusive screening capability with excellent sensitivity for internal discharges in metal-clad equipment, while ultrasonic detection offers precise source localization and immunity to electromagnetic interference. The most effective field programs employ both methods complementarily, using TEV for initial screening and trending, followed by ultrasonic techniques for precise diagnosis and localization.

As power distribution networks face increasing demands for reliability while operating aging asset bases, the strategic deployment of PD detection technologies becomes essential for informed asset management decisions. The integration of these detection methods into comprehensive online monitoring platforms represents the current state of the art in distribution equipment condition assessment, enabling utilities to maximize asset life while minimizing the risk of in-service failures.

Related Products from QDBRITOP:

• Integrated Online Monitoring Device for PD, Temperature and Leakage Current – Combines TEV and ultrasonic dual detection with temperature and leakage current monitoring for comprehensive switchgear condition assessment
• SCYC-HLJC2304 Integrated Online Monitoring for HV Cable Sheath and PD – Advanced cable monitoring solution with PD detection, sheath current monitoring, and temperature sensing
• DT801 Online Monitoring Device for Substation Surge Arresters – Continuous monitoring of zinc oxide surge arresters with real-time condition assessment
• SCYC-CW30 Passive Wireless Temperature Monitoring for RMU – Battery-free temperature monitoring that complements PD detection systems

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