TEV and Ultrasonic Detection Methods for Switchgear PD Monitoring
Partial discharge monitoring in medium-voltage switchgear relies predominantly on two complementary detection technologies: Transient Earth Voltage (TEV) sensing and airborne ultrasonic detection. While both methods detect the secondary effects of discharge activity, they operate on fundamentally different physical principles, have distinct sensitivity profiles, and are suited to different types of PD sources. Understanding these differences is essential for specifying an effective monitoring system for distribution switchgear and ring main units.
Physical Principles of TEV Detection
When a partial discharge event occurs inside a metal-clad switchgear compartment, the rapid current pulse generates an electromagnetic wave that propagates within the enclosed volume. This wave induces transient voltages on the internal surfaces of the metal enclosure. At any discontinuity in the metal shielding — cable entry points, gasket joints, viewing windows, ventilation grilles — a small portion of this electromagnetic energy escapes and appears as a transient voltage on the external grounded surface. This is the TEV signal.
The TEV phenomenon was first characterized in the 1980s by researchers at the UK’s Electricity Council Research Centre. The signal is typically a damped oscillatory pulse with a rise time of 1-5 nanoseconds and amplitude ranging from a few millivolts to several volts, depending on PD magnitude and sensor proximity. The frequency content spans from approximately 3 MHz to 100 MHz, with peak energy typically in the 10-30 MHz band.
TEV sensors are capacitive coupling plates — essentially flat copper electrodes — that are placed on the external surface of the switchgear panel. They are connected to a high-impedance amplifier (typically 50 ohms terminated into an integrating circuit) and bandpass-filtered to reject power-frequency interference. Modern digital TEV monitors sample at rates exceeding 100 MS/s to capture the fast transient waveforms accurately.
Ultrasonic Detection Physics
Ultrasonic PD detection operates on a completely different principle: acoustic emission. When ionization occurs within an air-filled void or along an insulation surface, the rapid heating and expansion of the ionized channel creates a pressure pulse. For discharges in air at atmospheric pressure, the acoustic spectrum extends from audible frequencies up to approximately 300 kHz, with peak energy typically in the 30-50 kHz range.
Piezoelectric ultrasonic sensors — usually resonant devices tuned to 40 kHz — convert these acoustic pressure waves into electrical signals. The sensors must be placed within the same air volume as the PD source because ultrasonic waves attenuate rapidly at air-solid interfaces. This is both a limitation and an advantage: while sensor placement is constrained, the acoustic isolation between compartments means that ultrasonic signals provide excellent spatial discrimination — a detected signal definitively originates from within the same compartment.
Detection Sensitivity Comparison
| PD Type | TEV Sensitivity | Ultrasonic Sensitivity | Best Method |
|---|---|---|---|
| Internal void discharge (solid insulation) | High | Low (acoustically shielded) | TEV |
| Surface discharge (tracking) | Medium-High | Medium | TEV + Ultrasonic |
| Corona discharge (air, sharp points) | Low-Medium | High | Ultrasonic |
| Floating electrode discharge | High | Low-Medium | TEV |
| Particle discharge (GIS) | Medium | High (contact) | Ultrasonic |
Internal void discharge in solid insulation — arguably the most dangerous PD type because it directly degrades the dielectric — generates strong TEV signals that propagate well through the metal enclosure but produces minimal airborne ultrasound because the acoustic energy is contained within the solid material. Conversely, corona discharge from sharp conductor edges — while generally less immediately harmful — produces strong ultrasonic emissions but relatively weak TEV signals because the discharge occurs in open air with good electromagnetic coupling to free space.
Installation Practicalities
TEV sensors are the easiest to install: they are placed externally on switchgear panel surfaces using magnetic mounts or adhesive, requiring no penetration of the enclosure and no outage for installation. This makes TEV the default choice for retrofit monitoring programs. The BRITOP integrated monitoring solution leverages this advantage, combining TEV sensors with wireless temperature and leakage current monitors in a single non-intrusive platform.
Ultrasonic sensors require access to the interior air volume or installation through existing apertures. For switchgear, they are typically mounted inside cable compartments or busbar chambers through pre-existing cable entry points. Factory-installed ultrasonic sensors in new equipment are increasingly common and provide optimal placement.
Signal Processing and Noise Discrimination
Both TEV and ultrasonic signals must contend with environmental noise. TEV measurements are susceptible to external electromagnetic interference from radio transmitters, switching operations in adjacent bays, and corona from overhead lines. Modern TEV monitors employ several discrimination techniques:
- Phase-resolved analysis: PD activity correlates with specific phase angles of the applied voltage waveform. By plotting signal amplitude against phase angle, genuine PD produces characteristic patterns (clustered around voltage zero-crossings for internal discharges, near voltage peaks for surface PD) while random noise is uniformly distributed.
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Pulse shape analysis: PD pulses have characteristic rise times and durations that differ from switching transients and radio interference. Time-domain waveform analysis can reject pulses that do not match PD signatures.
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Multi-sensor coincidence: When TEV and ultrasonic sensors detect signals simultaneously, the probability of genuine PD is very high, as random electromagnetic and acoustic noise are unlikely to coincide temporally.
The SCYC-HLJC2304 cable monitoring system from BRITOP exemplifies multi-sensor fusion, combining PD sensors with temperature, vibration, and circulating current measurements.
Practical Deployment Recommendations
For most distribution switchgear monitoring applications, a combined TEV + ultrasonic approach provides optimal coverage:
- Install TEV sensors on each switchgear panel to monitor internal PD activity in solid insulation
- Supplement with ultrasonic sensors in cable compartments and busbar chambers where surface PD and corona are likely
- Use wireless temperature sensors at critical connection points for complementary thermal monitoring
- Configure the central monitoring unit to cross-correlate TEV, ultrasonic, and temperature data for high-confidence fault detection
The comprehensive PD monitoring platform available from BRITOP integrates all these capabilities in a single, scalable system suitable for deployment across multiple substations.
Related Products from BRITOP
- Integrated Online Monitoring Device for Switchgear and RMU
- SCYC-CW30 Passive Wireless Temperature Monitoring
- SCYC-HLJC2304 HV Cable Monitoring System
- DT801 Surge Arrester Online Monitoring
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