Partial discharge (PD) is a small electrical discharge that bridges only part of the insulation between conductors — across a void inside a cast-resin part, along a contaminated surface, or from a sharp edge into air. Each discharge is tiny, but over months it erodes the insulation until it fails completely. Because PD starts long before failure, finding it on live equipment is one of the most useful tools in condition-based maintenance of medium and high voltage plant.
A discharge releases energy in several forms at once: an electromagnetic pulse, radio-frequency radiation, sound and ultrasound, light, heat and chemical by-products. Each handheld PD sensor picks up one of these.
Transient Earth Voltage (TEV)
A discharge inside metal-clad switchgear produces a fast electromagnetic pulse. Part of it escapes through gaps in the enclosure, such as joints and gasketed panels, and appears as a brief voltage on the outside of the earthed metal surface. A capacitive TEV sensor pressed against the panel detects it. TEV is the standard first check for internal discharge in metal-clad switchgear and ring main units, and readings are usually expressed in dB relative to 1 mV (dBmV).
Airborne Ultrasonic
Surface discharge and corona in air produce ultrasound. A contact-free ultrasonic sensor tuned to around 40 kHz picks this up through vents and gaps, or from a distance with a parabolic concentrator. The instrument converts it to an audible signal, and experienced users learn to recognise the characteristic crackle. Ultrasonic detection is best for surface tracking and corona: cable terminations, insulators, bushings and air-insulated busbars.
UHF
Discharges in SF6 have very fast rise times and radiate in the ultra-high-frequency band, broadly hundreds of MHz to a few GHz. UHF sensors fitted at dielectric windows or spacers on GIS, or placed near transformer openings, pick up this radiation while rejecting much lower-frequency noise. UHF is the usual method for GIS and is also used on transformers.
High-Frequency Current Transformer (HFCT)
An HFCT is a split-core current sensor clamped around an earth connection, typically a cable screen earth or a transformer neutral or core earth. It sees the PD current pulses as they flow to earth. HFCTs are the main tool for power cables and their accessories, and can be applied to transformers, generators and reactors.
Choosing Sensors
| Sensor | Detects | Best suited to |
|---|---|---|
| TEV | Pulse voltage on earthed enclosures | Internal PD in metal-clad switchgear and RMUs |
| Ultrasonic (contact-free) | Ultrasound in air | Corona and surface tracking, terminations, insulators |
| Ultrasonic (contact) | Ultrasound in solid enclosures | GIS and transformer tanks |
| UHF | Radio-frequency radiation | GIS, transformers |
| HFCT | Pulse currents in earth leads | Cables, transformers, rotating machines |
MEWOI's handheld PD detectors follow this pattern. The entry-level M9002 combines TEV and ultrasonic sensing for switchgear and RMUs; the supplier gives a TEV range of −5 to 60 dB and an ultrasonic range of −11 to 68 dB with a 40 kHz sensor and parabolic concentrator. The M9002C and M9003F are two-channel instruments that add GIS contact ultrasonic, UHF and HFCT sensors for transformers, GIS, cables, generators and reactors.
Interpreting Readings
Handheld PD surveys are comparative. Before measuring the equipment, take background readings on nearby earthed metal that is not part of the switchgear, and compare panels of the same type against each other. A reading clearly above background and above similar panels, or one that rises between surveys, justifies closer investigation. Calibrated apparent-charge measurement in picocoulombs, as defined by IEC 60270, belongs to factory and offline testing with a coupling capacitor; handheld survey values in dB are not directly comparable with it.
Frequently Asked Questions
Is the equipment tested live? Yes. These methods are designed for surveys of energised equipment, so normal safe approach distances and site rules still apply.
Why use two sensor types? Internal and surface discharges show up on different sensors. Checking with both TEV and ultrasonic distinguishes them and reduces false alarms from noise.