Partial discharge (PD) monitoring only works if the right sensor is coupled to the right equipment. Every detection method has a physics behind it, and each one sees a different class of defect. Selecting the wrong technology is the most common reason monitoring projects produce either noise or false alarms. This guide compares the four mainstream coupling methods – TEV, UHF, HFCT and ultrasonic (acoustic emission) – and explains how to choose between them, including the increasingly popular combined sensors.
The four coupling methods in one picture
TEV (transient earth voltage) measures the voltage pulse that a discharge inside a switchgear panel induces on the earthed metalwork. TEV sensors are cheap, easy to retrofit on the outside of panels, and very effective for air-insulated switchgear (AIS) and ring main units (RMU). Their weakness is that they pick up any pulse travelling through the earthing system, so they see cross-talk between adjacent panels.
UHF (300-1500 MHz) detects the electromagnetic wave emitted by a discharge inside a metal enclosure. It is the most sensitive method for GIS and transformer tanks, with excellent immunity to external corona. UHF sensors must be mounted on a dielectric window or built into the enclosure, which makes them slightly more complex to install than TEV but far more selective.
HFCT (high-frequency current transformer) clamps around the earth strap or the cable screen and measures the high-frequency current pulse of the discharge. It is the standard method for cable terminations and for transformer bushing and neutral connections. HFCT is broadband, robust and easy to install, but it is a single-point measurement: the sensor has to be on the right conductor to see the discharge.
Ultrasonic / acoustic emission (AE) detects the airborne or structure-borne ultrasound generated by a discharge. Handheld ultrasonic detectors are excellent for live-line surveys because they also locate the defect by sound intensity. For fixed monitoring, AE sensors are used where electrical coupling is difficult, for example on transformer tanks, and they are insensitive to electrical noise – but they are affected by acoustic background and by the distance between the sensor and the source.
Which sensor for which equipment?
Air-insulated switchgear panels (AIS, 10-35 kV): TEV is the first choice. A TEV sensor on each panel, optionally combined with ultrasonic and temperature sensing in a single unit, gives a cost-effective online picture of the whole switchboard.
Ring main units (RMU): the same logic applies – TEV-based sensors are the standard, and a combined TEV + ultrasonic + temperature sensor fits the confined space of an RMU compartment.
GIS: use UHF, preferably a built-in sensor for new equipment (see the article on built-in UHF sensors) or an external UHF sensor clamped to a dielectric window for retrofit projects. Spatial UHF sensors mounted on the enclosure surface are an alternative when no internal port is available.
Power cables and terminations: HFCT around the cable screen or earth strap is the workhorse. For cable systems that cannot be taken out of service, an oscillating wave test (OWTS) performed during planned outages complements continuous HFCT monitoring.
Transformers: HFCT on bushings and neutral plus acoustic sensors on the tank. UHF can be added if the transformer has a dielectric port. For generators, dedicated rotor inter-turn short-circuit testing (RSO) covers a different failure mode and should be planned separately.
Surge arresters (MOA): the relevant measurement is not PD but leakage current; a dedicated arrester leakage-current sensor with a lightning-strike counter is the correct choice.
When a combined sensor is the better investment
In real switchgear, one technology alone misses defects. A discharge inside a vacuum interrupter may produce almost no TEV but a clear ultrasonic signature, while a bushing defect may be seen in TEV but not by ultrasound. Combined sensors mount two or more coupling elements in one housing and feed them to one channel of the monitoring system.
HUWOR’s combined sensor family shows the pattern:
- HMJ1010-AT: TEV + ultrasonic, the standard two-in-one for switchgear panels
- HMJ1010-WAT: TEV + ultrasonic + temperature/humidity, for panels where climate is a stress factor
- HMJ1010-WGAT: UHF + TEV + ultrasonic + temperature/humidity, the four-in-one for GIS and large switchgear bays where all coupling methods are relevant
- HMJ1010-WGP: UHF + pulse-current coupling for combined GIS and cable circuits
A combined sensor costs only slightly more than a single-technology unit but gives the monitoring platform three or four independent views of the same compartment. In a monitoring project, the incremental cost of the extra sensing elements is usually much smaller than the cost of a second site visit to add a missed technology later.
Practical selection checklist
Before specifying a sensor, answer these questions:
- What is the equipment type and its rated voltage?
- Is the equipment metal-enclosed (UHF works) or air-insulated (TEV is the natural fit)?
- Can the sensor be installed without an outage? (External TEV/HFCT/AE yes; built-in UHF no.)
- Is the defect believed to be internal (UHF, TEV) or surface/corona (ultrasonic)?
- Does the site have strong electrical noise (favours UHF and ultrasonic)?
- Is this a retrofit or a new-build project (built-in sensors possible)?
- Will the sensor feed an existing monitoring IED or gateway, and what interface does it accept?
Documenting these answers before purchase avoids the classic mistake of installing a sensitive sensor in a location where it cannot physically couple to the discharge.
Summary
There is no single best PD sensor – there is a best sensor for each equipment type and defect. TEV is the economical default for air-insulated switchgear, UHF is the sensitivity leader for GIS and transformer tanks, HFCT is the standard for cables and bushings, and ultrasonic adds the acoustic view that catches what electrical methods miss. Combined sensors reduce project risk by giving several technologies in one installation. HUWOR manufactures the full sensor family plus handheld detectors and online monitoring systems, with OEM/ODM support. Contact the HUWOR sales team for a sensor selection proposal based on your equipment list.