Gas-insulated switchgear (GIS) hides its insulation inside a sealed metal enclosure, which makes PD testing both essential and specific: the enclosure screens external signals, so the detection method must match how PD signals escape – or the sensor must be inside. This article compares the methods available and the test regimes they fit, to help asset managers choose the right mix.
The signal paths in GIS
UHF electromagnetic
PD pulses radiate in the UHF band inside the enclosure. UHF sensors – internal couplers or external spacer/window couplers – capture these signals with high sensitivity and good immunity to outside noise.
Acoustic
PD produces acoustic emission that propagates through the gas and enclosure wall. Contact ultrasonic sensors on the enclosure detect it, often used to localise a source found by UHF.
Chemical
In severe internal discharge, decomposition products form in the gas; gas analysis can confirm significant discharge, though it is slow compared with electrical methods.
Sensor options compared
| Sensor type | Installation | Sensitivity | Use |
|---|---|---|---|
| Internal UHF coupler | Inside compartment (outage) | Highest | New build, planned outages |
| Dielectric-window / external UHF coupler | On window or spacer (often live) | High | Retrofit on operating GIS |
| Contact ultrasonic sensor | On enclosure (live) | Medium, localises | Confirm and locate PD |
Test regimes compared
Online testing and monitoring
- Portable online survey – an engineer visits with UHF or acoustic instruments, measures at sensor positions, compares with baselines.
- Permanent online monitoring – sensors installed on critical bays feed a monitor continuously, detecting fast-developing defects between surveys.
Offline testing
- Offline PD test – the bay is isolated and energised from an external source at moderate overvoltage while PD is measured. Finds defects more sensitively under controlled stress, at the cost of an outage and test equipment setup.
Choosing the mix
- New GIS – specify internal UHF couplers at manufacture for bays that justify monitoring.
- Operating GIS with outage windows – retrofit internal couplers on high-risk bays; use external couplers and acoustic methods for the rest.
- GIS without outages – external UHF and acoustic surveys establish baselines; permanent monitoring covers critical bays.
- Fleet screening – portable online surveys on a schedule, with offline tests at defined intervals or after significant events.
Why baseline matters in GIS PD
GIS PD levels vary by design and operating history. A baseline per bay, measured with the same instrument and procedure, is what makes later readings interpretable – the foundation of any GIS PD programme.
About HUWOR
Zhuhai Huawang Technology Co., Ltd. (HUWOR) manufactures internal and external UHF sensors, acoustic sensors, portable PD instruments and online monitoring systems for GIS, supporting utilities and OEMs worldwide with OEM/ODM supply and application guidance. Contact the HUWOR sales team for a GIS testing configuration.