Gas-insulated switchgear (GIS) is among the most reliable assets in a substation – and the hardest to inspect visually. Partial discharge (PD) inside the gas compartment is the early sign of insulation defects, and the most sensitive way to detect it on installed GIS is with UHF sensors. Retrofitting UHF couplers onto operating GIS is a proven, non-disruptive upgrade path.
Why UHF for GIS
PD pulses inside a GIS compartment radiate electromagnetic energy in the UHF band (roughly 300 MHz – 3 GHz). UHF sensors mounted inside or on the enclosure pick up these signals with high sensitivity and good immunity to external interference, because the metal enclosure screens outside noise. That combination – sensitivity plus noise immunity – is why UHF is the standard sensing technology for GIS PD monitoring.
Sensor types for retrofit
- Internal couplers – disc-shaped sensors installed inside the compartment through an access port or spare flange, offering the highest sensitivity. They are installed during outage windows or when a compartment is opened for other work.
- Dielectric-window couplers – sensors placed over an inspection window or dielectric barrier, detecting UHF signals through the window without entering the gas space. Retrofittable without opening the compartment in many designs.
- External (spacer-mounted) couplers – sensors clamped on the outside of an insulator or spacer that transmits UHF signals. The least invasive option, with somewhat lower sensitivity than internal couplers.
Installation considerations
Selection of monitoring points
Prioritise bays and sections where a failure would be most disruptive: transformer and cable connections, busbar sections and bays with a known installation or operational history. A multi-point configuration also enables PD source localisation by time-of-arrival comparison between sensors.
Live vs outage installation
Dielectric-window and external couplers can be fitted while the GIS is energised, following the manufacturer’s live-working procedure. Internal couplers require the compartment to be opened, so they are typically combined with planned outages or major maintenance windows.
Noise and grounding
Correct grounding of the sensor and its cable shield is essential to preserve the noise immunity that makes UHF useful. Cable runs should avoid paralleling power cables and pass through the monitoring cabinet with proper surge protection.
Integrating UHF PD data
UHF sensors feed a monitoring unit that measures PD amplitude, discharge rate and phase-resolved patterns. Practical integration options:
- Standalone PD monitoring cabinet with local alarming
- Data flow into the substation SCADA or asset management platform via an IoT gateway
- Combined monitoring with SF6 density, so insulation defects and gas loss are watched together
For utilities running retrofits in phases, start with the highest-risk bays, establish baselines and alarm on trends rather than absolute values.
About HUWOR
Zhuhai Huawang Technology Co., Ltd. (HUWOR) manufactures UHF PD sensors – including built-in and spatial UHF types – PD monitoring systems and IoT gateways for GIS applications, and supports retrofit projects worldwide with OEM/ODM supply and application guidance. Contact the HUWOR sales team for sensor selection for your GIS type.