Online partial discharge (PD) monitoring is the most widely used early-warning technology for metal-enclosed high-voltage equipment. For gas-insulated switchgear (GIS) and large transformers, the most sensitive and noise-immune coupling method is the ultra-high-frequency (UHF) technique. When the switchgear is still at the manufacturer’s factory, there is a unique opportunity to fit the sensor inside the equipment enclosure, where it can couple to the discharge signal with maximum sensitivity. This article explains the built-in UHF partial discharge sensor (HUWOR model HMJ1010-WGN), how it works, and what to consider when specifying it for a new GIS or transformer project.
Why UHF for GIS partial discharge monitoring?
A partial discharge emits a short burst of electromagnetic energy across a very wide spectrum, from a few kilohertz up to several gigahertz. Inside a metal-enclosed GIS compartment, the enclosure acts as a waveguide: the electromagnetic wave travels along the compartment and can be picked up at almost any accessible port. UHF sensors detect the 300-1500 MHz band. This band is well above most external corona and switching noise, which is why UHF monitoring inside GIS and transformer tanks is considered the gold standard for sensitivity and immunity to interference.
The main limitation of UHF is that the wave cannot pass through solid metal. The sensor therefore has to be mounted either on a dielectric window (such as the inspection/viewing port of GIS) or on an internal electrode that reaches into the enclosure. A built-in sensor takes the second approach: it is installed during factory assembly, so the sensing element sits inside the compartment from day one.
Built-in versus external UHF sensors
External UHF sensors are clamped or bonded onto the outside of a dielectric spacer or a cast-resin inspection window. They are easy to retrofit, but their sensitivity depends on the thickness and material of the window and on the quality of the coupling. A built-in sensor avoids these losses because it is electrically and mechanically integrated with the equipment, giving a stronger and more repeatable signal.
For OEM buyers this matters in two ways. First, the sensor is calibrated against the actual enclosure, so the baseline sensitivity is known before the equipment leaves the factory. Second, the sensor can be factory-tested together with the GIS, which significantly reduces commissioning time on site.
HMJ1010-WGN at a glance
The HUWOR HMJ1010-WGN is a built-in UHF partial discharge sensor designed for GIS and transformer PD monitoring. Its published specification covers a frequency range of 300-1500 MHz, which is the standard UHF band used for GIS partial discharge detection worldwide. The sensor is connected directly to the built-in UHF ports of the equipment and works with the HUWOR online monitoring system or with third-party monitoring platforms via standard interfaces.
Because the sensor is installed inside the equipment, the exposed part of the device must meet the dielectric and sealing requirements of the compartment. The HMJ1010-WGN is supplied as a factory-installed component, and the mounting position is agreed with the switchgear manufacturer so that the sensor sees the largest possible volume of the compartment.
What can be detected
UHF monitoring inside GIS is sensitive to the defects that matter most in service:
- Floating or loose metallic particles inside the compartment
- Protrusions and surface defects on the high-voltage conductor
- Insulator defects, voids and surface contamination on spacers
- Faults associated with poor contact of the moving contacts of disconnectors and earthing switches
Each of these defects produces a characteristic UHF pattern that a trained analyst, or the pattern-recognition software of the monitoring platform, can distinguish. Continuous monitoring also makes it possible to track the trend of the discharge over time, which is more informative than a single spot measurement.
Integration into an online monitoring system
In a typical GIS project, several HMJ1010-WGN sensors are distributed along the bays. The sensors are cabled back to a monitoring IED or an IoT gateway (for example the HUWOR HMJ1020-WJM485), which digitises the signals, applies time-domain and phase-resolved analysis, and reports to the control room. The system can be configured to raise alarms when a discharge crosses a set level, and to archive phase-resolved PD (PRPD) patterns for later diagnosis.
Because the same UHF port can also be used for periodic live testing with a handheld instrument, a built-in sensor gives the utility both continuous online coverage and an access point for portable verification.
Specification considerations for project engineers
When specifying a built-in UHF sensor, the key points to confirm with the supplier are:
- The sensor frequency band and its overlap with the monitoring platform’s acquisition range
- The mechanical interface with the GIS compartment (flange, diameter, depth)
- The dielectric withstand of the exposed electrode and the sealing class of the joint
- Whether the sensor is delivered calibrated and with a sensitivity certificate
- Compatibility with the existing monitoring IED or gateway
HUWOR supports OEM/ODM supply of the HMJ1010-WGN, including private labelling, custom cable lengths and integration with the customer’s monitoring platform. The reference values in the datasheet (such as the 300-1500 MHz band) are measured under the conditions stated in the datasheet; project-specific acceptance criteria can be agreed during the site survey.
Summary
Built-in UHF partial discharge monitoring gives GIS and transformer operators the most sensitive, most repeatable and most noise-immune PD data available. Fitting the sensor at the factory removes the coupling losses of external sensors and shortens site commissioning. For new GIS projects, specifying a built-in UHF sensor such as the HUWOR HMJ1010-WGN is a small incremental cost that pays back in asset visibility for the whole life of the equipment.
Zhuhai Huawang Technology Co., Ltd. (HUWOR) manufactures UHF, TEV, ultrasonic, HFCT and combined partial discharge sensors, handheld PD detectors and online monitoring systems, and supports OEM/ODM supply. For technical specifications or a project proposal, contact the HUWOR sales team.