Power transformers are the highest-value assets in most substations, and insulation defects inside them are hard to detect without taking the unit out of service. Field partial discharge (PD) testing on energised transformers – using HFCT clamps on the grounding and neutral leads, plus ultrasonic sensors on the tank – finds developing defects early, without an outage.
Why field PD testing works on transformers
PD in transformer insulation – winding defects, bushing problems and moisture-affected insulation – generates current pulses and mechanical (acoustic) waves:
- HFCT clamps measure the high-frequency current pulses flowing through the grounding and neutral connections, catching PD activity in the winding and bushing region.
- Ultrasonic sensors on the tank surface detect the acoustic emission from internal discharge, localising it by comparing signal amplitude at different tank positions.
Used together, the two methods confirm PD presence and approximate its location inside the tank.
Sensor placement
HFCT clamps
- Clamp on the transformer grounding strap or neutral lead where the configuration allows safe access
- Also measure the cable sheath/earth connection at the bushing terminals where accessible
Ultrasonic sensors
- Mount on the tank wall, bottom and top positions – discharge from winding and core defects propagates to the tank surface
- On oil-filled transformers, acoustic signals from internal discharge travel well through the oil and tank wall
Dealing with interference
Substation environments contain corona, switching and communication noise. Practical mitigation:
- Compare signals with background: measure the same setup on an adjacent calm unit or at a distant point on the tank
- Correlate HFCT and ultrasonic: a real internal discharge usually produces both signals; external corona is more often picked up by only one method
- Use phase correlation where the instrument supports it – PD pulses align with the AC cycle in characteristic patterns
Interpreting results
- Both HFCT and ultrasonic activity – strong indication of internal PD; escalate to a detailed investigation, including DGA sampling if not already scheduled.
- Ultrasonic only, localised to one tank region – likely surface or bushing-related discharge; inspect that region specifically.
- HFCT only, near background on ultrasound – investigate further; could be noise or PD in a location that transmits poorly to the tank surface.
Escalating to permanent monitoring
When field testing confirms rising PD on a critical transformer, move from periodic testing to continuous supervision: install HFCT and/or UHF sensors with an online monitor feeding the asset management platform. Trending then drives the maintenance decision – whether to plan an outage, dry the insulation or prepare for replacement.
About HUWOR
Zhuhai Huawang Technology Co., Ltd. (HUWOR) supplies HFCT clamps, contact ultrasonic sensors, handheld PD detectors and online monitoring systems for transformer applications, supporting utilities and contractors worldwide with OEM/ODM supply. Contact the HUWOR sales team for a transformer PD testing configuration.