Why Online PD Monitoring for Power Transformers
Power transformers are the most critical and most expensive single asset in a substation, and their insulation systems can fail with little warning. Partial discharge (PD) is the earliest reliable indicator of insulation deterioration in transformer windings and bushings: voids, moisture, contamination, and mechanical damage all produce detectable discharge long before a catastrophic fault.
An online partial discharge monitoring system for transformers keeps sensors installed permanently and records PD activity while the transformer stays in service. Continuous measurement and trend analysis turn sporadic readings into an early-warning timeline, which is exactly what asset managers need to plan interventions instead of reacting to failures.
PD Detection Methods for Power Transformers
| Method | Sensing Principle | Installation | Best For |
|---|---|---|---|
| UHF (in-oil / drain valve) | Electromagnetic pulses in oil and windings | Sensor through oil drain valve or access hatch | High sensitivity, online monitoring, locating |
| Acoustic emission (AE) | Ultrasonic pressure waves in oil and tank wall | Mounted on tank wall | Detection and source location; retrofit-friendly |
| HFCT (bushing tap / earth) | High-frequency currents on bushing tap or earth lead | Clamp around bushing tap, neutral, or earth conductor | Retrofit without tank modification |
| DGA (dissolved gas) | Gas by-products of insulation stress | Online DGA monitor or periodic oil sampling | Complementary long-term trending |
Most effective online systems combine two methods â typically UHF or HFCT with AE â so that a reading from one sensor can be confirmed by another before an alarm is raised.
System Architecture of an Online PD Monitoring System
- Sensors. UHF, AE, or HFCT sensors installed on the transformer, connected by screened cables or wireless links.
- Acquisition unit. A concentrator or data acquisition unit digitises signals, applies noise suppression, and computes PD parameters such as apparent charge, pulse count, and PRPD patterns.
- Communication. Data is sent to a local server or cloud platform over RS485, Ethernet, 4G, or IoT protocols.
- Analysis platform. Software displays trends, alarms, PRPD patterns, and defect classification, with remote access for engineers.
This architecture is the same one used across GIS and switchgear, which means a utility can standardise on one platform for all its assets.
Key Selection Criteria for a Transformer PD Online Monitoring System
- Sensitivity. The system should reliably resolve discharges from a few pC upward â the levels relevant to transformer insulation.
- Noise discrimination. Transformers operate beside noisy busbars and communication signals; proven anti-interference and pattern recognition are essential to avoid false alarms.
- Retrofit-friendliness. Installation without taking the transformer out of service, using drain valves or clamps, reduces project risk.
- Integration. The platform should accept data from multiple sensor types and ideally multiple assets.
- Standards alignment. Equipment should follow IEC 60270 / IEC 62478 practice and utility specifications such as Q/GDW 12082 where required.
HUWOR Online PD Monitoring for Transformers
HUWOR designs and manufactures online partial discharge monitoring systems and sensors for transformers, GIS, switchgear, and cables, together with portable live-line detectors such as the DAC-M30. The monitoring platform combines PD data with IoT connectivity, supporting multi-channel acquisition and remote analysis. HUWOR participates in drafting industry standards including Q/GDW 12082-2021 for wireless sensors in the power IoT, and T/EPTC 002-2022.
- View the HUWOR monitoring and detection product range
- IoT concentrator and monitoring platform white paper
- Partial discharge monitoring white paper
- All HUWOR white papers
Frequently Asked Questions
Can partial discharge in transformers be monitored online without an outage?
Yes. UHF sensors can be installed through oil drain valves and AE sensors on the tank wall without de-energising the transformer, so an online PD monitoring system can be retrofitted during normal operation.
Which PD method is best for power transformers, UHF or acoustic?
Both are valuable. UHF offers high sensitivity and works well for internal discharges, while acoustic emission helps with source location and is easy to retrofit. Many systems use them together for confirmation.
How does online PD monitoring differ from periodic PD testing on transformers?
Online monitoring records continuously and provides trend data, catching defects that develop between inspection cycles. Periodic portable testing gives a snapshot at a point in time and is used to screen many assets or verify repairs.
What standards apply to transformer PD online monitoring equipment?
International practice follows IEC 60270 and IEC 62478. Utility specifications such as Q/GDW 12082 for wireless sensors in the power IoT also apply in China’s grid projects, and HUWOR participates in drafting such standards.
For system design support, a quotation, or white papers, contact us at hezhiqiang@huwor.com.cn.
Related Reading
Transformer Partial Discharge (PD) Online Monitoring System
How to Choose a Partial Discharge Detector for GIS Substations
All-in-One PD Testing System: UHF, TEV, AE & HFCT Methods Explained
Cable Partial Discharge Testing: HFCT, Tan Delta and Field Practice – related testing practices for cables.
HUWOR Online PD Monitoring Systems
HUWOR supplies the Multi-Channel Partial Discharge Diagnosis System for transformer and switchgear online monitoring, together with UHF, HFCT, TEV and AE sensors and the IoT Gateway for data aggregation. OEM/ODM supply is supported for utilities and system integrators.