Maintaining the reliability of a power transformer goes far beyond oil sampling or routine electrical checks. One diagnostic technique that has gradually become indispensable is Partial Discharge (PD) testing. Although PD itself is not always catastrophic, its early detection can prevent insulation breakdown, unplanned outages, and expensive transformer failures.
This article provides an engineer-friendly, experience-based explanation of partial discharge testing—how it works, why it matters, and what field engineers should actually look for during evaluation.
What Is Partial Discharge in a Transformer?
Partial discharge is a localized electrical breakdown that occurs within a small portion of the insulation system, typically caused by voids, moisture, aging, or manufacturing defects. PD does not bridge the entire insulation, but the ionization process generates electrical pulses, heat, and chemical by-products that gradually deteriorate the insulation.
In other words, PD is a symptom of insulation distress that evolves quietly, sometimes for years, before a transformer finally fails.
Why PD Matters More Than Ever
- Modern grids run closer to their thermal and loading limits
- Renewables introduce more fluctuation and dynamic stress
- Utilities demand longer asset life cycles
- Failures are far more expensive due to downstream outages
This is why PD testing has become a critical component of transformer asset management.
Common Sources of Partial Discharge
Partial discharge can occur almost anywhere in the insulation system. Field experience shows several recurring locations:
- Insulation paper voids
- Moisture pockets within oil-impregnated paper
- Loose clamping structures creating sharp electric field gradients
- Degraded bushings or cable terminations
- Surface discharge along contaminated insulation
How Partial Discharge Testing Works
PD testing focuses on detecting the small electrical pulses emitted during discharge activity. These pulses can be captured through various measurement methods. Each technique has its own ideal use case depending on site conditions and transformer design.
Simplified diagram of PD pulse propagation inside a transformer winding
Key Methods for PD Testing
1. IEC 60270 Conventional PD Measurement
This is the most widely recognized method for factory acceptance tests and high-accuracy measurements. The test measures charge magnitude (in pC) using a coupling capacitor, PD detector, and an impedance matching network.
Advantages:
- High sensitivity and accuracy
- Well-established acceptance criteria
Limitations:
- Difficult for field/on-site transformers
- Affected by strong electromagnetic noise
2. Ultrasonic / Acoustic Partial Discharge Detection
PD activity generates acoustic waves that can be detected by sensors placed on the transformer tank.
Benefits:
- Excellent for on-site diagnosis
- Noise-resistant
- Helps pinpoint the physical PD location
Typical Use Cases:
Localizing internal discharge without intrusive testing.
Technician applying an acoustic PD sensor to the transformer tank wall
3. UHF (Ultra High Frequency) PD Detection
Transformers with UHF sensors can detect electromagnetic radiation produced by PD.
Advantages:
- Immune to external interference
- High-resolution detection
- Often used in GIS and modern power transformers
4. Online PD Monitoring Systems
Online PD monitors continuously collect data while the transformer remains energized.
Why asset managers prefer online monitoring:
- Real-time alarms
- Long-term trending
- Detection of intermittent PD events
Online systems have become a core component of predictive maintenance strategies, especially for large utility transformers.
Interpreting PD Test Results: What Engineers Should Look For
Partial discharge test data must be interpreted in context. Engineers usually evaluate the following:
1. PD Magnitude Trends
A rising pattern is more concerning than a one-time spike.
2. Phase-Resolved PD Patterns (PRPD)
Different discharge types create distinct phase patterns, helping identify:
- Internal void discharge
- Surface discharge
- Floating electrode effects
3. PD Location
Acoustic triangulation or UHF time-of-flight methods can determine where the PD originates.
4. Correlation With Dissolved Gas Analysis (DGA)
PD activity often correlates with rising hydrogen or acetylene levels.
When PD Requires Immediate Action
Not all PD means imminent failure. However, action is required when:
- PD magnitude increases rapidly
- PRPD pattern indicates internal insulation voids
- DGA gases rise together with PD
- Discharge is detected near bushings or leads
- UHF signals suggest internal arcing activity
In such cases, utilities often schedule:
- Thermal imaging
- Additional acoustic scans
- Offline high-voltage diagnostic tests
- Bushing replacement or re-tightening
- Insulation drying or oil reclamation
Best Practices for Reliable PD Testing
- Perform combined diagnostics (PD + DGA + thermal)
- Record baseline PD values during commissioning
- Use shielded measurement cables to reduce noise
- For online systems, monitor long-term trends rather than single-day events
- Re-test after windowed maintenance or bushing replacements
Conclusion
Partial discharge testing is no longer an optional diagnostic—it is one of the most important tools available for preventing catastrophic transformer failures. By understanding PD behavior, choosing the right measurement technique, and interpreting results in context, engineers can significantly extend transformer life and avoid unplanned outages.
Reliable PD testing doesn’t just detect problems—it buys time, reduces risk, and protects some of the most valuable assets in the power grid.