Partial Discharge Testing on Power Transformers: Case Study

Partial discharge (PD) is one of the leading causes of insulation failure in power transformers. Detecting PD early can prevent catastrophic failures, extend asset life, and ensure grid reliability. This long-form article explores the use case of PD testing on power transformers—guided by key standards from IEEE, CIGRE, and IEC—and presents a detailed case study of field implementation.

What Is Partial Discharge?

Partial discharge is a localized dielectric breakdown of a small portion of the insulation system under high electrical stress. It occurs when voids, cracks, or defects within solid insulation (paper, pressboard, epoxy) or gas gaps in oil-filled transformers allow micro-discharges. Over time, PD erodes insulation, producing chemical by-products and acoustic, electromagnetic, or chemical signals detectable with specialized equipment.

Why PD Matters

• Insulation Degradation: PD erodes insulation fibers, eventually leading to full breakdown.

• Unplanned Outages: Transformer failures can trigger blackouts, equipment damage, and safety hazards.

• Maintenance Costs: Early PD detection allows scheduled repairs rather than emergency replacements.

Standards & Best Practices

PD testing protocols are governed by international standards:

• IEC 60270 (High-Voltage Test Techniques – Partial Discharge Measurements): Defines test circuits, measurement methods, and calibration procedures for PD detection in electrical apparatus.

• IEEE Std C57.125 (Guide for Partial Discharge Measurement in Liquid-Immersed Transformers): Outlines PD measurement techniques in oil-filled transformers, mounting sensor types, and data interpretation guidelines.

• CIGRE TB 095 (Application Guide for On-Line Partial Discharge Measurements on Rotating Machines):While focused on rotating machines, many principles apply to transformers, including sensor placement and data analysis for on-line PD monitoring.

Adhering to these standards ensures consistency, accuracy, and comparability of PD test results across utilities and manufacturers.

Use Case: Field PD Survey on a 110 kV Power Transformer

Scenario

A regional utility’s maintenance team schedules a preventive PD survey on a 110 kV oil-immersed distribution transformer nearing 20 years of service. The goal is to detect early PD activity and decide whether to rewind the transformer or apply other remedial measures.

Equipment & Setup

1. Sensor Selection:

   • Ultrasonic sensor clamped to the tank wall to detect acoustic PD signals.

   • High-voltage coupling capacitor sensor attached to the transformer bushing for electrical PD pulses, per IEEE C57.125 recommendations.

2. Test Circuit:

   • Transformer de-energized and isolated.

   • Bushing sensor grounded via coupling network.

   • Calibration with known PD source (IEC 60270) verifies measurement chain.

3. Data Acquisition:

   • Portable PD measurement unit records phase-resolved PD patterns (PRPD).

   • Synchronized phase reference from bushing voltage tap.

Procedure

1. Calibration: Inject a calibration pulse at the coupling capacitor to set sensitivity (IEC 60270), ensuring PD pulse magnitude corresponds to picocoulombs (pC).

2. Baseline Measurement: With no voltage applied, confirm background noise level.

3. Step-Voltage Test: Apply AC voltage in 10 kV increments up to 110 kV, recording PD inception and extinction voltages.

4. Data Logging: At each voltage step, record PRPD patterns, counting discharge events and charge magnitude.

5. Analysis: Compare PD patterns against known defect signatures (e.g., voids within pressboard, surface discharges on windings) using CIGRE guidance.

Interpreting PD Data

• Inception Voltage (PDIV): Voltage at which PD first occurs; lower PDIV indicates severe defects.

• Pulse Magnitude (pC): Larger magnitudes can signal major voids or surface discharges.

• PRPD Patterns: Clusters in specific phase angles help distinguish between internal voids, surface discharges, or corona.

According to IEEE C57.125, PD above 50 pC at service voltage warrants further investigation and potentially partial or full rewind.

Case Study: Algiers Substation Transformer PD Remediation

Background

At the El Harrach substation in Algiers, a 110 kV/33 kV, 40 MVA transformer built in 2005 began exhibiting erratic temperature rises during heavy load. No visible oil leaks or mechanical faults were found.

PD Survey & Findings

• Method: On-site PD survey using the steps above.

• Results:

  • PDIV detected at 75 kV (below expected ~90 kV).

  • Pulse magnitudes reached 120 pC at 110 kV.

  • PRPD patterns showed symmetrical pulses around 180° phase, indicating internal voids in the winding insulation.

Remedial Action

• Offline Partial Discharge Mitigation:

  • Vacuum impregnation repair of internal windings.

  • Replacement of suspect pressboard spacers.

• Follow-up PD Test: Post-repair, PDIV rose to 105 kV, and maximum pulses fell below 20 pC at full voltage—within acceptable limits per IEC 60270 criteria.

Outcome

• Extended Service Life: Transformer returned to service with a projected additional 15 years of reliable operation.

• Cost Savings: Avoided unplanned outage and full rewind cost of ≈ $300,000 by applying targeted repairs.

• Reliability Improvement: Subsequent monitoring showed no temperature anomalies or PD resurgence after six months of operation.

Benefits of Routine PD Testing

1. Early Fault Detection: Identify insulation defects before they escalate.

2. Optimized Maintenance: Targeted repairs instead of full equipment rebuilds.

3. Improved Reliability: Reduce unplanned outages and safety risks.

4. Standards Compliance: Adherence to IEC, IEEE, and CIGRE best practices ensures consistent, defensible test results.

Frequently Asked Questions

1. What is the main standard for PD measurement in transformers?

   • IEC 60270 defines partial discharge measurement methods, calibration, and test circuits.

2. Why perform step-voltage PD tests?

   • Step-voltage tests reveal PD inception and extinction voltages, indicating insulation health under different stress levels.

3. How do you distinguish internal PD from surface discharges?

   • Phase-resolved PD patterns (PRPD) show signature clusters: internal voids cluster tightly around certain phase angles, while surface discharges spread more broadly.

4. What PD level is considered critical in a power transformer?

   • Generally, PD pulses above 50 pC at nominal voltage signal significant defects requiring remedial action, per IEEE C57.125 guidance.

5. Can PD be detected on energized transformers?

   • Yes—on-line PD monitoring uses permanent sensors per CIGRE recommendations to track PD during normal operation, reducing the need for outages.

Internal Links:

• Learn more about our Insulation Testers for off-line PD detection.

• Explore Power Quality Analyzers to combine PD and waveform analysis.

By integrating PD testing into your preventive maintenance program—aligned with IEC 60270, IEEE C57.125, and CIGRE recommendations—you’ll catch insulation faults early, optimize repair costs, and ensure transformer reliability for years to come.