A Comprehensive Guide to Partial Discharge in Power Transformers

Partial discharge in a power transformer refers to a localized electrical breakdown within a small portion of the insulation material. This discharge does not completely bridge the gap between conductors but occurs due to intense electric fields within voids, cracks, or other imperfections in the insulation.

By Lamothe Paris
7 min read
A Comprehensive Guide to Partial Discharge in Power Transformers

Power transformers are vital components in electrical grids, and their reliable operation is crucial for maintaining a stable power supply. Partial discharge (PD) is a key indicator of the health of a transformer's insulation system. Understanding what PD is, why it occurs, and how to test for it is essential for effective maintenance and preventing costly failures.1

What is Partial Discharge in Power Transformers?

Partial discharge in a power transformer refers to a localized electrical breakdown within a small portion of the insulation material. This discharge does not completely bridge the gap between conductors but occurs due to intense electric fields within voids, cracks, or other imperfections in the insulation.1 The IEC 60270 standard defines PD as a "localized dielectric discharge in a partial area of a solid or liquid electrical dielectric insulation system under high-voltage field stress." These discharges can occur in various parts of the transformer, including windings, bushings, and insulating oil.4

Key Characteristics of Partial Discharge:

  • Localized: Occurs in a specific, limited area of the insulation.1

  • Incomplete Breakdown: Does not fully bridge the insulation between conductors.4

  • Transient Pulses: Manifests as short bursts of electrical energy.7

  • Early Indicator: Often appears before a complete insulation failure.5

Why is Partial Discharge Testing Important for Power Transformers?

Regular partial discharge testing of power transformers offers numerous benefits:

  • Early Fault Detection: Identifies insulation defects in their initial stages, often before other methods can detect them.8

  • Preventing Catastrophic Failures: Allows for timely intervention, preventing minor issues from escalating into major breakdowns and costly repairs.9

  • Extending Equipment Lifespan: By addressing PD early, the degradation of insulation can be slowed down, prolonging the operational life of the transformer.10

  • Cost-Effective Maintenance: Early detection and repair of PD are significantly cheaper than dealing with major failures and unplanned outages.10

  • Ensuring Reliability: Helps maintain the reliability and availability of the power supply by preventing unexpected transformer failures.1

  • Quality Assurance: Used during manufacturing and commissioning to ensure the transformer meets insulation standards.12

Common Types of Partial Discharge in Power Transformers

Partial discharges in power transformers can be classified into several types based on their location and cause:7


Type of PD

Description

Common Location(s)

Corona Discharge

Occurs at sharp points or edges of conductors where the electric field is highly concentrated, often in air or oil.7

Around sharp edges of windings, high-voltage connections, and bushings.

Surface Discharge

Travels along the surface of insulating materials, often due to contamination, moisture, or imperfections on the surface.7

Across the surface of bushings, spacers, and winding insulation.

Internal Discharge

Occurs within voids, gas bubbles, or delaminations inside solid or liquid insulation materials.7

Within the windings, solid insulation layers, and insulating oil.

Electrical Treeing

A progressive degradation of the insulation material in a branching pattern caused by the repetitive impact of partial discharges.7

Initiates at weak points within the insulation and propagates over time.

Barrier Discharge

Occurs at the interface between different insulating materials, such as oil and paper insulation.7

At the boundaries between oil and solid insulation components within the transformer.

Performing Electrical Partial Discharge Testing on Power Transformers

The electrical (conventional) method, as per IEC 60270, is a common technique for PD testing. Here's a step-by-step guide:12


Step

Description

Key Considerations

1

Safety First

Ensure the transformer is de-energized and isolated. Ground all test equipment.12

2

Connect Measurement System

Connect a high-voltage coupling capacitor in parallel with the transformer. Connect the PD detector using shielded cables to the bushing taps.12

3

Calibration

Calibrate the PD measuring system by injecting a known charge pulse into the circuit. This establishes a reference for accurate measurements.12

4

Apply Test Voltage

Gradually increase the AC voltage from below the expected PD inception voltage (PDIV) in stages. Follow relevant standards (IEC 60076-3, IEEE C57.12.90) for voltage levels.5

5

Monitor PD Activity

Observe the PD detector for discharge pulses. Record the PDIV and PD extinction voltage (PDEV), as well as the magnitude and frequency of PD pulses.17

6

Analyze PRPD Patterns

Utilize phase-resolved partial discharge (PRPD) patterns to visualize PD pulses relative to the AC voltage phase. These patterns help identify the type of defect.1

7

Evaluate Results

Compare the measured PD levels with acceptable limits from standards or the transformer's factory test report. High PD activity indicates potential issues.20

Key Parameters to Monitor:

  • Apparent Charge (q): Magnitude of the PD pulse, measured in picocoulombs (pC).9

  • Partial Discharge Inception Voltage (PDIV): The lowest voltage at which PD starts.17

  • Partial Discharge Extinction Voltage (PDEV): The voltage at which PD ceases when the voltage is lowered.17

  • Phase Angle (φ): The point in the AC cycle when PD occurs.17

  • PRPD Patterns: The overall distribution of PD pulses across the AC cycle, which can indicate the type of defect.1

Other Partial Discharge Testing Methods for Power Transformers

Besides the electrical method, other techniques are used for PD assessment in power transformers:21

  • Acoustic Emission (AE): Detects ultrasonic waves generated by PD using sensors on the transformer tank. Useful for on-line testing and locating PD sources.21

  • Ultra-High Frequency (UHF): Detects electromagnetic waves emitted by PD in the UHF range using antennas inside or outside the tank. Offers high sensitivity and noise immunity.21

  • Dissolved Gas Analysis (DGA): Identifies gases produced by the breakdown of insulation due to PD and other faults. A crucial complementary diagnostic tool.18

Interpreting Partial Discharge Test Results

Interpreting PD test results requires analyzing the measured parameters and patterns. Higher apparent charge values generally indicate more severe discharges.9 The PDIV and PDEV provide insights into the voltage levels at which PD activity starts and stops. PRPD patterns are particularly valuable for identifying the type of insulation defect. For example, void discharges often appear around the rising voltage, while corona discharges might occur near the voltage peaks.31 Trend analysis of PD activity over time is also crucial for assessing the progression of insulation degradation.4

It's important to compare the results with relevant standards and previous test data. Experienced analysts are often needed for accurate diagnosis and to differentiate between genuine PD signals and noise.1

Conclusion

Partial discharge testing is a critical tool for maintaining the health and reliability of power transformers. By understanding the basics of PD, the importance of testing, and the various methods available, you can proactively manage the condition of your transformers, prevent failures, and ensure a consistent power supply.4 Regular PD testing, conducted by qualified professionals, is a vital component of a comprehensive predictive maintenance program for power transformers.4

Works cited

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