Understanding Partial Discharge in Transformer Bushings and How to Prevent It

Partial discharge (PD) is one of the earliest indicators that insulation inside a transformer bushing may be experiencing electrical stress in localized regions. Over time, this activity can influence insulation behavior and gradually reduce bushing service life. For this reason, understanding PD in transformer systems remains an essential part of condition assessment.

PD may appear in internal voids, at material interfaces, or along external surfaces where moisture or contamination changes electrical conditions. Although each event is extremely short, their cumulative effects can alter insulation performance. Clear knowledge of how PD develops, how it is detected, and how it can be controlled helps maintain stable and predictable insulation behavior across different operating environments.

What Is Partial Discharge in Transformer Bushings?

Partial discharge is a localized electrical event that occurs when the electric field inside a bushing becomes concentrated in a small region of the insulation. Instead of forming a full breakdown, the discharge bridges only a portion of the insulation system. Its presence may indicate issues such as internal voids, gas pockets, or irregular interfaces where the field strength becomes unusually high.

Depending on the bushing construction, PD may form in several locations:

• In paper or resin-based insulation, microscopic voids may act as stress concentration points.

• At internal interfaces or joints, small gaps can influence how the field distributes across layers.

• On the external surface, moisture, dust, or pollution may promote surface discharge under elevated electrical stress.

Each pattern provides engineers with insight into how the insulation system is reacting in service.

Why Partial Discharge Occurs

Internal Material Factors

Bushings contain insulation that must remain uniform to support predictable electrical behavior. Any deviation, including residual moisture, incomplete impregnation, or microscopic cavities, may create small zones where electrical stress concentrates. These zones allow PD to form when the field strength exceeds what the material can withstand locally.

Design-Related Factors

Bushings depend on controlled electric-field distribution around the conductor. Key design related factors that contribute to PD formation include inadequate electric field grading due to improper foil design or termination, insufficient or mismatched insulation thickness and dielectric properties, and poorly designed interfaces that trap air or gas. Thermal design limitations, moisture diffusion paths, insufficient sealing design, and external insulation or creepage design can also initiate discharge activity.

Operational and Environmental Factors

Temperature cycles cause expansion and contraction, which may influence material interfaces over time. Humidity, pollution, or surface contamination can promote external discharge in outdoor installations. Occasional overvoltages from switching operations or grid disturbances may further elevate electrical stress inside the bushing.

Effects and Technical Implications of Partial Discharge

Partial discharge influences how insulation behaves over time by creating localized electrical activity within internal gaps or interfaces. These events can gradually alter material properties, affecting dielectric uniformity and surface condition. PD does not immediately result in a breakdown but can deteriorate insulation properties over time, leading to catastrophic bushing failure.

Detection and Monitoring Techniques

Electrical Detection at the Test Tap

Many high-voltage bushings include a test tap that allows engineering teams to observe leakage current. The waveform of this current contains information about how the insulation responds to electrical stress. Advanced monitoring systems can detect subtle changes in the high-frequency components of this signal, which may indicate PD activity. This method supports routine assessment within ongoing partial discharge testing programs without interrupting equipment operation.

Ultra High Frequency (UHF) and High Frequency (HF) Detection Methods

Partial discharge produces short electromagnetic pulses, which UHF or HF sensors can capture from inside the transformer tank or around the bushing flange. These methods help locate internal activity and provide a clearer understanding of how intense or frequent it is. Because the signals are brief, the sensors are designed to distinguish PD pulses from ordinary electrical noise within the installation.

Acoustic and Complementary Techniques

Some PD events generate small mechanical vibrations within the insulation. Acoustic sensors mounted at strategic points can detect these patterns and help identify the approximate region of activity. Acoustic measurements are often reviewed alongside electrical data for accurate evaluation.

Role of Online Monitoring

Continuous observation supports early awareness of insulation changes, especially in strategic installations. Online systems review trends in leakage current, capacitance, tan delta, and PD indicators. These trends help identify developing transformer insulation problems by showing how the bushing responds under routine load. Because the information is collected during real operation, it reflects actual electrical stress rather than ideal test conditions. This approach strengthens long-term maintenance planning.

Prevention Strategies

Design and Material Considerations

A well-designed bushing controls electric-field distribution through properly configured grading foils, conductor interfaces, and insulation geometry. Material selection must support stable performance under thermal and electrical variations.

Manufacturing and Quality Controls

During production, insulation systems must be processed to minimize voids, residual moisture, and contamination. Impregnation methods for paper, resin, or synthetic systems ensure complete filling of internal spaces. Routine factory tests, including power-factor tests and PD measurements, verify insulation integrity before installation.

Installation and Commissioning Practices

Correct handling and installation help protect insulation integrity. Clean mounting surfaces, aligned hardware, and proper torque settings reduce the chance of creating gaps or contamination. Commissioning measurements establish baseline values for capacitance, power factor, and PD activity, helping teams evaluate insulation behavior over time and identify early signs of transformer bushing defects.

Operational Practices and Monitoring

Routine inspection and cleaning of external surfaces support stable performance in polluted or coastal environments. Online monitoring tools help track behavior without removing the bushing from service. Regular data review allows teams to identify developing issues early and intervene before deterioration becomes significant.

FAQs

1. Is partial discharge always visible during commissioning?

Not always. Early partial discharge in bushings may fall below detection limits and become clearer once the equipment stabilizes under normal operating voltage and temperature.

2. Which environmental conditions influence PD behavior?

Humidity, temperature shifts, and airborne contaminants can affect insulation response, guiding teams to review data patterns when assessing long-term discharge activity.

3. Do acceptable PD limits vary between bushing designs?

Yes. Acceptable PD thresholds depend on insulation design and geometry, helping engineers assess readings accurately when evaluating transformer insulation problems.

4. How does transformer loading affect PD activity?

Changes in loading affect transformer temperature, which can influence insulation behavior and local electrical stress. Understanding load conditions helps engineers interpret PD measurements more accurately over time.

Conclusion

Partial discharge provides early insight into how insulation behaves in service and whether localized electrical activity requires attention. Understanding how PD forms, how it is detected, and how it can be reduced helps manage insulation performance confidently.

At Yash Highvoltage®, partial discharge behavior is examined under controlled laboratory conditions using advanced measurement systems. Our in-house, ILAC-accredited test laboratory allows for precise dielectric verifications, ensuring every unit adheres to stringent global quality benchmarks. These capabilities ensure that every bushing design performs reliably under defined electrical stresses throughout its operational lifecycle.

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