Practical Partial Discharge Measurement: A Key to High-Voltage Equipment Reliability

Introduction Partial Discharge (PD) is a critical phenomenon in the operation and maintenance of high-voltage equipment, such as transformers, cables, switchgear, and motors. Though often imperceptible, PD can lead to insulation degradation and eventual equipment failure if left unchecked. The significance of PD measurement has grown over the decades, transitioning from a specialized diagnostic tool to an essential part of quality assurance and maintenance strategies for electrical systems.

This article explores the fundamentals, applications, and practical insights into PD measurement, based on the authoritative work "Practical Partial Discharge Measurement on Electrical Equipment" by Greg C. Stone, Andrea Cavallini, Glenn Behrmann, and Claudio Angelo Serafino.

What Is Partial Discharge? Partial discharge is a localized electrical discharge that partially bridges the insulation between conductors without causing immediate breakdown. PD typically occurs in areas of high electric stress, such as voids within solid insulation, along surfaces where gas meets insulation, or in gaps between conductors.

IEC 60270 defines PD as “a localized electrical discharge that only partially bridges the insulation between conductors.” It differs from corona discharge, which occurs in gaseous media around conductors. PD can occur at relatively low voltages in certain scenarios but becomes a concern in high-voltage systems where insulation integrity is paramount.

Why Measure Partial Discharge? PD is both a symptom of and a contributor to insulation degradation. Measuring PD can:

1. Ensure Manufacturing Quality: Factory PD tests verify that equipment meets manufacturing standards and is free from critical insulation defects.
2. Detect Aging and Degradation: Onsite PD testing identifies insulation wear and aging in operational equipment, helping predict failures before they occur.
3. Improve Maintenance Planning: By assessing the condition of insulation systems, PD testing helps prioritize maintenance activities and reduce unexpected outages.

Applications of PD Measurement PD testing is integral to the lifecycle of high-voltage equipment, from manufacturing to in-service operation.

1. Factory Testing

Factory PD tests are a standard quality assurance measure for high-voltage equipment rated above 3 kV. These tests ensure that insulation systems are free from defects that could lead to premature failure. For example:

• Transformers: PD testing identifies voids or manufacturing defects in oil-paper composites.
• Cables: Ensures proper extrusion and curing processes in XLPE and other organic insulations.
• Switchgear: Detects potential issues in epoxy spacers and other components.

2. Onsite and Offline Testing

Some equipment, such as gas-insulated switchgear (GIS) and large transformers, requires onsite testing after assembly. These tests confirm that installation processes have not introduced defects. Offline testing is also used to evaluate equipment condition during scheduled outages.

3. Online PD Monitoring

With advancements in digital systems, online PD monitoring has become increasingly prevalent. Continuous monitoring enables real-time condition assessment without interrupting equipment operation. This approach is especially valuable for critical assets like power cables and rotating machines.

Techniques for PD Measurement Modern PD measurement employs a range of techniques to capture and analyze discharge signals. The choice of technique depends on the equipment and the specific conditions of the test.

1. Electrical Detection

Electrical detection remains the most widely used method. It involves capturing current pulses generated by PD using coupling capacitors or high-frequency current transformers (HFCTs).

2. Acoustic Detection

Acoustic sensors detect sound waves produced by PD, making them suitable for enclosed equipment like transformers and GIS. This method is particularly effective in noisy environments where electrical signals might be obscured.

3. Radio-Frequency Detection

RF sensors measure electromagnetic signals emitted during PD. This technique is commonly applied in GIS and rotating machines, where high-frequency emissions are prevalent.

4. Optical and Chemical Detection

PD can also be detected through light emissions or chemical byproducts like ozone or dissolved gases. These methods are often used as supplementary tools in specific applications.

Standards and Guidelines PD measurement is governed by numerous standards developed by organizations like the IEC and IEEE. These standards provide guidance on test procedures, equipment requirements, and interpretation of results. Key standards include:

• IEC 60270: Governs the measurement of PD in high-voltage equipment.
• IEEE C57.124: Focuses on PD testing in transformers.
• CIGRE Technical Reports: Offer consensus-based guidelines for specific applications.

These standards are critical for ensuring consistency and reliability in PD measurement across industries.

Challenges in PD Testing Despite its importance, PD testing is not without challenges. Engineers often face issues like:

1. Electrical Noise and Interference: External electrical disturbances can obscure PD signals, leading to false positives or missed defects. Techniques like electromagnetic shielding and signal filtering are used to mitigate these issues.
2. Interpreting Results: PD patterns can vary widely depending on the equipment and defect type. Accurate interpretation requires expertise and, increasingly, advanced software tools.
3. Evolving Technology: As equipment designs and operating conditions change, PD testing techniques must adapt. For example, HVDC systems present unique challenges compared to traditional AC equipment.

Emerging Trends and Future Directions The field of PD measurement is evolving rapidly, driven by advances in technology and the growing demand for reliable energy systems.

1. Digital Hardware and AI

Digital hardware has revolutionized PD testing, enabling real-time data processing and advanced signal analysis. Artificial intelligence (AI) is being explored to automate pattern recognition and fault diagnosis, reducing the reliance on human expertise.

2. Integration with Asset Management

PD testing is increasingly integrated with broader asset management strategies. Continuous monitoring systems provide valuable data for predictive maintenance, helping utilities optimize their resources.

3. DC and Impulse Testing

The rise of HVDC systems has prompted new approaches to PD testing. Techniques for measuring PD under DC excitation and short-risetime voltage impulses are under development, addressing the unique characteristics of these systems.

Conclusion Partial discharge measurement is an indispensable tool for ensuring the reliability and safety of high-voltage equipment. From factory quality assurance to in-service monitoring, PD testing provides valuable insights into the health of insulation systems, helping prevent costly failures and extend equipment life.

As the field continues to evolve, the integration of digital technologies and advanced analytics promises to further enhance the accuracy and utility of PD measurement.