Partial Discharge in a Critical GIS Installation: A Detailed Case Study

Gas-Insulated Switchgear (GIS) has become the cornerstone of modern high-voltage substations, prized for its compact design, high reliability, and minimal maintenance requirements compared to air-insulated switchgear (AIS).1 However, despite its robust nature, the insulation system within GIS is susceptible to degradation over time due to various electrical, thermal, and environmental stresses, as well as potential manufacturing or installation defects.3 Partial discharge (PD) is a critical indicator of these developing insulation weaknesses within GIS, acting as an early warning system for potential failures that could lead to significant power outages and economic losses.5 This in-depth case study examines a real-world scenario involving the detection, diagnosis, and management of partial discharge activity within a vital GIS installation at a major metropolitan substation, illustrating the importance of proactive condition monitoring and the sophisticated techniques employed to ensure the continued reliable operation of this critical infrastructure.

1. Introduction: The Imperative of Monitoring GIS Health

The increasing demand for uninterrupted power supply necessitates the highest levels of reliability from electrical transmission and distribution networks. Gas-Insulated Switchgear plays a crucial role in these networks, providing safe and efficient switching and protection of high-voltage circuits within a confined space, making it particularly suitable for urban environments where land is scarce.7 The primary insulation medium in GIS is sulfur hexafluoride (SF6) gas, chosen for its excellent dielectric properties. However, the enclosed nature of GIS, while advantageous, can also make the detection of internal insulation problems more challenging.8 Partial discharge, defined as a localized electrical breakdown that only partially bridges the insulation between conductors, is a key indicator of developing faults within the SF6 gas or the solid insulation components of GIS. Continuous PD activity can progressively degrade the insulation, eventually leading to flashovers and catastrophic equipment failure.5 Therefore, the ability to detect, locate, and classify partial discharge within GIS is of paramount importance for proactive maintenance and ensuring the long-term health and reliability of the power grid.5 This case study will detail an instance where on-line partial discharge monitoring identified a developing issue within a critical GIS installation, outlining the diagnostic process and the steps taken to mitigate the risk of failure.

2. Background: The Critical GIS Installation

The subject of this case study is a 275 kV GIS installation located at a major substation serving a large metropolitan area. This GIS is a critical node in the transmission network, responsible for interconnecting several high-voltage transmission lines and feeding power to downstream distribution substations. The installation had been in service for approximately 12 years and was considered to be in good overall condition based on routine maintenance inspections and standard electrical tests. However, as part of a system-wide initiative to enhance asset management through continuous condition monitoring, an on-line partial discharge monitoring system was recently installed on this GIS. The monitoring system utilized permanently installed Ultra-High Frequency (UHF) sensors within various compartments of the GIS to continuously monitor for electromagnetic emissions indicative of PD activity.11

3. Initial Detection: Anomaly Alert from On-Line Monitoring

Several weeks after the installation of the on-line PD monitoring system, the system generated an alert indicating the detection of unusual UHF activity within one of the busbar sections of the GIS. The alert was triggered by a sustained increase in the amplitude and repetition rate of UHF pulses detected by multiple sensors within that specific section. The monitoring system software also provided preliminary analysis of the Phase-Resolved Partial Discharge (PRPD) patterns associated with the detected signals, suggesting characteristics consistent with a "floating potential" type of defect.12 This initial alert prompted an immediate review of the monitoring data by the substation maintenance team and remote diagnostic experts.

4. Diagnostic Testing: Confirming and Characterizing the PD

Upon receiving the alert, a team of high-voltage diagnostic engineers was dispatched to the substation to conduct further on-site testing and analysis. The primary objective was to confirm the presence of partial discharge, further characterize its nature, and attempt to pinpoint its precise location within the affected busbar section. The diagnostic testing involved a combination of advanced UHF analysis and acoustic emission measurements.

4.1. Advanced UHF Data Analysis

The on-line monitoring system provided a wealth of data, including continuous recordings of UHF signals and corresponding PRPD patterns. The diagnostic team utilized specialized software to perform a more detailed analysis of this data. This involved:

• Frequency Spectrum Analysis: Examining the frequency spectrum of the detected UHF signals to identify dominant frequency components, which can sometimes be indicative of the PD source mechanism.13

• Pulse Shape Analysis: Analyzing the shape and duration of individual UHF pulses to look for specific characteristics associated with different defect types.

• Trend Analysis: Reviewing the historical data from the on-line monitoring system to understand the evolution of the PD activity over time. The increasing trend in amplitude and repetition rate was a key factor in raising concern.14

• Comparison with Defect Libraries: Comparing the observed PRPD patterns with established libraries of patterns associated with known GIS defect types, such as floating particles, protrusions, voids in insulation, and surface discharges.12 The patterns in this case strongly suggested a floating potential, possibly due to a loose metallic component or a conductive particle with a fluctuating electrical potential.

4.2. Acoustic Emission (AE) Measurements

To complement the UHF analysis and aid in source localization, acoustic emission (AE) testing was also performed. AE sensors were temporarily attached to the exterior of the GIS enclosure around the affected busbar section.15 These sensors are designed to detect the ultrasonic waves generated by the mechanical vibrations resulting from partial discharge events within the SF6 gas or solid insulation.15 The AE data acquisition system recorded the amplitude and time of arrival of the acoustic signals at each sensor. By analyzing the time differences in the arrival of the signals, the diagnostic team was able to triangulate the approximate location of the PD source within the busbar section, corroborating the compartment identified by the UHF sensors.

5. Internal Inspection: Visual Confirmation of the Defect

Based on the consistent findings from both the UHF and acoustic emission testing, which strongly indicated a floating potential within a specific compartment of the busbar section, a decision was made to take the GIS out of service during a planned maintenance window for an internal inspection. Strict safety protocols were followed to de-energize and ground the GIS section before opening it. Upon gaining access to the interior of the affected compartment, a thorough visual inspection was conducted. This inspection revealed a small metallic component, believed to be a part of a cable shield that had become detached, resting loosely near one of the busbar conductors. The component showed signs of electrical tracking and minor surface damage, consistent with intermittent electrical discharges occurring between the component and the high-voltage conductor.18

6. Resolution and Remedial Actions: Eliminating the Floating Potential

The detached metallic component was carefully removed from the GIS compartment. The area around the defect was thoroughly cleaned to remove any conductive particles or contamination that might have contributed to the PD activity.18 The insulation surfaces in the vicinity were inspected for any signs of more extensive damage, but none were found. The GIS compartment was then reassembled and filled with SF6 gas to the specified pressure.

7. Post-Repair Testing: Verifying the Effectiveness of the Remedial Actions

Following the repair, comprehensive partial discharge testing was conducted again using both the electrical (conventional) method and the on-line UHF monitoring system. The electrical method, performed according to IEC 60270 standards, showed PD levels well below the acceptable limits, confirming the significant reduction in discharge activity.7 The on-line UHF monitoring system also indicated a complete absence of the previously detected anomalous signals in the repaired busbar section. These results confirmed the successful elimination of the floating potential defect and the restoration of the insulation integrity within the GIS.

8. Lessons Learned and Recommendations: Enhancing Condition Monitoring Practices

This case study provided several valuable lessons and reinforced the importance of proactive condition monitoring for GIS:

• On-Line PD Monitoring is a Powerful Tool: The on-line UHF monitoring system provided early detection of a developing fault that might have gone unnoticed during traditional periodic inspections.

• Multi-Method Diagnostics Enhance Accuracy: The combination of UHF analysis and acoustic emission measurements provided a more comprehensive understanding of the PD activity and aided in accurate source localization.22

• PRPD Patterns are Indicative of Defect Types: The ability to analyze PRPD patterns allowed for a preliminary diagnosis of a floating potential defect, guiding the subsequent inspection efforts.24

• Internal Inspections are Crucial for Confirmation: While on-line monitoring and non-invasive testing provide valuable insights, internal inspections remain essential for visually confirming the nature and location of defects.25

• Timely Intervention Prevents Escalation: The early detection and removal of the floating metallic component prevented further degradation of the insulation and the potential for a more serious failure.26

Based on this experience, the following recommendations were implemented:

• Expand On-Line PD Monitoring: Extend the deployment of continuous on-line PD monitoring systems to other critical GIS installations within the network.28

• Enhance Data Analysis Capabilities: Invest in advanced software tools and training for personnel to improve the analysis and interpretation of PD monitoring data, including automated defect classification.31

• Regular Review of Monitoring Data: Establish protocols for the regular review of on-line monitoring data by both substation personnel and remote diagnostic experts to identify and address any anomalies promptly.

• Integrate PD Data with Asset Management Systems: Integrate the data from the PD monitoring systems with the overall asset management system to facilitate risk-based maintenance planning and prioritization of interventions.

9. Advanced Analysis Techniques for GIS Partial Discharge

Beyond the basic analysis of PD parameters, several advanced techniques are employed to gain deeper insights into the health of GIS insulation:

• Time-Resolved Partial Discharge (TRPD) Analysis: Analyzing the sequence of PD pulses over time can reveal patterns related to the defect's behavior and the influence of factors like voltage stress and temperature.33

• Statistical Analysis of PD Data: Examining statistical distributions of PD pulse amplitudes, repetition rates, and energy can provide quantitative measures of the severity of insulation degradation.

• Artificial Intelligence (AI) and Machine Learning (ML) for Pattern Recognition: AI and ML algorithms are increasingly used to automatically classify PD patterns and identify defect types with high accuracy, even in the presence of noise.35 These algorithms can be trained on large datasets of PD signals from various defect scenarios to improve their diagnostic capabilities.21

• Wavelet Transform for Noise Reduction and Feature Extraction: Wavelet transform techniques can effectively denoise PD signals and extract relevant features for more accurate analysis and classification.

10. Emerging Trends in GIS Partial Discharge Monitoring

The field of GIS condition monitoring is continuously evolving, with several key trends emerging:

• Increased Adoption of Permanent On-Line Monitoring: Continuous monitoring systems are becoming more cost-effective and easier to deploy, allowing for real-time assessment of GIS insulation health.37

• Integration of Wireless Sensor Networks: Wireless PD sensors offer greater flexibility in deployment and can reduce installation costs, particularly for large GIS installations.

• Cloud-Based Data Management and Analytics: Cloud platforms enable centralized storage, management, and analysis of PD monitoring data from multiple substations, facilitating remote diagnostics and trend analysis across the entire network.

• Development of Multi-Sensor Fusion Techniques: Combining data from different types of PD sensors (e.g., UHF, acoustic, TEV) can provide a more comprehensive and reliable assessment of insulation condition.4

• Standardization of UHF PD Measurement: Efforts are underway to develop more standardized procedures for UHF PD measurement in GIS, including guidelines for sensitivity verification and correlation with defect severity.

11. Economic Benefits of Partial Discharge Testing for GIS

Investing in partial discharge testing and monitoring for GIS offers significant economic advantages:

• Preventing Catastrophic Failures: Early detection of insulation defects can prevent major equipment failures, which can result in substantial repair or replacement costs and prolonged power outages.38

• Reducing Maintenance Costs: Condition-based maintenance based on PD data allows for targeted interventions only when necessary, optimizing maintenance schedules and reducing unnecessary costs associated with time-based maintenance.38

• Extending Equipment Lifespan: By addressing PD activity in its early stages, the degradation of insulation can be slowed, extending the operational life of expensive GIS assets.38

• Improving Power System Reliability: Preventing unexpected GIS failures contributes to a more stable and reliable power supply, minimizing disruptions for customers and avoiding potential penalties for utilities.2

12. Impact of Environmental Factors on GIS Partial Discharge

Environmental conditions can influence partial discharge activity within GIS:

• Temperature: Temperature variations can affect the density and dielectric strength of the SF6 gas, potentially influencing PD inception and propagation.

• Pressure: The pressure of the SF6 gas is critical for its insulating properties. A decrease in gas pressure can lower the breakdown voltage and increase the likelihood of PD.40

• Humidity: While GIS is a closed system, moisture ingress over time can contaminate the SF6 gas and the solid insulation, potentially exacerbating PD activity.

• Contamination: The presence of conductive particles (e.g., metal shavings) or insulating particles within the GIS can initiate or intensify partial discharges.40

13. Specific PD Characteristics for Different GIS Defect Types

Different types of defects in GIS tend to produce characteristic partial discharge signatures:

• Floating Particles: Typically generate erratic, burst-like PD patterns that are not consistently synchronized with the AC voltage waveform.41

• Protrusions on Conductors or Enclosure: Often cause PD concentrated around the peaks of the AC voltage waveform due to high electric field stress at the sharp points.41

• Voids in Solid Insulation: Tend to produce repetitive PD pulses occurring around the rising and falling edges of the AC voltage waveform.41

• Surface Discharge: Can exhibit a variety of PRPD patterns depending on the location and nature of the surface defect or contamination.41

• Spacer Defects (e.g., cracks, delamination): May produce PD patterns similar to voids or surface discharges, depending on the specific nature of the defect.

14. Integration with Other GIS Diagnostic Methods

Partial discharge testing is often used in conjunction with other diagnostic techniques to provide a more comprehensive assessment of GIS health:

• Gas Analysis: Analyzing the composition of the SF6 gas can detect the presence of decomposition products (e.g., SO2, SOF2) that are generated by partial discharge or other fault conditions.44

• Insulation Resistance Testing: Measuring the insulation resistance can help identify significant insulation degradation, although it may not be sensitive to localized PD activity.45

• Frequency Response Analysis (FRA): While primarily used for detecting mechanical or structural issues, FRA can sometimes indicate changes in the electrical characteristics of the GIS that might be related to insulation problems.

15. On-Line vs. Off-Line PD Testing for GIS

Both on-line and off-line PD testing methods have their place in GIS condition monitoring:

• On-Line Testing: Performed while the GIS is in service, allowing for the detection of PD activity under normal operating conditions, including the effects of load and temperature. It is particularly valuable for continuous monitoring and detecting intermittent faults. However, on-line testing can be more susceptible to background noise.

• Off-Line Testing: Conducted when the GIS is de-energized, providing a controlled environment with potentially lower noise levels and the ability to vary the applied voltage to investigate PD inception and extinction voltages. Off-line testing is often performed during commissioning or maintenance outages.3

16. Conclusion: Ensuring GIS Reliability Through Proactive PD Management

The case study of the 275 kV GIS installation highlights the critical role of partial discharge monitoring in ensuring the continued reliable operation of these essential components of the power grid. The early detection of a floating potential defect through on-line UHF monitoring, followed by thorough diagnostic testing and targeted repair, prevented a potential insulation failure and underscored the value of proactive condition-based maintenance strategies. By understanding the principles of partial discharge in GIS, employing appropriate testing techniques, and continuously monitoring the health of these assets, utilities can minimize the risk of unexpected outages, optimize maintenance efforts, and ensure a stable and dependable power supply for the communities they serve.2 The integration of advanced analysis techniques, the adoption of emerging monitoring trends, and a clear understanding of the economic benefits further solidify the importance of partial discharge management as a cornerstone of modern GIS maintenance practices.

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