Partial Discharge in a Critical Power Transformer: A Detailed Case Study

Power transformers are often considered the heart of electrical substations, playing a pivotal role in voltage transformation and power delivery. Their failure can result in significant financial losses, prolonged outages, and potential safety hazards.

Par Lamothe Paris
12 min de lecture
Partial Discharge in a Critical Power Transformer: A Detailed Case Study

The reliable operation of power transformers is fundamental to maintaining the integrity of electrical grids and ensuring a consistent power supply to consumers and industries alike. These high-value assets are subjected to a multitude of stresses throughout their operational lifespan, which can lead to the gradual degradation of their insulation systems. Partial discharge (PD) is a critical indicator of such degradation and can serve as an early warning sign of potential failures if left undetected and unaddressed.1 This comprehensive case study delves into a real-world scenario involving the detection, analysis, and resolution of partial discharge activity within a large, critical power transformer located at a major industrial facility. By examining the methodologies employed, the findings obtained, and the subsequent actions taken, this study aims to underscore the importance of proactive PD monitoring and the multifaceted approaches necessary for effective asset management.

1. Introduction: The Silent Threat to Transformer Reliability

Power transformers are often considered the heart of electrical substations, playing a pivotal role in voltage transformation and power delivery. Their failure can result in significant financial losses, prolonged outages, and potential safety hazards.3 Insulation failure is one of the most common causes of transformer breakdowns, and partial discharge is a primary mechanism that contributes to this degradation.5 PD refers to localized electrical discharges that occur within the insulation system but do not completely bridge the gap between conductors.7 These discharges, though often small in magnitude, can progressively erode the insulation material over time, leading to a reduction in dielectric strength and eventually culminating in catastrophic failure.8 Recognizing the significance of PD as a precursor to insulation breakdown, regular monitoring and diagnostic testing are essential for ensuring the long-term reliability and operational efficiency of power transformers.2 This case study will explore a specific instance where PD activity was detected in a critical power transformer, highlighting the steps taken to identify the issue, locate its source, and implement appropriate remedial measures.

2. Background: The Monitored Asset and Initial Concerns

The subject of this case study is a 250 MVA, 230/13.8 kV step-down power transformer that serves as a critical link in the power supply for a large-scale manufacturing plant. This transformer had been in service for approximately 18 years and was considered a vital asset due to its direct impact on the plant's production capacity. As part of the facility's proactive maintenance program, the transformer was subjected to regular dissolved gas analysis (DGA) of its insulating oil. DGA is a well-established technique for detecting the presence of fault gases generated by the thermal and electrical degradation of insulation materials within a transformer.10 Routine DGA results had historically been within acceptable limits. However, a recent analysis revealed a notable increase in the concentration of hydrogen (H2) and methane (CH4), which are often indicative of partial discharge or arcing activity within the oil or solid insulation.13 This change in the gas profile raised concerns about the potential presence of developing insulation faults and prompted further investigation using partial discharge testing.

3. Partial Discharge Testing: A Multi-Method Approach

To thoroughly assess the condition of the transformer's insulation system, a comprehensive partial discharge testing program was implemented, utilizing a combination of different detection methods. This multi-method approach was chosen to leverage the strengths of each technique and gain a more complete understanding of the PD activity.16 The primary testing methods employed were the electrical (conventional) method, the acoustic emission (AE) method, and the ultra-high frequency (UHF) method.

3.1. Electrical (Conventional) Method

The electrical method, in accordance with IEC 60270 standards, directly measures the electrical pulses generated by partial discharges.2 A high-voltage coupling capacitor was connected to the transformer's bushing tap, and the signals were fed into a specialized PD detector. The system was calibrated using a pulse calibrator to ensure accurate measurement of the apparent charge in picocoulombs (pC).2 The test voltage was gradually increased up to the transformer's operating voltage, and PD activity was monitored throughout the voltage ramp and during a sustained period at the operating voltage. Phase-resolved partial discharge (PRPD) patterns were also recorded to provide insights into the type of defect causing the discharges.1

3.2. Acoustic Emission (AE) Method

The acoustic emission method detects the ultrasonic waves produced by the mechanical vibrations resulting from partial discharge events.16 Several piezoelectric AE sensors were strategically placed on the outer surface of the transformer tank, using magnetic mounts and acoustic couplant to ensure good signal transmission.10 The sensors were connected to a multi-channel AE data acquisition system, which recorded the amplitude, frequency, and time of arrival of the acoustic signals. By analyzing the time difference of arrival of the signals at different sensors, it was possible to estimate the location of the PD source within the transformer tank using triangulation techniques. This method is particularly useful for on-line testing and for pinpointing the physical location of discharge activity.10

3.3. Ultra-High Frequency (UHF) Method

The ultra-high frequency method detects the electromagnetic waves emitted by partial discharges in the UHF range (300 MHz to 3 GHz).16 A specialized UHF antenna, designed for insertion through an oil drain valve on the transformer tank, was used to capture these high-frequency signals.11 The UHF signals were then analyzed using a spectrum analyzer to identify the frequency components and amplitude of the emissions. PRPD patterns were also generated from the UHF data to correlate the electromagnetic activity with the phase of the applied voltage.31 The UHF method is known for its high sensitivity to internal discharges and its ability to filter out lower-frequency electrical noise.25

4. Test Results: Unveiling the Presence and Characteristics of PD

The partial discharge testing revealed significant PD activity within the power transformer. The results from each testing method provided complementary information about the nature and location of the discharges.

4.1. Electrical Method Findings

The electrical PD test detected consistent discharge activity with an apparent charge magnitude ranging from 150 to 300 pC at the transformer's operating voltage. The partial discharge inception voltage (PDIV) was recorded at approximately 85 kV, and the partial discharge extinction voltage (PDEV) was around 60 kV. The PRPD pattern exhibited characteristics indicative of internal void discharges, with pulses occurring predominantly around the rising and falling edges of the AC voltage waveform.34 This pattern suggested the presence of small air pockets or voids within the solid insulation of the transformer windings or bushings.

4.2. Acoustic Emission Method Findings

The acoustic emission testing detected multiple sources of ultrasonic activity within the transformer tank. Analysis of the time difference of arrival of the acoustic signals indicated that the primary PD source was located in the vicinity of the high-voltage winding, towards the top of the transformer. The acoustic signals had a frequency range between 30 kHz and 150 kHz, which is typical for partial discharges in oil-paper insulation. The amplitude of the acoustic signals correlated with the magnitude of the electrical discharges detected by the conventional method, suggesting a common origin.

4.3. Ultra-High Frequency Method Findings

The UHF testing also detected significant electromagnetic emissions in the frequency range of 400 MHz to 800 MHz. The PRPD pattern obtained from the UHF data corroborated the findings of the electrical method, showing discharge activity synchronized with the AC voltage cycle and consistent with internal void discharges. The amplitude of the UHF signals increased with the applied voltage, further confirming the presence of PD. The fact that the UHF signals were readily detected through the oil drain valve suggested that the discharge source was located within the main tank and not in an external component like a bushing.

5. Data Analysis and Interpretation: Pinpointing the Defect

The combined analysis of the results from the three PD testing methods provided a comprehensive picture of the insulation condition within the power transformer. The consistency in the PRPD patterns obtained from both the electrical and UHF methods strongly indicated that the primary source of PD was internal void discharges.34 The location of the acoustic emission source, identified near the top of the high-voltage winding, provided a physical area of focus for further investigation. The magnitude of the discharges, while not immediately critical, was significant enough to warrant concern and the need for remedial action to prevent further insulation degradation.15 The initial increase in hydrogen and methane levels in the DGA results served as a crucial trigger for the PD testing and aligned with the findings of electrical and electromagnetic activity associated with insulation breakdown.

6. Source Localization and Verification: Confirming the Defect Location

To further pinpoint the exact location and nature of the defect, an internal inspection of the transformer was scheduled during a planned outage. The oil was drained, and the transformer was carefully opened. Visual inspection of the high-voltage winding revealed evidence of discoloration and potential thermal stress in a specific section near the top, consistent with the location identified by the acoustic emission testing. Closer examination of this area revealed the presence of small voids and delaminations within the paper insulation between the winding layers. These voids were likely formed during the manufacturing process or developed over time due to thermal cycling and electrical stress, creating areas of high electric field concentration where partial discharges could initiate.8 Samples of the insulation paper from the affected area were taken for laboratory analysis to confirm the presence of degradation products associated with PD activity.

7. Resolution and Remedial Actions: Addressing the Insulation Issue

Based on the findings of the PD testing and the internal inspection, a decision was made to undertake repairs on the affected section of the high-voltage winding. The damaged insulation was carefully removed and replaced with new material, ensuring proper layering and impregnation to eliminate the voids and delaminations. The transformer was then reassembled, the insulating oil was processed to remove any dissolved gases and moisture, and the transformer was subjected to post-repair PD testing. The results of the follow-up testing showed a significant reduction in PD activity, with the apparent charge magnitude dropping to below 50 pC, and the PRPD patterns indicating a substantial improvement in the insulation condition. The acoustic emission and UHF testing also confirmed the absence of significant discharge activity in the previously identified location. Subsequent DGA results showed a stabilization and eventual decrease in the levels of hydrogen and methane, further validating the effectiveness of the repair work.

8. Lessons Learned and Recommendations: Enhancing Future Practices

This case study provided valuable insights into the detection, analysis, and resolution of partial discharge activity in a critical power transformer. Several key lessons were learned:

  • Proactive Monitoring is Crucial: Regular DGA serves as an effective initial screening tool for identifying potential insulation issues, prompting further investigation with PD testing.

  • Multi-Method Testing Provides a Comprehensive View: Utilizing a combination of electrical, acoustic, and UHF PD testing methods offers a more complete understanding of the nature and location of discharge activity, enhancing diagnostic accuracy.36

  • PRPD Patterns are Essential for Defect Identification: Analyzing PRPD patterns provides valuable information about the type of insulation defect causing the partial discharges, guiding the diagnostic process.38

  • Acoustic Emission Aids in Source Localization: Acoustic emission testing is highly effective in pinpointing the physical location of PD sources within the transformer tank, facilitating targeted inspections and repairs.

  • Internal Inspections are Necessary for Verification: In cases of significant PD activity, internal inspections are crucial for visually confirming the defect location and assessing the extent of the damage.40

  • Timely Remedial Actions Prevent Escalation: Addressing PD activity in its early stages through appropriate repairs can prevent further insulation degradation and potentially catastrophic failures, extending the operational life of the transformer.41

Based on the experience gained from this case, the following recommendations were made:

  • Implement continuous on-line PD monitoring systems for critical power transformers to provide real-time data and detect any sudden changes in insulation condition.43

  • Establish baseline PD signatures for new transformers to facilitate the identification of any developing issues over time.4

  • Develop and maintain a comprehensive database of PRPD patterns associated with different types of defects to aid in the interpretation of test results.

  • Ensure that maintenance personnel are adequately trained in the principles of partial discharge testing and the interpretation of test data.4

  • Integrate PD testing with other diagnostic methods, such as frequency response analysis (FRA) and insulation resistance testing, for a more holistic assessment of transformer health.

9. Conclusion: Safeguarding Critical Assets Through Vigilance

The case study of the power transformer at the industrial facility underscores the critical role of partial discharge testing in maintaining the reliability and longevity of high-voltage electrical equipment. The proactive approach, involving regular DGA and comprehensive PD testing, enabled the early detection of a developing insulation fault, preventing a potential catastrophic failure and ensuring the continued operation of a vital asset. The successful resolution of the PD issue through targeted repairs highlights the importance of a thorough diagnostic process and the value of investing in condition-based maintenance strategies. By embracing vigilance and implementing robust PD monitoring programs, organizations can effectively safeguard their critical power transformers, minimize downtime, reduce maintenance costs, and ensure a stable and reliable power supply for their operations. The insights and recommendations derived from this case study serve as a testament to the significance of partial discharge management in the realm of power system maintenance and asset management.

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