Understanding Partial Discharge and How to Perform Testing on Power Transformers and GIS Switchgear

How to Perform PD Testing on Power Transformers and GIS Switchgear

The increasing demand for a consistent and reliable electricity supply places immense pressure on power infrastructure. Critical components like power transformers and Gas-Insulated Switchgear (GIS) are essential for maintaining this supply. A common indicator of potential issues within these systems is partial discharge (PD), a localized electrical breakdown in the insulation that can ultimately lead to significant problems.¹Detecting and understanding PD through regular testing is therefore paramount for ensuring the longevity and reliability of these vital assets.²

What Exactly is Partial Discharge? Breaking Down the Basics

Partial discharge refers to small, localized electrical sparks or discharges that occur within the insulation system of medium- and high-voltage electrical equipment.² These discharges are termed "partial" because they do not completely bridge the insulation between two conducting electrodes.⁶ The International Electrotechnical Commission (IEC) 60270 standard defines partial discharges as “localized electrical discharges that only partially bridge the insulation between conductors and which may or may not occur adjacent to a conductor”. This highlights the contained nature of the electrical activity within the insulating material. Partial discharge is a complex physical phenomenon resulting from local electrical stress concentration in the insulation.¹¹ It is a widely accepted indicator for defects .

Several factors can initiate partial discharge by creating areas of concentrated electrical stress within the insulation or on its surface.¹² These include inhomogeneities in the dielectric material such as voids or air pockets in solid insulation, cracks, and gas bubbles or particulate contaminants in liquid insulation. Contaminants like dust, dirt, or moisture on the insulation surface can also disrupt the electric field and lead to discharges. Furthermore, defects introduced during manufacturing or installation, as well as the natural aging and deterioration of insulation materials over time, can create weak points susceptible to PD. Over-stressing of the insulation due to high voltage fields or transient events can also trigger PD. Poor electrical connections or loose components within the equipment can also contribute to partial discharge activity.² The presence of air bubbles or other impurities inside insulators can lead to local electrical breakdown and PD.¹⁴ Understanding partial discharge characteristics is crucial to prevent the degradation of electrical insulation performance.⁸ PD can occur in systems operating at voltages of 3000V and above .

Partial discharge can manifest in various locations within power transformers and GIS switchgear. In power transformers, common sites include gas-filled voids within solid insulation found in windings or bushings, gas bubbles within the insulating oil, along the interfaces of different insulating materials, and around sharp edges of conductors where the electric field is most intense. Issues like poor electrical connections or movement of windings can also generate PD.² For GIS switchgear, PD often occurs due to the presence of particles, such as metal shavings, floating within the sulfur hexafluoride (SF6) insulating gas. It can also arise around loose or broken components or within voids present in solid insulation parts like spacers. Surface discharges along the surfaces of insulators are another potential location for PD in GIS. Other causes of PD include surface contamination, workmanship issues, and material defects.¹⁷ In GIS, protrusions or fixed particles, moving particles, voids in insulators, and poor contact can all lead to PD . Loss of insulation integrity due to loss of gas pressure or density can also be a factor .

Even though individual partial discharges may be small, their continuous occurrence can pose a significant threat to the long-term health of the equipment.⁷ These discharges act like tiny, repeated sparks that gradually erode and degrade the insulation material. This process can lead to the formation of electrical trees, which are branching patterns of degradation within the insulation , as well as tracking along insulation surfaces. Over time, this cumulative damage weakens the dielectric strength of the insulation, eventually leading to complete insulation failure and potentially catastrophic breakdown of the equipment. The total charge redistributed within the barrier is a good indicator of the number of voids and their likelihood of becoming a failure.⁶ Setting a low limit on allowable partial discharge during testing provides confidence that a high voltage failure will not occur over time.⁶

The Importance of Partial Discharge Testing: Protecting Your Assets

Partial discharge (PD) testing is an indispensable diagnostic method for ensuring the reliability and longevity of both power transformers and GIS switchgear.² It acts as an early warning system, enabling the detection of insulation degradation before it culminates in major failures.

For both types of high-voltage equipment, PD testing offers a multitude of crucial benefits. It allows for the early detection of insulation defects, often identifying minor flaws or weaknesses before they can be detected by other diagnostic techniques. By addressing PD activity in its initial stages, catastrophic failures, which can result in significant equipment damage, prolonged power outages, and serious safety hazards, can be effectively prevented. Regular PD testing provides valuable insights into the rate at which insulation is degrading, facilitating timely maintenance interventions that can substantially extend the operational lifespan of expensive assets like transformers and GIS. Furthermore, the early identification and rectification of insulation issues through PD testing prove to be significantly more cost-effective than dealing with major equipment breakdowns and the associated downtime. Predictive maintenance strategies informed by PD data enable planned outages and more efficient repairs. Ultimately, by preventing unexpected failures, PD testing directly contributes to the overall reliability and availability of the power supply network, minimizing disruptions for consumers. In addition to monitoring equipment in service, PD testing plays a vital role in quality assurance during the manufacturing process, ensuring that new equipment adheres to insulation standards and is free from defects. It is also essential during on-site commissioning to detect any damage that might have occurred during transportation or installation. On-line PD testing of MV and HV plant gives an advance warning of pending insulation failure, allowing plant owners to take remedial maintenance action during planned outages.¹⁰ Unlike off-line testing, on-line PD testing and monitoring provide an accurate picture of the HV plant's health and performance under normal service conditions, including the effect of load, temperature, and humidity.¹⁰ Qualification of PD 'criticality' within the plant owner's HV network can be achieved quickly and easily using on-line, screening, and diagnostic PD test technology to provide an 'early warning system' for incipient insulation faults.¹⁰

The significance of PD testing is underscored by the fact that it is often a mandatory factory acceptance test for high-voltage equipment. The fundamental drive behind PD measurement is to proactively prevent unplanned failures, particularly in power transformers. International standards, such as IEC 60270, provide comprehensive guidelines for conducting partial discharge measurements, ensuring consistency and comparability across different tests and equipment. Additionally, IEEE standards offer valuable guidance on PD testing methodologies and their applications in the power industry.

Investing in PD testing is therefore a proactive and strategic approach to asset management. The ability to identify and address potential insulation issues early offers substantial returns in terms of preventing costly failures, ensuring the longevity of critical equipment, and maintaining a reliable power supply. Adherence to established international standards further validates the reliability and widespread acceptance of PD testing practices.

Step-by-Step Guide: Performing Partial Discharge Testing on Power Transformers

Several methods are available for performing partial discharge testing on power transformers, each detecting different physical manifestations of PD. The most common techniques include the electrical (conventional) method, the acoustic emission (ultrasonic) method, the ultra-high frequency (UHF) method, and dissolved gas analysis (DGA). The electrical method, as defined by IEC 60270 , directly measures the electrical pulses produced by PD using a coupling capacitor and a specialized PD detector, quantifying the apparent charge in picocoulombs (pC). This method allows for precise calibration, especially in controlled laboratory environments.¹⁹ The acoustic emission (AE) method detects the ultrasonic pressure waves generated by PD events using sensors placed on the transformer tank's exterior. Being non-invasive, it's suitable for on-line testing and can help locate PD sources using multiple sensors. The ultra-high frequency (UHF) method detects the electromagnetic waves emitted by PD in the UHF range (300 MHz to 3 GHz) using antennas placed inside or outside the transformer tank. The tank itself provides shielding against external noise, and this method offers high sensitivity and potential for PD localization. While not a direct PD test, dissolved gas analysis (DGA) is a vital complementary technique that identifies gases produced by the breakdown of insulation due to PD and other faults. Other methods include radio interference voltage (RIV) testing ⁴, optical detection , and chemical detection . Hybrid methods combining different techniques are also employed .

Here is a simplified, step-by-step procedure for conducting a basic PD test on a power transformer using the electrical (conventional) method ²:

1. Prioritize Safety: Ensure the power transformer is completely de-energized and isolated from the power system following strict safety protocols. Ground all components of the test setup to prevent any electrical hazards. Safety is paramount when working with high voltage equipment .
2. Connect the Measurement System: Establish a connection between the PD measuring instrument and the transformer. This typically involves connecting a high-voltage coupling capacitor in parallel with the transformer under test. The signals from this capacitor are then transmitted to the PD detector via a shielded cable to minimize interference.² Connections are often made at the transformer's bushing taps. The measuring system typically includes a coupling device, a transmission system, and a measuring instrument .
3. Calibration is Key: Calibrate the PD measuring system before commencing the test. This is achieved by injecting a pulse of known charge (from a calibrator) into the test circuit and observing the corresponding reading on the PD detector.² This calibration step establishes a reference, allowing for accurate measurement of the apparent charge of any PD events in picocoulombs (pC). Calibration ensures that the apparent charge measured at the test terminals is representative of the actual discharges .
4. Apply the Test Voltage: Gradually increase the AC voltage applied to the transformer, starting from a level below the anticipated partial discharge inception voltage (PDIV). The voltage is typically raised in stages, with a brief pause at each level. The maximum test voltage and the duration for which it is applied are usually specified in relevant standards such as IEC 60076-3 or IEEE C57.12.90, or as recommended by the transformer manufacturer.²² The partial discharge inception voltage (PDIV) is the lowest voltage at which PD pulses are observed .
5. Monitor for PD Activity: Continuously observe the output of the PD detector for any indications of discharge pulses. Note the voltage level at which PD first appears (PDIV) and the voltage at which it disappears when the voltage is lowered (PD extinction voltage, PDEV). Record the magnitude (in pC) and the frequency of occurrence of the PD pulses at various voltage levels. The partial discharge extinction voltage (PDEV) is the voltage at which PD ceases .
6. Analyze PRPD Patterns: Most modern PD detectors offer the capability to display phase-resolved partial discharge (PRPD) patterns. These patterns visualize the PD pulses in relation to the phase angle of the applied AC voltage, providing valuable insights into the nature of the defect causing the discharge. Carefully observe and record these patterns for subsequent analysis. Different types of insulation defects often produce unique PRPD patterns .
7. Evaluate the Findings: Compare the measured PD levels (magnitude, rate, and patterns) with the acceptable limits outlined in the relevant standards or in the transformer's factory test report.²⁴ Excessive PD activity suggests potential insulation issues that warrant further investigation. The apparent charge (q) of the PD pulses, measured in pico-coulombs (pC), is a key parameter to evaluate .

Key parameters to monitor during PD testing of power transformers include the apparent charge (q) of the PD pulses, the partial discharge inception voltage (PDIV), the partial discharge extinction voltage (PDEV), the phase angle (φ) at which the PD pulses occur, and the overall characteristics of the discharges as depicted in the PRPD plot. The discharge power, generally expressed in watts (W), and the quadratic rate (D), which is the sum of the squares of the individual apparent charge magnitudes, are also important parameters .

Step-by-Step Guide: Performing Partial Discharge Testing on GIS Switchgear

Partial discharge testing of Gas-Insulated Switchgear (GIS) often employs techniques tailored to its unique design, where high-voltage conductors are housed within a grounded metal enclosure filled with insulating SF6 gas. Common PD testing methods for GIS include the ultra-high frequency (UHF) method, the acoustic emission (ultrasonic) method, the electrical (conventional) method, and the transient earth voltage (TEV) method. The ultra-high frequency (UHF) method is particularly effective for GIS due to the enclosed environment, which minimizes external interference. PD events in GIS emit electromagnetic waves in the UHF range (100 MHz to 2 GHz) , which can be detected by UHF sensors placed internally or externally. This method offers high sensitivity and potential for on-line monitoring and PD source localization. The acoustic emission (ultrasonic) method uses sensors on the GIS enclosure to detect sound waves generated by PD within the SF6 gas or solid insulation. It's advantageous in high electromagnetic interference environments and can aid in locating PD sources. While applicable, the electrical (conventional) method (IEC 60270) can be challenging on-site due to the capacitance of GIS and susceptibility to noise. Coupling capacitors are used to extract PD signals. Transient earth voltage (TEV) sensors on the GIS surface can detect high-frequency voltage pulses caused by internal PD, offering a non-intrusive on-line testing option. Other methods include gas analysis to detect SF6 decomposition products , and optical methods .

Here's a simplified step-by-step procedure for conducting a basic PD test on GIS switchgear using the Ultra-High Frequency (UHF) method ²⁵:

1. Adhere to Safety Protocols: Strictly follow all safety regulations when working with energized high-voltage equipment. Ensure access to the designated UHF sensor ports or windows on the GIS enclosure. Safety is paramount when dealing with high voltage .
2. Install UHF Sensors: Connect UHF sensors to the designated ports on the GIS. Many GIS installations come equipped with pre-installed UHF couplers. Ensure the sensors are properly connected to the UHF PD detection instrument using coaxial cables.²⁵ Various types of UHF sensors exist, including disc-type, monopole-type, and spiral-type sensors.²⁷
3. Initiate Data Acquisition: Power on the UHF PD detection system and configure the necessary measurement parameters, such as the frequency range and recording duration.²⁵ Begin the data acquisition process to capture any UHF signals originating from within the GIS compartments. This testing can be performed on-line while the GIS is in service, making it a valuable tool for continuous condition monitoring.
4. Filter Noise and Process Signals: Utilize the software associated with the UHF PD detection system to filter out any background noise and analyze the captured signals. Look for transient pulses within the UHF frequency range that are indicative of partial discharges. Noise suppression techniques are crucial for accurate PD detection .
5. Analyze PRPD/TRPD Patterns: Many UHF PD analysis systems can generate phase-resolved partial discharge (PRPD) patterns by synchronizing the detected UHF pulses with the phase of the power frequency voltage. Analyzing these patterns can assist in identifying the type of insulation defect causing the PD, such as floating particles, surface discharge, or voids. Time-resolved partial discharge (TRPD) patterns can also be analyzed, particularly for DC GIS applications.
6. Localize the Discharge (If Possible): If multiple UHF sensors are strategically placed in different compartments of the GIS, the time difference of arrival (TDOA) of the PD signals at these sensors can be used to estimate the location of the discharge source within the GIS. Specialized software is often employed for this localization analysis.
7. Evaluate and Track Trends: Compare the detected PD activity, including its amplitude, rate, and patterns, with baseline measurements obtained during the initial commissioning of the GIS or from previous tests.²⁸ Also, compare the current activity with any established alarm thresholds within the monitoring system. An increase in PD activity or the emergence of new or more severe PD patterns may signal a developing insulation problem. Trend analysis over time is important for assessing insulation degradation .

Key parameters to monitor during UHF PD testing of GIS include the amplitude and repetition rate of the UHF pulses, the characteristics of the PRPD or TRPD patterns, and the location of the discharge if localization techniques are used. The frequency spectrum of the PD signals can also provide valuable information .

Interpreting Partial Discharge Test Results: What Do the Numbers Mean?

Interpreting the results of partial discharge tests, whether conducted on power transformers or GIS switchgear, necessitates a careful examination of several key parameters. The apparent charge (q), measured in picocoulombs (pC), indicates the magnitude of individual discharge events. Generally, higher charge values suggest a more energetic discharge and potentially a more severe defect. However, the actual significance of a specific charge value can vary depending on the equipment type, its voltage rating, and the precise location of the discharge. The partial discharge inception voltage (PDIV) is the voltage level at which PD activity begins as the applied voltage increases, while the partial discharge extinction voltage (PDEV) is the level at which PD ceases as the voltage decreases. A lower PDIV may indicate a higher susceptibility to PD, and a significant difference between PDIV and PDEV can be indicative of certain types of defects. The repetition rate, or frequency, of PD pulses provides information about the activity level of the discharge source. A high repetition rate might suggest a rapidly deteriorating insulation condition.

Phase-resolved partial discharge (PRPD) patterns, which plot PD pulses against the phase angle of the applied AC voltage, are highly valuable for diagnosis. Different types of insulation defects often produce unique PRPD patterns that experienced analysts can recognize. For instance, discharges in voids frequently appear concentrated around the rising portions of the voltage waveform, whereas corona discharges might be more prevalent near the voltage peaks.²⁹ Monitoring changes in PD parameters over time, known as trend analysis, is crucial for assessing the progression of insulation degradation. An increasing trend in PD magnitude or repetition rate, or a change in the PRPD pattern, can signal a worsening condition that requires attention. Correlating PD activity with operational factors like load and temperature can also provide important context.⁶

While these parameters offer significant insights, accurate diagnosis and evaluation of PD test results often require the expertise of trained analysts. Experienced professionals can consider the specific characteristics of the equipment, the testing method employed, and the overall context of the results to determine the severity of the PD and recommend appropriate actions. They are also skilled at distinguishing between genuine PD signals and noise or other forms of interference.

Conclusion: Investing in Partial Discharge Testing for Long-Term Reliability

Understanding partial discharge and implementing routine PD testing programs for power transformers and GIS switchgear represents a critical investment in the long-term reliability and safety of your electrical infrastructure. The early detection of insulation defects through PD testing can prevent catastrophic equipment failures, significantly reduce costly downtime, extend the operational life of your valuable assets, and ultimately contribute to a more stable and dependable power supply. By grasping the fundamental principles of PD and the common testing methodologies, technical managers and maintenance engineers can make well-informed decisions regarding their maintenance strategies, ensuring the continued safe and efficient operation of their power systems. It is highly recommended to incorporate regular partial discharge testing as a vital component of your predictive maintenance program to proactively address potential insulation issues and safeguard your critical electrical assets for years to come. For comprehensive PD testing and analysis, always seek consultation from qualified professionals who possess the necessary expertise and specialized equipment to accurately assess the condition of your power transformers and GIS switchgear.

Explore our comprehensive range of partial discharge testing services and solutions, meticulously designed to assist you in maintaining the health and reliability of your high-voltage equipment. Contact us today for a consultation to discover how we can partner with you to protect your critical assets and ensure the uninterrupted operation of your power systems.

Works cited

1. Interpretable Detection of Partial Discharge in Power Lines with Deep Learning - PMC, accessed May 18, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC8003486/

2. The Evolution of Partial Discharge Testing in Electrical Equipment - NETA World Journal, accessed May 18, 2025, https://netaworldjournal.org/the-evolution-of-partial-discharge-testing-in-electrical-equipment/

3. Overview and Partial Discharge Analysis of Power Transformers: A Literature Review - SciSpace, accessed May 18, 2025, https://scispace.com/pdf/overview-and-partial-discharge-analysis-of-power-5fz54kob3r.pdf

4. Online Partial Discharge Monitoring and discharge localization on transformers by means of UHF method - Qualitrol Corp, accessed May 18, 2025, https://www.qualitrolcorp.com/wp-content/uploads/2018/01/Online-Partial-Discharge-Monitoring-and-discharge-localization-on-transformers-by-means-of-UHF-method-final-Rev.-1.pdf

5. On-site Partial Discharge Testing of Transformers - Type here the title of your Paper, accessed May 18, 2025, https://cigre.ca/papers/2021/paper%20441.pdf

6. A Review Partial Discharge Activity in Electrical Insulation - CiteSeerX, accessed May 18, 2025, https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=7fc4ce0e48c1e4ac1cfac2978c2991eb34823973

7. Basics of Partial Discharge - TEquipment, accessed May 18, 2025, https://assets.tequipment.net/assets/1/26/Phenix_Basics_of_Partial_Discharge.pdf

8. Journal Paper on Partial Discharge Monitoring? - ResearchGate, accessed May 18, 2025, https://www.researchgate.net/post/Journal-Paper-on-Partial-Discharge-Monitoring

9. Introduction to Partial Discharge (Causes, Effects, and Detection), accessed May 18, 2025, https://site.ieee.org/sas-pesias/files/2020/05/IEEE-Alberta_Partial-Discharge.pdf

10. Guide for Field Test of Partial Discharge in Power Transformers - of IEEE Standards Working Groups, accessed May 18, 2025, https://grouper.ieee.org/groups/transformers/subcommittees/dielectric_test/F16-PAR%20Request_Guide%20for%20Field%20Test%20of%20Partial%20Discharge.pdf

11. Identification of Partial Discharge Sources by Feature Extraction from a Signal Conditioning System - PMC, accessed May 18, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC11014056/

12. Partial Discharge | Encyclopedia MDPI, accessed May 18, 2025, https://encyclopedia.pub/entry/7528

13. Offline Partial Discharge Measurement and Analysis - OMICRON, accessed May 18, 2025, https://www.omicronenergy.com/en/application/offline-testing/offline-partial-discharge-measurement-and-analysis/

14. (PDF) Estimation of partial discharge parameters in GIS using acoustic emission techniques, accessed May 18, 2025, https://www.researchgate.net/publication/238951105_Estimation_of_partial_discharge_parameters_in_GIS_using_acoustic_emission_techniques

15. A Novel Method for Pattern Recognition of GIS Partial Discharge via Multi-Information Ensemble Learning - MDPI, accessed May 18, 2025, https://www.mdpi.com/1099-4300/24/7/954

16. The Basics of Partial Discharge Testing - HV TECHNOLOGIES, Inc., accessed May 18, 2025, https://www.hvtechnologies.com/the-basics-of-partial-discharge-testing/

17. The Importance of Partial Discharge Testing on Power Transformers - OMICRON, accessed May 18, 2025, https://www.omicronenergy.com/en/news/coverstory/the-importance-of-partial-discharge-testing-on-power-transformers/

18. Transformer Partial Discharge Monitoring | PD Monitoring - Dynamic Ratings, accessed May 18, 2025, https://www.dynamicratings.com/solutions/transformer-monitoring/partial-discharge-monitoring/

19. Guide for Field PD Tests for Liquid-filled Power Transformers - of IEEE Standards Working Groups, accessed May 18, 2025, https://grouper.ieee.org/groups/transformers/subcommittees/dielectric_test/F16-Presentation_Field%20Guide%20for%20Partial%20Discharge%20Measurement.pdf

20. ABC Partial Discharge Measurement as per IEC 60270 - YouTube, accessed May 18, 2025, https://www.youtube.com/watch?v=jznaR-BNLxI

21. Guide to Transformer Testing Standards, accessed May 18, 2025, https://www.maddox.com/resources/articles/guide-to-transformer-testing-standards

22. Partial Discharge Recognition in Gas Insulated Switchgear Based on Multi-information Fusion - ResearchGate, accessed May 18, 2025, https://www.researchgate.net/publication/274838121_Partial_Discharge_Recognition_in_Gas_Insulated_Switchgear_Based_on_Multi-information_Fusion

23. HOW DOES IT WORK? PARTIAL DISCHARGE TESTING EXPLAINED - Pulse Electronics, accessed May 18, 2025, https://www.pulseelectronics.com/wp-content/uploads/2021/06/Partial-Discharge.pdf

24. Partial Discharge Measurement (GIS) - OMICRON, accessed May 18, 2025, https://www.omicronenergy.com/en/solution/partial-discharge-measurement-gis/

25. A Meticulous Method for the Measurement of Partial Discharges in Gas Insulated Switchgears - Warse, accessed May 18, 2025, http://www.warse.org/IJETER/static/pdf/file/ijeter06932021.pdf

26. How to write a blog post that will actually get read. Really!, accessed May 18, 2025, https://blog.thebrokerlist.com/how-to-write-a-blog-post-that-will-actually-get-read-really/

27. Partial Discharge Based Risk Assessment Framework From MV Switchgear Containing Electrical Defects | Request PDF - ResearchGate, accessed May 18, 2025, https://www.researchgate.net/publication/372791579_Partial_Discharge_Based_Risk_Assessment_Framework_From_MV_Switchgear_Containing_Electrical_Defects

28. Research of GIS partial discharge type evaluation based on convolutional neural network, accessed May 18, 2025, https://pubs.aip.org/aip/adv/article/10/8/085305/992500/Research-of-GIS-partial-discharge-type-evaluation