This report provides a comprehensive summary of the technical webinar, “Mastering Tan Delta (Power Factor) Measurement – Session 2,” of Wisegrid Energy. The objective of this document is to distill the critical methodologies, safety protocols, data interpretation techniques, and practical case studies for testing power transformers and high-voltage bushings as detailed in the presentation. The report will cover essential topics ranging from meticulous test preparation and analysis of results for different equipment types to the application of advanced diagnostic methods like frequency and voltage sweeps. These tests are cornerstones of modern asset management, providing deep insights into the health of critical insulation systems, and this summary serves as a practical guide to their effective implementation.
1. The Foundational Principles of Tan Delta and Capacitance Measurement
In modern electrical asset management, Tan Delta (also referred to as Dissipation Factor or Power Factor) and Capacitance measurements are of strategic importance for evaluating the insulation condition of high-voltage equipment. These tests provide a non-intrusive method to detect subtle degradation and mechanical changes that could lead to catastrophic failure. The core reasons for performing these tests are twofold:
- Tan Delta / Dissipation Factor: This measurement quantifies the extent of energy loss within an insulation system. Problems such as aging, insulation decomposition, and moisture ingress increase these dielectric losses, which manifest as additional heat within the insulation. A rising Tan Delta value is therefore a primary indicator of deteriorating insulation quality.
- Capacitance: This measurement serves a dual diagnostic role. For power transformers, a change in capacitance reveals physical winding displacement or deformation, often resulting from mechanical stress or through-fault events. For high-voltage bushings, it is a critical tool to identify short circuits that may have occurred between the internal capacitive grading layers.
The following sections will detail the specific application of these principles to power transformers and high-voltage bushings, outlining the distinct procedures and interpretation criteria for each.
2. Diagnostic Testing of Power Transformers
Accurate Tan Delta testing on power transformers requires a systematic approach to avoid incorrect diagnoses. This process encompasses meticulous preparation to eliminate external interference, a clear understanding of the measurement strategy based on the transformer’s winding configuration, and a nuanced interpretation of the results that goes beyond simple pass/fail values.
2.1. Test Preparation Protocol
Three mandatory preparation steps must be followed to ensure the integrity and accuracy of transformer test results:
- De-energize and Isolate: The transformer must be completely isolated from the grid. This includes disconnecting any connected current transformers (CTs) or circuit breakers. Even short lengths of connected conductors can introduce parasitic capacitance to ground, which will skew the measurement results.
- Ensure Proper Grounding: It is crucial to verify that the transformer tank is properly connected to the substation ground. This is an especially important check during the commissioning of new equipment.
- Short-Circuit Windings: All terminals belonging to the same winding must be connected together and short-circuited. Critically, this includes the neutral point, which must be isolated from ground and then short-circuited with the other bushings of that winding to prevent measurement errors.
2.2. Analyzing Two-Winding and Three-Winding Transformers
The complexity of Tan Delta testing increases with the number of windings in a transformer. Two-winding and three-winding transformers require different measurement strategies to accurately assess their internal capacitances.
| Feature | Two-Winding Transformers | Three-Winding Transformers |
| Primary Targets | The three main target capacitances are CHL (High-to-Low), CHG (High-to-Ground), and CLG (Low-to-Ground). | The six main target capacitances are CHL (High-to-Low), CHT (High-to-Tertiary), CHG (High-to-Ground), CLT (Low-to-Tertiary), CLG (Low-to-Ground), and CTG (Tertiary-to-Ground). |
| Measurement Strategy | Additional measurements are taken beyond the primary three main tests in order to double-check the initial results. | The measurement between the low-voltage and tertiary windings (CLT) is expected to yield a very low value. This occurs because the middle winding acts as an internal guard, shielding the other two windings from each other. |
To simplify these complex procedures, Wisegrid’s CAPTAN 12 software provides dynamic wiring diagrams that update in real-time to guide the operator on the correct connections for each specific measurement.
2.3. Interpreting Transformer Test Results
Evaluating the health of a transformer requires a comprehensive assessment of the test data against established standards and historical trends.
- Temperature Correction: While older IEEE C57 standards provided generic temperature correction factors, this practice is no longer recommended in newer versions. Due to the wide variation in modern insulation materials, it is now best practice to obtain specific correction factors directly from the transformer manufacturer.
- Dissipation Factor (DF) Limits: According to the IEEE C57.152 standard, the typical maximum DF for a new transformer with a voltage rating below 230 kV is 0.5%. For service-aged transformers, a DF up to 1% can be acceptable.
- Capacitance Change (Critical Indicator): Changes in capacitance are often a more urgent indicator of mechanical problems than DF. The assessment criteria are tiered:
- < 5%: The change is considered acceptable.
- 5% to 10%: The situation requires investigation, and testing intervals should be reduced.
- > 10%: The change is critical, and the transformer should not be energized until the cause is identified and resolved.
- Rate of Change: The most powerful analysis comes from trend analysis over time. The rate of change, comparison with similar assets (sister units), and correlation with complementary tests like Dissolved Gas Analysis (DGA) are more important than a single absolute value.
3. Diagnostic Testing of High-Voltage Bushings
While physically smaller than transformers, high-voltage bushings have unique construction characteristics and failure modes that demand specialized Tan Delta and capacitance testing approaches. Understanding their design is the first step toward effective diagnosis.
3.1. Bushing Construction and Critical Terminals
Modern condenser-type bushings consist of capacitive grading layers, typically made of aluminum foil and impregnated paper, designed to control the electrical field. For testing, two primary types of measurement taps are used:
- Test Tap (or Tan Delta Tap): This is the connection to the last aluminum foil layer of the bushing. The capacitance between the main conductor and this tap (C1) is the primary measurement, while the capacitance from this tap to ground (C2) is typically in the 500-600 pF range. Importantly, the measured C2 value will change when the bushing is installed on a transformer. This is due to stray capacitance effects from the transformer’s internal structure, a key reason why nameplate C2 values cannot be directly compared to in-situ measurements.
- Potential Tap (or Voltage Tap): This tap connects between the last and second-to-last conductive layers. It has a much higher capacitance (1-5 nF), and its value does not change upon installation, making it a more stable reference point if available.
A critical safety warning was issued regarding these taps: leaving a test tap ungrounded while the bushing is in service can induce dangerously high voltages on the tap. For example, a 220 kV bushing could develop 61.1 kV on an open test tap. This will lead to arcing, partial discharge, overheating, and ultimately, catastrophic failure of the bushing. The tap must always be properly grounded after testing is complete.
3.2. Common Fault Signatures in C & DF Measurements
Different internal bushing faults produce distinct and identifiable changes in capacitance and dissipation factor, enabling technicians to perform a targeted diagnosis.
| Fault Type | Effect on Capacitance (C1) | Effect on Dissipation Factor (DF) |
| Interturn Short Circuit | Increases | May remain constant |
| Moisture Ingress | Increases | Increases |
| Contamination | Depends on the type of contaminant | Increases |
| Oil Leakage | Decreases | Decreases at a low-test voltage but increases at a high-test voltage due to partial discharge. |
3.3. Test Methodologies and Safety Protocols
The standard test for an installed bushing is the C1 measurement, performed in Ungrounded Specimen Test (UST) mode. However, executing this test safely and accurately requires adherence to crucial protocols.
For safety, all unused windings on the transformer must be grounded. Failure to do so can create a hazardous induced voltage on other terminals due to capacitive coupling. For instance, applying 10 kV to the high-voltage (HV) side for testing can induce a dangerous 5 kV on the ungrounded low-voltage (LV) terminals.
For accuracy, it is essential that the bushing under test be short-circuited with the other bushings of the same winding. Attempting to test a single bushing in isolation will produce erroneously high dissipation factor readings. The webinar provided a clear example of this procedural error: a new bushing measured a normal 0.25% DF before installation. After installation, when tested in isolation, the same bushing measured 0.617%—nearly 2.5 times higher—incorrectly suggesting a problem where none existed.
A supplementary diagnostic method is the “Hot Collar” test. This involves placing a conductive collar around the bushing and is used to verify oil levels, especially in bushings that do not have a sight glass. The primary acceptance criterion mentioned for this test is a watt loss of less than 0.1 watts.
3.4. Interpreting Bushing Test Results
The condition of a bushing is assessed using criteria similar to those for transformers, but with specific thresholds for dissipation factor based on nameplate values.
- Capacitance Change (C1): The limits are identical to those for transformers. A change of less than 5% is acceptable, 5-10% requires investigation, and a change greater than 10% is critical.
- Dissipation Factor (C1): The assessment is made relative to the nameplate reference value (DF_ref):
- < 2 x DF_ref: Acceptable
- 2 to 3 x DF_ref: Requires investigation
- > 3 x DF_ref: Critical
- Temperature & Moisture Influence: The dissipation factor is strongly dependent on temperature, particularly when high moisture content is present. An ABB recommendation to test a bushing immediately after de-energizing (while it is hot) and then test it again several hours later (after it has cooled). A large change in the DF value between these two tests is a strong indicator of an internal problem.
4. Advanced Diagnostic Techniques
Beyond standard 50/60 Hz testing, frequency and voltage sweeps offer powerful, non-intrusive methods for diagnosing subtle or emerging insulation problems that might otherwise go undetected.
4.1. Frequency Sweep Analysis
The principle of frequency sweep analysis is that decreasing the test frequency has a similar effect on the dissipation factor as increasing the insulation’s temperature. The key diagnostic takeaway is that a sharp increase in the dissipation factor as the test frequency is lowered (e.g., from 60 Hz down to 1 Hz) is a strong indicator of insulation degradation, high moisture content, or other contaminants.
4.2. Voltage Sweep (Tip-Up) Testing
A voltage sweep, also known as a “Tip-Up” test, involves incrementally increasing the test voltage (e.g., from 2 kV up to 12 kV) while measuring the dissipation factor. A significant change in the dissipation factor as the voltage changes is a primary indicator of issues such as external contamination on the bushing’s surface, bad electrical connections, or internal partial discharge activity.
5. Featured Technology in Practice: The CAPTAN 12 System
The webinar featured demonstrations using the CAPTAN 12 test system, highlighting its capabilities for performing the tests described. Key features and specifications of the system include:
- Voltage/Current: An output of up to 12 kV and 300 mA.
- Frequency Range: A wide range from 1 Hz to 500 Hz, enabling advanced frequency sweep diagnostics.
- Design: A modular design that allows for easy field servicing of components like the power supply and measurement modules.
- Software: The system includes a workspace for project management and provides dynamic wiring diagrams to guide the user through the correct test connections for various measurements.
6. Conclusion: Key Takeaways for Field Professionals
This technical webinar provided a wealth of actionable information for technicians and engineers responsible for high-voltage asset management. The most critical insights and best practices can be distilled into a set of “golden rules” for field professionals:
- Always Prioritize Capacitance: Winding deformation and bushing short-circuits are often revealed by significant changes in capacitance long before the dissipation factor is affected. Always check the capacitance (‘C’) value before focusing on the dissipation factor (‘DF’).
- Preparation is Paramount: The root cause of incorrect conclusions is often poor test preparation. Ensure the asset is completely isolated, the grounding is correct, and all windings are short-circuited properly before starting any measurement.
- Heed All Safety Protocols: Ungrounded test taps on energized bushings and ungrounded transformer windings during testing are severe safety hazards. These risks must be managed vigilantly through strict adherence to established safety procedures.
- Leverage Advanced Diagnostics: When standard tests are inconclusive or point to a subtle issue, use frequency and voltage sweeps. These tools can uncover insulation degradation, contamination, or connection problems that a single-frequency test might miss.
- Contextualize Your Data: While absolute values have their place, the most powerful analysis comes from context. This includes analyzing trend data over time, comparing results with sister units, and correlating findings with other diagnostic tests like DGA.
If you have any questions about the webinar, feel free to contact us:
📞 +1 (647) 300-8836 | 📞 +1 (949) 434-6077
📧 info@wisegridenergy.com
