The turns ratio test of a power transformer is a vital diagnostic electrical test used to verify the ratio of the primary to secondary windings.
This blog post provides a detailed overview of its purpose, methodology, and key considerations.
Purpose of the Turns Ratio Test
The primary objectives of the turns ratio test are to:
- Validate Design Specifications: Ensure the transformer’s turns ratio matches the specified values.
- Detect Manufacturing Defects: Identify issues such as incorrect winding turns or coil connections.
- Assess Transformer Performance: Verify that the voltage transformation aligns with the intended design.
- Monitor Degradation: Detect changes in winding integrity due to aging or damage.
Type of Test
The turns ratio test is a low-voltage test conducted as part of routine, type, and acceptance testing. It is performed in factory settings during production and in the field during maintenance or commissioning.
Problems Identified by the Test
The turns ratio test can uncover:
- Incorrect Winding Turns: Detects mismatches in the number of turns between windings.
- Shorted Turns: Identifies internal winding faults caused by insulation failure.
- Connection Errors: Detects improper or incomplete connections in transformer windings.
- Tap Changer Malfunctions: Verifies the functionality of the tap changer and its impact on voltage ratios.
Test Setup
Performing a turns ratio test involves the following steps:
- Equipment Required:
- Turns ratio meter or TTR device.
- Insulated test leads and clamps.
- Preparation:
- De-energize the transformer and isolate it from the system.
- Identify and label all primary and secondary terminals.
- Ensure safety precautions to avoid accidental energization.
- Testing Procedure:
- Connect the turns ratio meter to the primary and secondary windings.
- Apply a low-voltage signal to the primary winding.
- Measure the voltage ratio between primary and secondary windings.
- Record readings for all tap positions (if applicable).
Passing Criteria and Tolerances
The turns ratio test results are usually compared to the initial factory test results or the nameplate ratings.
- Ratio Accuracy: The measured turns ratio should match the design specifications within a tolerance of 0.5%.
- Tap Changer Functionality: Ratios for all tap positions should align with manufacturer specifications.
- Phase Relationship: Ensure proper phase alignment between primary and secondary windings.
International Standards
The turns ratio test should adhere to standards such as:
- IEC 60076: Power Transformers—Part 1, General.
- IEEE C57.12.90: Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers.
These standards define the testing procedures and acceptable tolerances for turns ratio verification.
Key Points and Practical Experiences
- Calibrated Instruments: Use a calibrated turns ratio meter for precise measurements.
- Environmental Considerations: Perform the test under stable temperature and humidity conditions.
- Safety First: De-energize and isolate the transformer before testing.
- Record Keeping: Maintain detailed records of test results for comparison over time.
- Skilled Operators: Conduct the test with experienced personnel to ensure accuracy and reliability.
Conclusion
The turns ratio test is a critical step in ensuring the reliability and performance of power transformers. By following standardized procedures and leveraging expert knowledge, operators can identify potential issues early, supporting efficient and safe transformer operation. This test is indispensable for both commissioning and maintenance activities, ensuring compliance with design and operational requirements.
References
- IEC60076, Power Transformers, https://webstore.iec.ch/en/publication/588
- EEE C57.12.90, Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers, https://standards.ieee.org/ieee/C57.12.90/7211/