The no-load loss test measures the iron losses in a power transformer when operating at rated voltage without any load connected. It is an essential diagnostic tool for evaluating transformer performance under minimal load conditions.
This post discusses the purpose, methodology, and best practices related to the no-load loss test of a power transformer.
The main no-load losses in a transformer are caused by hysteresis and eddy currents in the core. An important measure is the Excitation Current, which is the current required to maintain the magnetic flux in the transformer and identifies the magnetic properties of the core.
Purpose of the No-Load Loss Test
The primary objective of the no-load loss test are to:
- Assess Core Losses: Measure iron losses in the transformer core due to hysteresis and eddy currents.
- Evaluate Magnetizing Current: Determine the current required to magnetize the core under no-load conditions.
- Ensure Design Compliance: Verify that the core and associated losses align with design specifications.
- Identify Core Defects: Detect issues such as core lamination damage or incorrect assembly.
Type of Test
The no-load loss test is conducted offline during factory acceptance, commissioning, and routine maintenance. It involves energising the transformer at rated voltage and frequency without connecting any load.
Problems Identified by the Test
This test can identify several potential issues, such as:
- Excessive Core Losses: Indicates design flaws or material degradation.
- Core Lamination Issues: Identifies defects such as insulation breakdown, e.g. weak insulation between silicon steel sheets, partial short-circuits between laminations, loose or misaligned laminations.
- Other Core Issues: failed insulation in core bolts, binding belts, and yokes or faults arising from multiple grounding points.
- Harmonic Distortion: Detects issues in the supply voltage causing non-linear core behavior.
Test Setup
Performing the no-load loss test involves the following steps using the test circuit in Figure 1.
- Equipment Required:
- Voltmeter, ammeter, and wattmeter.
- Variable voltage source.
- Safety and grounding equipment.
- Preparation:
- De-energize and isolate the transformer.
- Connect the primary winding to the variable voltage source.
- Leave the secondary winding open-circuited.
- Testing Procedure:
- Gradually apply voltage to the primary winding up to the rated level.
- Measure and record the input power, voltage, and current.
- Calculate core losses and magnetizing current based on readings.
Passing Criteria and Tolerances
The test results should meet the following criteria:
- Core Loss Limits: Losses should not exceed the values specified in the transformer design or nameplate data.
- Magnetizing Current: Should fall within acceptable limits as per manufacturer specifications.
- Voltage and Frequency Accuracy: Ensure the applied voltage and frequency are stable and match rated values.
International Standards
The no-load loss test adheres to the following standards:
- IEC 60076: Power Transformers—Part 1, General.
- IEEE C57.12.90: Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers.
Key Points and Practical Experiences
- Stable Input Supply: Use a stable and distortion-free voltage source to avoid inaccurate results.
- Temperature Considerations: Perform the test at standard ambient temperature for consistent results.
- Monitoring Harmonics: Check for harmonics in the input supply, as they can affect core loss readings.
- Regular Testing: Conduct periodic no-load loss tests to monitor core health over time.
- Skilled Operators: Ensure the test is performed by experienced personnel to maintain accuracy and safety.
Conclusion
The no-load loss test is an important test for verifying transformer performance and ensuring system reliability. By adhering to standardised procedures and maintaining accurate records, operators can proactively identify potential issues and optimise transformer operations, ensuring long-term efficiency and safety.