Transformers are critical components of modern electrical systems, ensuring efficient power distribution across industries. Given their vital role, transformer testing is essential for maintaining reliability, safety, and performance throughout their operational lifespan and can be likened to other equally beneficial solutions like electric motor rewinds.
Testing typically falls into three key categories: commissioning, routine screening, and diagnostic testing. Each serves distinct purposes and involves specific methodologies to address potential vulnerabilities.
Commissioning
Commissioning occurs when a new transformer is installed. This process verifies the transformer’s performance after transportation and ensures it aligns with manufacturer specifications. Moisture ingress and physical stress during transit are common concerns addressed during commissioning. Additionally, commissioning establishes a performance baseline for future comparisons, aiding in early fault detection. Key tests during commissioning include, but are not limited to:
1. Power Factor Testing
Power factor testing is a crucial diagnostic tool that evaluates the condition of a transformer’s electrical insulation. It achieves this by applying a known voltage and measuring the leakage current resulting from imperfections in the insulation.
The power factor represents the cosine of the phase angle between voltage and current. Ideally, a transformer’s power factor would be zero, indicating perfect insulation with no leakage current. However, because no insulation is flawless, small deviations from zero are expected.
2. Dielectric Frequency Response (DFR) and Variable Frequency Power Factor Testing
While conventional power factor testing is invaluable, it may not always offer a complete picture of your transformer’s insulation health. Advanced techniques like Dielectric Frequency Response (DFR) and Variable Frequency Power Factor Testing bridge this gap.
Variable Frequency Power Factor Testing:
This method complements conventional power factor tests by determining the system’s Individual Temperature Correction (ITC) factor. Accurate ITC values allow engineers to normalise test results across varying temperatures, improving diagnostic reliability.
Dielectric Frequency Response (DFR):
The DFR test is a powerful tool for moisture and insulation diagnostics. By analysing how dielectric properties change over a range of frequencies, this test delivers a detailed assessment of insulation moisture content and oil conductivity. High moisture levels not only reduce the transformer’s load-handling capacity but also accelerate ageing, emphasising the importance of proactive moisture management.
3. Transformer Turns Ratio (TTR) Testing
The TTR test ensures that a transformer maintains the correct ratio of primary to secondary turns—a critical factor for proper functionality. It verifies that the transformer’s turns ratio is consistent with design specifications, allowing the transformer to step voltage up or down accurately. This test can also reveal shortened turns or open windings, incorrect winding connections, manufacturing defects or mechanical damage.
4. Winding Resistance Testing
Poor design, manufacturing errors, transportation damage, environmental factors, and overloading can all cause winding faults or compromised connections. Winding resistance tests serve to measure the resistance of windings to identify these faults and verify the integrity of internal connections. Specifically, it can detect shorted or open windings, loose connections in welded joints or switches, and degradation in mechanical or electrical contacts. Overall, winding resistance testing not only ensures proper transformer operation but also provides peace of mind by confirming that all connections are secure and that insulation resistance is robust.
5. Exciting Current Test
The exciting current test is a crucial diagnostic procedure used to identify defects in a transformer’s core and coils. By measuring and recording individual phase excitation current values, this test can uncover a range of issues, including loose tap changer contacts, shorted open turns, and improper wiring. The test is typically conducted on the high-voltage winding to mitigate current flow during measurement. For three-phase transformers, measurements are made one phase at a time to ensure accuracy and prevent interference
6. Dielectric Breakdown Test on Oil Samples
Clean insulating transformer fluids or oil plays a critical role in ensuring efficient operation and insulation. Over time, contaminants like moisture, gasses, and solid particles can accumulate, degrading the oil’s quality and reducing its insulation effectiveness. This test involves extracting a sample of transformer oil, applying a test voltage between two electrodes within the sample, and gradually increasing the voltage until an electrical breakdown occurs. The breakdown voltage value is then compared to industry standards to evaluate the oil’s condition and identify if maintenance is needed to restore oil quality.
7. Bushing CT Ratio Testing
For bushing current transformers, this test determines the ratio of primary to secondary turns, similar to a Transformer Turns Ratio (TTR) test. It verifies whether the bushing’s ratio aligns with specifications, helping to detect issues such as incorrect winding connections or manufacturing defects. Accurate bushing CT ratios ensure proper energy transfer, maintaining the transformer’s overall efficiency and reliability.
8. Surge Arrester Loss and Current Testing
A surge arrester is a critical component of modern transformers, designed to protect against over-voltage transients caused by lightning strikes, switching surges, or other electrical disturbances. These devices function as insulators under normal conditions, but they safely divert excessive voltage to the ground when needed. Measuring the arrester’s losses (in watts) and current using a power factor test provides a reliable indication of its condition. This test helps identify contamination or deterioration in your arrester. While this test does not directly measure the arrester’s fault-to-ground capability, it serves as an early warning for insulation failure. If the insulation is compromised, the grounding capacity is likely impaired as well.
Routine Screening
Routine or preventative maintenance is essential in ensuring transformer health and operational efficiency. However, running every available test is neither practical nor cost-effective. Instead, the focus should be on tailoring the testing strategy to the specific needs of the transformer and its operational context.
Some of the key questions to guide test selection are:
- Are you addressing a specific issue?
- Are you conducting a general assessment to preempt potential problems?
Once the purpose of the maintenance is clear, select from the tests outlined previously.
Diagnostic Testing
Diagnostic testing is essential when identifying or troubleshooting specific transformer issues. While all electrical tests are theoretically on the table, the key lies in selecting those most relevant to the symptoms or conditions at hand.
Steps to Effective Diagnostics:
- Review the transformer’s operational performance and test history.
- Consider the systems reliant on the transformer and the information they provide.
- Select tests that align with observed symptoms and past maintenance records.
Available Tests:
In addition to the arsenal of potential tests listed above, diagnostic scenarios may call for specialised evaluations, such as:
- Partial Discharge Testing:
Detects partial discharges caused by air bubbles or insulation gaps. Though low in energy, these discharges generate thermal stress that can accelerate insulation ageing or cause damage. - Frequency Response Testing for Stray Losses:
This test identifies short circuit conditions within conductor bundles and evaluates structural components contributing to leakage flux. It is particularly useful for assessing the mechanical integrity of windings and core assemblies.
Conclusion
Transformer testing is essential to ensuring the reliability, efficiency, and safety of electrical systems. Commissioning tests verify initial performance and safety, routine tests maintain operational standards, and diagnostic tests predict and mitigate potential failures. Together, these testing stages form a comprehensive approach to lifecycle management, safeguarding transformers against unforeseen issues, and optimising their long-term performance.
