Common questions when using Tan Delta Tester
The following is a compilation of questions we have received from customers who have purchased Wrindu Tan Delta Tester. We hope this information will be helpful.
Q: Why are my Tan Delta readings much higher on a short cable compared to a long one?
A: Short cables often show higher readings due to false leakage current from the surface of the cable. Clean the cable ends and use a guard wire to bypass surface leakage and get more accurate readings.
Q: Can I perform a Tan Delta test in a high-interference energized substation?
A: Yes, but interference may cause sensor drift or inaccurate readings. To minimize this, use a test set with interference suppression and disconnect adjacent bus work if possible. If readings remain unstable, consider using a frequency-shift method like testing at 45Hz and 55Hz for a 50Hz system.
Q: Why does the insulation look "good" at 5kV but "highly degraded" at 10kV?
A: A sudden increase in readings at higher voltages can indicate moisture or aging issues, such as "water trees." If the readings spike at higher voltages, stop the test to prevent in-service failure.
Q: How does ambient temperature affect my results, and should I correct them?
A: Higher temperatures can artificially increase dielectric losses, making insulation appear worse than it is. Always normalize results to 20°C (68°F) using correction factors for accurate trending.
Q: What if I get a negative Tan Delta reading? Does that mean the insulation is perfect?
A: No, negative readings are impossible and indicate a setup error. Check for external interference or poor grounding, as these are often the causes of negative readings.
Q: My test results fluctuate wildly during the first few minutes. Is the tester faulty?
A: No, the initial fluctuations are normal as the voltage stabilizes. Wait for stable readings for 2–3 minutes at each voltage step before recording data.
Q: Why does my XLPE cable show a sharp "tip-up" even though it passed the withstand test?
A: A "tip-up" indicates localized moisture or water trees, which the withstand test might miss. If the Tan Delta values change sharply between 0.5U₀ and 1.5U₀, investigate immediately.
Q: Can I test mixed cables (e.g., XLPE spliced with PILC) using one Tan Delta reading?
A: Yes, but the results will be an average of both materials, potentially masking defects. Test sections individually if possible, and use the combined result for long-term trending rather than a pass/fail decision.
Q: How do I handle testing on a high-humidity day (RH >80%)?
A: High humidity can cause surface leakage, leading to falsely high readings. Clean the terminations and dry them with a heat gun before testing to ensure accurate results.
Q: Why do I see different results when using different test frequencies (e.g., 0.1Hz vs. 0.02Hz)?
A: Tan Delta values change with frequency; lower frequencies typically yield higher numbers. Always record the test frequency and compare results taken at the same Hz for consistency.
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