Ten Important Points about Tan Delta Test of Transformer
Here are several important points about the transformer tan δ (dissipation factor) test. Many people often overlook these points, yet they are very important for correct understanding and evaluation. Even engineers with many years of testing experience may not fully notice these details.
Is tan delta the lower, the better?
Absolutely not, tan delta is not simply “the lower the better”. A healthy insulation system shows a stable trend. A single low value does not tell the full story. Slow and steady growth over many years is acceptable.
Does the temperature affect the tan delta results cleary?
Yes, temperature changes the tan delta result very clearly. Tan delta rises fast when the insulation gets warmer. A 10°C rise can increase the value a lot. It always compare results at the same temperature.
Does moisture in the oil have influence?
Yes, moisture in the oil has a very strong influence. Even a small moisture increase can double tan delta. The high-voltage winding reacts more strongly to moisture. So please check oil moisture first when you see abnormal data.
Does oil-paper "delamination" can cause hidden problems?
Yes, Oil-paper delamination can cause many hidden problems. Aging insulation may lose contact between oil and paper. This issue reduces proper polarization inside the system. Tan delta rises slowly when this problem develops.
Can capacitance distribution inside the windings affect the test?
Yes. HV, LV, and inter-winding capacitance may shift over time. Mechanical movement or thermal stress can change the layout. These changes can raise or lower tan delta unexpectedly.
Can high oxygen levels increase tan delta?
Yes. Oxygen speeds up oil oxidation inside the tank. Oxidation creates polar compounds that increase losses. This effect grows faster in poorly sealed transformers.
Can early partial discharge fail to change tan delta?
Yes. PD often starts in a very small area. Tan delta shows the condition of the whole insulation. And It rises only when local damage spreads more widely.
Can surface leakage produce false high readings?
Yes. Dust, moisture, or oil films guide unwanted current paths. These paths raise tan delta during the test. A guard electrode helps remove these false effects.
Can a new transformer show a higher tan delta?
Yes.New insulation needs time to stabilize in service. Fresh oil additives can raise the initial value. Tan delta often becomes lower after months of operation.
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What is the tan delta on a transformer?
