Regarding Tandelta of Power Transformer

There's been a lot written about here on CR$'s Electrical Engineering forum about tan-delta transformer testing, let's review some of it.

As kvsridhar: suggested, the 02/12/2009 thread is a good place to start, with myn1996's GA=3 drawing and explanation, with the loss resistance current I_R causing a phase shift away from the perfect winding-capacitance current I_C and its 90° phase shift. "The greater the angle, the worse is the insulation."

In a 09/18/2010 thread, Hello Dear, I Want Understand Transformer Tan Delta Test, mzfh2020 gets justinanto1987's crisp answer, "Can be found out by dividing leakage current by capacitive current. Actually it determines how much % leakage current flows of capacitive current. Device which shows less value will have better insulation."

zm_eng explains in a 11/27/2008 thread, "The insulation resembles a capacitor in that it is a conductor separated by insulation from the neutral and in a perfect capacitor the current leads the voltage by 90 degrees. Any deterioration in insulation will result in an increase in resistance and therefore the angle between current and voltage will be less than 90 degrees (because of resistive current component in phase with voltage). The resistive current and capacitive current are measured and Ir/Ic gives the tangent of angle delta. Delta being the angle between the capacitive current component and the resultant current. The angle delta is zero for a perfect capacitor and any increase indicates insulation contamination."

An 08/13/2008 thread tops CR4's search list, with a GA=4 post by napin, in which he reproduces the High Voltage Inc publication, Tan_Delta_FAQ.pdf

He apologizes for not reproducing the figures in his post, so it may be better to get the pdf file, which is about cable testing, and points out the need for performing tan delta tests at high voltages.

In a 02/03/2008 thread, Transformer Oil Testing (tan delta), a Guest points out that "Tan delta (dissipation factor) or Power Factor of the transformer oil (New) is usually less than 0.5% at 100C," and that this is "a routine test for acceptance of new or used cable oil."

In a 01/31/2009 thread by myn1996, asking if they should accept a new substation transformer with a 0.457% test result, Siswanto posts a lengthy GA=4 discussion of transformer oil assessment, with multitudinous tables, plots and formulas.

A guest said, "I strongly suggest you do not accept the result - it will cause problems over the long term." Later in the thread, myn1996 told us, "during the SFRA test the txfr failed due to severe deformation in the x0-x3 sec winding plus the impact recorder showed a 10G vertical acceleration. The manufacturer decided to remove the txfr and bring it back to the factory to be disassembled, checked and tested."

In a 01/26/2009 thread, by myn1996, nithya_btl answers a question about of tan delta testing vs frequency, showing the effect of moisture in two interesting 10 to 330Hz plots, with and without moisture.

Later in the same thread paiya80 posts a method to deal with interference in tan delta measurements in high voltage sub stations, by reversing the line and neutral and taking an average, thereby avoiding frequency sweeps to solve interference issues.

In a 06/03/2010 thread, scooby1981 asked about temperature correction of tan-delta values, and debasis.nandy answered, "A paper on the subject says that IEEE method of temp. correction does not give a correct picture as it has been revealed by DFR test results that tan delta of winding will be dependent on moisture content of solid insulation, i.e. different temp correction factors for transformers with different moisture content of solid insulation."

In a 12/09/2009 post, nssudhakar1968 asks if frequent high-voltage-insulation tests will degrade old equipment, e.g., "132 kV / 11 kV oil filled transformers, 11 kV generators, 11 kV motors, 6.6 Kk motors, etc." The only answer he gets is a reference to CR4's search engine.

In a 08/17/2008 thread, Partial Discharge vs. Tan Delta Test, Faiz Ahmad asks about the "difference between partial discharge and tan delta test" and if there are similarities. A CR4 search on partial discharge test (PD tests), brings up a new long list of interesting threads, such as this 02/14/2009 thread.

OK, folks, that's just a small peek at the CR4 threads and postings on this subject.