Q and A: Variable frequency tests

Power transformers are costly, difficult to replace assets that are often used in critical applications. It’s unsurprising therefore that many different techniques have been developed for diagnostic and routine testing of transformers. So many techniques, in fact, that callers to the Megger helpline are often confused, especially when it comes to variable frequency tests. Here are some of the questions we’re regularly asked.

Q: Is variable frequency testing of a power transformer the same thing as an SFRA test?

A: Yes – and no! SFRA (sweep frequency response analysis) testing is the best known of the variable frequency transformer testing techniques, but it’s definitely not the only one. There are many other transformer diagnostic techniques based on frequency, and each of them has unique diagnostic functions and values. Other widely used techniques include DFR (dielectric frequency response),  narrow-band DFR, and FRSL (frequency response of stray losses).

Q: What is SFRA testing and why is it useful?

A: SFRA testing, which can be conveniently performed with Megger FRAX test sets, is used to check the mechanical integrity of transformer components such as the core, the windings and the clamping structures. To carry out the test, a low-voltage signal is injected into one end of a winding and measured at the other end so that the electrical transfer function of the transformer can be determined. The test is typically repeated over a range of frequencies from 20 Hz to 2 MHz. The results are compared with a reference curve that was produced using the same technique when the transformer was new or known to be undamaged. This technique reveals many types of fault, including core movements, faulty core grounds, winding deformations, winding displacements, partial winding collapse, hoop buckling and shorted turns. It’s important to note that SFRA is essentially a comparative test. Without a reference curve for the transformer, the information provided by the test is much less useful.

Q: Is DFR testing similar to SFRA testing?

A: No, it’s quite different. DFR testing, which is sometimes also known as frequency domain spectroscopy (FDS), is essentially a series of tan delta tests carried out over a range of frequencies. The frequencies used are much lower than those used for SFRA – typically 1 mHz (millihertz!) to 1 kHz. The results are usually presented in the form of a capacitance and/or dissipation factor/power factor curve and, when they are used in conjunction with insulation modelling they provide invaluable information about the condition of the transformer’s insulation system, particularly the moisture content of cellulose insulation and the oil conductivity. Megger offers DFR testing with instruments in the IDAX range.

Q: What’s the difference between DFR measurements and narrow-band DFR measurements?

A: The measuring techniques used are similar but, as the name implies, narrow-band DFR uses a much more restricted range of frequencies – usually from around 1 Hz to 500 Hz. Also, the results are analysed directly rather than by using modelling techniques. It takes much less time to carry out a narrow-band DFR test than a full DFR test – around two minutes compared with twenty minutes or more – but the narrow-band test doesn’t provide the estimated moisture content for the cellulose insulation. What it does do is to provide an earlier indication of problems than traditional power dissipation factor (PF/DF) tests performed only at power frequencydo. It also confirms that seemingly good PF/DF delta values really are good and allows the transformer’s individual temperature correction (ITC) factor to be determined. The Megger DELTA and Megger TRAX test instruments are used to perform narrow-band (NB) DFR testing in addition to PF/DF tests.

Q: What is FRSL testing and what is it used for?

A: FRSL stands for frequency response of stray losses, and it’s the only test that can detect strand-to-strand short circuits in a conductor bundle. It is also sensitive to problems that lead to increased losses in transformer structural components such as the tank, the clamping structures and the tie plates. Problems of this type may produce overheating gases in the oil. FRSL measurements are made with the Megger TRAX test instrument. Also, if you’re using a Megger FRAX test set for SFRA testing, FRSL test results are made available by the software at the completion of the SFRA tests.