Frequently Confused by Frequency Tests?

By Jill Duplessis, Megger

Testing transformers at non-traditional frequencies-frequencies other than 50 Hz or 60 Hz direct current (DC)-can provide invaluable diagnostic information. This has led to the development of a range of novel test techniques. Unfortunately, but unsurprisingly, many of these tests have the word “frequency” in their titles, which often has led to confusion and, sometimes, to missed diagnostic opportunities. I’d like to dispel that confusion by taking a look at some of the most popular of these tests, many of which have evolved from long-established tests. They’ve been created by simply repeating the traditional measurements as the frequency of the test source is changed.

The most notable example is the power factor or tan delta test. Traditionally, this has been performed at line frequency (50 Hz or 60 Hz) only. It is a useful tool for assessing the health of most transformer insulation systems, but it has several shortcomings. Many of these can be eliminated by repeating the test over a range of frequencies, as you’ll discover shortly when DFR and narrowband DFR testing are explained.

A less well-known example is a short-circuit impedance measurement. This test is typically used to look for winding deformation by evaluating the so-called leakage reactance. When this measurement is repeated at several frequencies, typically between 1 Hz and 500 Hz, with the focus on the resistive component of the results, unique diagnostic information becomes available. This is the FRSL technique and also will be further explained further.

It is common for the person doing the testing to confuse dielectric frequency response (DFR) and frequency response of stray losses (FRSL) tests with sweep frequency response analysis (SFRA) tests. This is because SFRA testing is much better known than the two other techniques and, as a result, many people upon hearing the word “frequency” assume that the reference must be to SFRA testing. The tests, however, are most definitely not the same and, while SFRA provides a lot of diagnostic information, it is not a substitute for the information provided by the other frequency-based tests.

So, next time you hear frequency mentioned in the name of a test, be wary of jumping to conclusions. Remember that there are many diagnostic tests based on frequency, and each has unique diagnostic value.

The following tests are the most common for transformers:

Sweep Frequency Response Analysis (SFRA): This test is used to evaluate the mechanical integrity of the core, windings and clamping structures within power transformers. A small voltage signal is injected into one end of a winding and measured at the other end of the winding so that the transformer’s electrical transfer function can be determined. This test is repeated over a frequency range from 20 Hz to 2 MHz. When the test results are compared with a reference “signature,” a wide range of fault types can be detected, including core movements, faulty core grounds, winding deformations, winding displacements, partial winding collapse, hoop buckling, broken clamping structures, shorted turns and open windings.

Dielectric Frequency Response (DFR), also known as Frequency Domain Spectroscopy (FDS): DFR, together with insulation modelling, is a preferred method for measuring moisture content in power transformers’ cellulose insulation. The results are normally presented as a capacitance or dissipation factor/power factor or both vs. a frequency curve. A typical measurement range for transformers is 1 mHz to 1 kHz.

A DFR measurement combined with modelling the response using the X-Y model can evaluate the insulation system’s condition. Using DFR to determine the level of moisture is based on comparing the transformer’s measured dielectric response to a modelled dielectric response. An analyzing algorithm rearranges the modelled response and produces a new curve that reflects the measured transformer. The results present the transformer’s moisture content and oil conductivity.

Narrow Band DFR, also known as Variable Frequency Power Factor: This is a DFR measurement-a series of power factor/tan delta measurements, each made using a voltage source at a different frequency-but over a narrower band of frequencies, from 1 Hz to 500 Hz. The analysis does not rely on modelling capabilities and it does not provide the cellulose insulation’s estimated moisture content. Rather, it is a much shorter DFR measurement (approximately two minutes per test) that provides earlier indication of problems than a traditional power factor/tan delta test. It also verifies that seemingly good power factor/tan delta values actually are good, and reveals when seemingly good values are not good. In addition, it allows the tester to determine a transformer’s unique individual temperature correction (ITC).

Frequency Response of Stray Losses (FRSL): This test stands alone in its ability to detect strand-to-strand short circuits in a conductor bundle. In addition, it is sensitive to problems that have resulted in increased losses of structural components such as the transformer tank, clamping structure and tie plates. These problems might cause overheating of gases in the oil. Some SFRA test sets, such as Megger’s FRAX, provides FRSL results concurrent with an SFRA short-circuit test measurement.

Frequency-based tests on transformers are some of the most potent, and convenient, diagnostic tools currently available. To get the best from them, however, it’s important to understand the unique benefits that each of the available tests can provide. The intent of this short article was to provide you with some useful guidance, as well as dispel the confusion that often surround these types of tests.

Jill Duplessis is global technical marketing manager for Megger.

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