Standards for transformer testing

Power transformers are costly and, in many cases, critical assets. This means that accurate information about their current condition is exceptionally useful, as it allows maintenance to be rationally planned and also allows incipient faults to be identified and remedied before they develop into outright failures.

In the past, however, building a comprehensive picture of the condition of a transformer involved carrying out a whole battery of tests and, even then, some types of potential problem could not be detected without actually dismantling the transformer, which is a very costly, time consuming and inconvenient process. It was to address these shortcomings of conventional test methods that sweep frequency response analysis (SFRA) testing was developed.

Electrically, a transformer is made up of multiple capacitances, inductances and resistances. It is in effect a very complex circuit that produces a unique “fingerprint” when test signals are injected over a range of frequencies and the results plotted as a frequency response curve.

Movement of the windings, which can be caused by electrical overloads, mechanical shocks etc will therefore alter the capacitances (and/or inductances) and change the shape of the frequency response curve. The SFRA test technique for transformers is based on comparisons between measured curves, which allow variations to be detected. An SFRA test involves multiple sweeps on a particular transformer and reveals whether the mechanical or electrical integrity of the transformer has been compromised.

SFRA testing has now been in use for well over a decade, and during that time it has amply proved its worth as an efficient and effective means of identifying faults and potential faults in power transformers. Nevertheless, there are still many engineers and technicians who see SFRA testing as a new and relatively untried technique. This view has, in part at least, been based on the perception that SFRA techniques are rarely referenced in national and international standards and guides.

Once, of course, this was true – because of the nature of the standardisation process, there is always a significant ‘waiting period” before major standards and guides catch up with any new form of technology no matter how useful it may be. In the case of SFRA testing, however, this waiting period is now over, and  the technique is making an appearance in the latest editions of many international standards and guides that relate to transformer testing, including 

• CIGRE TB 342, “Mechanical-Condition Assessment of Transformer Windings” (2008)
• CIGRE TB 445, “Guide for Transformer Maintenance” (2011) 
• IEEE PC57.149, “Guide for the Application and Interpretation of Frequency Response Analysis for Oil Immersed Transformers” (2012)
• IEC 60076-18, “Power transformers Part 18: Measurement of frequency response” (2012)
• IEEE C57.152, “IEEE Guide for Diagnostic Field Testing of Fluid-Filled Power Transformers, Regulators, and Reactors” (2013)

Given this wide recognition of SFRA testing, potential users need have no further qualms about the validity or usefulness of this technique. SFRA testing is no longer an unknown application but a recognized, described, standardized and recommended method for transformer diagnostics.