Winding resistance testing is a very revealing electrical diagnostic test for the routine screening of power transformers to determine their “state of health”. In fact, it’s an essential test when dissolved gas analysis indicates overheating in the oil or of the paper, and no condition assessment is complete without it.

Moreover, if you were to ask experienced substation maintenance engineers for their thoughts about which electrical tests should be done on a routine basis (which is not necessarily the same, of course, as the tests they are actually doing!), the winding resistance test would be likely to take second place only to the transformer turns ratio (TTR) test.

The TTR test typically takes top honours, not necessarily because of its diagnostic capabilities, but because it provides validation and reassurance that the transformer is actually doing what it’s supposed to do - transforming voltage. Nevertheless, the DC winding resistance test would be a very strong contender for the top spot if the list of important transformer tests were ever re-ordered purely on the basis of diagnostic value and screening prowess.

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Nikola Tesla’s name is synonymous with pioneering electrical developments, and he is accepted as the originator of many devices – not the least of which is the AC induction motor – which we now take for granted. His inventions form the basis of much of the technology we currently use and although controversial, his life is now celebrated by engineers and history pundits alike.

Introduction

Wherever electrical equipment and systems are installed, electrical hazards exist. The most common of these hazards are shock, arc flash, and arc blast, and they must be identified and assessed to determine if and where each of them exist. In addition, businesses must assess the potential for exposure of personnel who work on, near, or interact with the electrical equipment. Electrical standards and regulations worldwide include requirements for assessing the workplace to identify hazards where employees would be required to wear personal protective equipment (PPE).

This article is intended to familiarise readers with frequency response of stray losses (FRSL) testing for transformers - an invaluable test technique that is rapidly gaining recognition in the industry because of its ability to reveal problems that would be missed by other electrical test methods.

FRSL as a test tool for transformers was first discussed and investigated by Hydro Quebec in the 1970’s because of its ability to detect winding deformation. Hydro Quebec concurrently examined the use of the sweep frequency response analysis (SFRA) method and leakage reactance testing for the same purpose, and ultimately SFRA, together with leakage reactance testing, became the accepted tools for confirming winding deformation.

For some time, FRSL went largely unused. That is, until its seemingly latent diagnostic strengths were exposed. Of particular note was the discovery that FRSL was very useful for revealing short-circuits between individual strands within a conductor bundle. This is a failure mode that, until the advent of FRSL, had been undetectable with electrical test methods.

A conductor bundle may be comprised of any number of individually insulated conductor strands. When two or more of these strands are shorted together, this is not a turn-to-turn fault or even a partial turn-to-turn short-circuit . The latter is a situation when one or more strands within a conductor bundle become short-circuited to one or more strands within an adjacent turn of the conductor bundle. The exciting current test, for example, can reveal a partial turn-to-turn short circuit but is not sensitive to a strand-to-strand short circuit.

A transformer turns ratio test provides a quick verification of the most fundamental operational characteristic of a transformer – its ability to transform voltage as anticipated. In doing so, the test provides invaluable reassurance to the operator. Open- and short-circuit conditions in transformer main and tap windings may cause the transformer turns ratio to change and therefore this test is at once providing useful diagnostic information.

Engineers have noted, however, that traditional "hand-crank" TTR instruments sometimes give values for turns ratio that are different from those given by modern automated TTR instruments. Indeed, many insist that the traditional instruments provide results that are more dependable. But is this actually true? And why do the instruments produce different results?

Mexico already has over 3 GW of installed wind power capacity and plans are in place to increase this to more than 9.5 GW by the end of 2018. This ambitious target can only be achieved if the installation and commissioning of a new plant proceeds smoothly and the plant proves reliable in operation.

Most engineers amass a collection of invaluable reference materials throughout their careers. Most have also dealt with the angst resulting when a valued reference piece goes M.I.A., which fuels subsequent reluctance to loosen a tight grip on these libraries. Since in most cases, personal financial investment has grown these libraries, which typically include university textbooks and the like, often when these valued employees move on, these treasure troves follow.

This highlights an interesting aspect to succession planning; that is, the advantages of establishing and maintaining a protected corporate library, accessible to all. A corporate library serves important roles, including: (1) spreading valued knowledge much further within the organization and (2) aiding with succession planning as an additional way to pass knowledge.