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Transformer screening: Is it time for a rethink?

Jill Duplessis - Global technical marketing manager

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.

P1000645 transformerThe 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.

A winding resistance test is used to assess the condition of the current carrying path between transformer bushing terminals. Problems such as loose, defective or incorrect connections; open/ partially open (broken strands) or short-circuited turns in windings; or high contact resistance in tap changers (DETC or OLTC) will result in a change in resistance, and therefore be indicated by an unexpectedly high, low (for short-circuits) or unstable resistance measurement.

Loose, defective or incorrect connections in the general current-carrying path (such as winding exit leads) cause enough transformer failures each year for these problems to be regarded collectively as a failure category (e.g., “Leads”) of its own. Of components that fail in transformers, meanwhile, tap changers constitute a major and, by some statistics – leading, percentage component category.

It’s interesting then to ask why it is that all transformer owners have not historically embraced winding resistance testing as a routine screening test. The main reasons are:

  • Winding resistance tests are simple in theory, but they are often challenging to execute. To obtain representative test results, the transformer core must first be driven into saturation. This is not always easy to do.
  • The test may take a considerable time to complete.
  • The transformer will need to be demagnetised after test, since a residually magnetised core may result in damaging inrush currents when the transformer is energised, with negative implications for the transformer and the power system. A magnetised core may also adversely affect certain offline diagnostic test techniques (such as SFRA and excitation current) making it impossible to draw meaningful conclusions about the condition of the transformer.
  • Demagnetisation further extends the time needed to perform the test, and some old methods of demagnetisation (for example, using a 12 V car battery) create safety concerns.

These reasons all have some validity, particularly for testing carried out with traditional test equipment. But times have changed—there’s now a faster and more convenient way to carry out TTR and winding resistance measurements (and demagnetize afterward) with just one easy-to-use instrument. This is the new MWA test set from Megger.

Designed specifically to make the measurement of ratio and winding resistance safer and faster, MWA test sets are 50% smaller and lighter than the two traditional instruments that would have previously been needed to perform these tests. They also use the same intuitive user interface for both tests and conveniently collate the results of the two tests on the same form. A further important feature is that they use only one lead set.

MWA test sets are suitable for testing all types of power transformers. Their applications are not, however, limited to transformers. They can also be used to test generators and motors, busbar connections and circuit breaker contacts. In addition to turns ratio and winding resistance, test facilities provided by MWA instruments include polarity, excitation, make/break transition and phase.

The single lead set used by MWA test sets is a major benefit. Not only does it save time, as the connections to the transformer only need to be made once for the full range of tests, it also brings important safety benefits. This is because power transformer connections are often inaccessible from the ground, and ladders are needed to reach them.

Every ladder climb is a potential safety hazard, so using the MWA, which only requires the connections to be made once, rather than traditional test sets where separate connections have to be made for each test, reduces the risk exposure for test technicians and engineers.

A further benefit is that the lead sets supplied with the instrument are fitted with Kelvin clamps, which simultaneously make voltage and current connections in one convenient operation. This saves time and virtually eliminates the risk of misconnection. The jaws of the clamps open a full 100 mm, which means that connections can be easily made to almost any transformer. MWA test sets also have banana plug sockets that make light work of connections to current transformer secondaries.

MWA test sets are available in two versions: the MWA300 which is designed to be used in conjunction with a Windows PC running the Megger’s acclaimed PowerDB power testing software, and the MWA330A which has a powerful integrated industrial PC running the same software, complemented by a 12-inch sunlight-readable colour touchscreen and a built-in thermal printer.

The instruments fully support both asset-based and manual test control, and the software provides a convenient master test form with an easy-to-use nameplate header. Entering the transformer parameters automatically fills in all of the form information.

The software includes a winding resistance test wizard that makes it easy to automate the test sequence by answering simple questions such as which windings are to be tested, the taps at which the tests should start and finish, and whether all phases or individual phases are to be tested. An option is provided within the software to automatically determine when a measurement stabilises, which allows accurate and dependable results to be obtained in the minimum time possible. The results from different tap settings are presented graphically, providing immediate visual confirmation that they are following the expected progression.

Automatic demagnetisation is a standard feature of MWA test sets and is typically carried out before the first TTR test and again after completion of winding resistance tests. Single- and three-phase demagnetisation are supported, with both essentially providing the same result of leaving the core fully demagnetised so that the transformer can be returned to service without risk of problems.

The new MWA test set from Megger successfully addresses the problems that have often led organisations to forego the benefits that winding resistance testing can provide. With the MWA, winding resistance tests can be made together with transformer turns ratio tests with minimal extra time, no additional test connections and automatic demagnetization of the core afterwards, so if winding resistance testing is not routine in your organisation, it is definitely time for a rethink on transformer screening testing!

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