It’s time to rethink your transformer screening tests
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.
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.
However, despite its importance, there are a number of reasons why winding resistance has not been widely embraced, all with 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.
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.
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. An important feature is that they use only one lead set. Not only does this 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 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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