Rethink your transformer testing

Rethink your transformer testing

When thinking about transformer testing, TTR ranks number one in the routine maintenance testing for most engineers.  It provides a good starting point for transformer diagnostics, in that it tests that the transformer is doing what it is supposed to do.

When it comes to uncovering problems that may not yet be impacting operations, DC winding resistance testing is regarded as another essential routine screening tool. It indicates problems such as loose, defective or incorrect connections, which cause enough transformer failures each year to be regarded as a failure category of its own. Winding resistance tests may also reveal problems with a transformer’s tap changer (s), on-load (OLTC) and/ or de-energized (DETC) – a major and, by some statistics – leading, component category that fails in transformers. However, the test tends to be neglected due to a number of concerns and challenges surrounding winding resistance.

But transformer testing has changed dramatically over recent years, so those concerns and challenges have become outdated.  Winding resistance isn’t as troublesome as it used to be.

What you thought about transformer testing


Winding resistance has a stigma attached to it due to a number of factors:

  • To obtain representative test results, the transformer core must first be driven into saturation, which can be difficult to execute
  • 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

That stigma is hard to shake, but things are different now.  While the above concerns are all very valid, new technologies have allowed both these tests to be carried out much quicker, safer and easier.

Introducing the MWA


Megger’s MWA test set performs both turns ratio and winding resistance tests from just one instrument, with just one connection.  Designed specifically to make the measurement of ratio and winding resistance safer and more efficient, without compromising on accuracy of the results.


  • With only one lead set, only one connection is required to perform both tests, eliminating the need for multiple ladder climbs, thus reducing risk of injury
  • Automatic demagnetisation is a standard feature, that is typically carried out before the first TTR test and again after completion of winding resistance
  • 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


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