Transformer measurement transformed

Andy Sagl - Product manager

Measuring the resistance of power transformer windings isn’t as easy as it sounds. Because of the very high inductance of the windings, the test current takes a fair bit of time to stabilise and, as the rate of change of current decreases over time, it’s not easy to decide when it has reached its final value.

With conventional ohmmeters, therefore, measuring transformer winding resistances can be a very slow and tedious process. Nevertheless, accurate resistance measurements can provide invaluable information about the condition of the transformer, quickly revealing, for example, the presence of shorted turns or loose connections.

Because of the usefulness of these measurements, Megger has developed the technologies used in the new MTO210 transformer ohmmeter to allow them to be made more conveniently.

The key technology is the patented “Quick Test” measurement technique. This works by applying test currents simultaneously to the primary and secondary windings in such a way that the magnetic effects in the transformer core oppose each other. This allows test results to be obtained up to ten times more quickly than with conventional instruments.

Further, the simultaneous testing of primary and secondary windings, in both single- and threephase transformers, means that testing time is reduced by a factor of two compared with performing the tests separately.

Also, the MTO210 provides a bi-directional test capability, which allows the test currents to be applied to the transformer in the direction that makes best use of any existing core magnetisation. This cuts the time needed to obtain accurate test results.

In addition to its main function of measuring winding resistances, Megger’s MTO210 is also invaluable for checking the contact resistance of voltage regulators and tap changers with make-before- break contacts. The instrument instantly gives a clear indication if pitted or misaligned contacts are detected.

Because transformers store large amounts of electrical energy, safety is an important concern when making resistance measurements. This concern is fully addressed in the MTO210 by making provisions for the stored energy to be automatically discharged on completion of the test, if the test leads are inadvertently disconnected, or if the power fails during a test.

Further, the MTO210 offers an integrated demagnetisation feature, which allows users to demagnetise the transformer core either prior to testing or after testing is complete.

Demagnetisation can also be carried out as a standalone operation, independent of resistance testing.

While primarily designed for use with transformers, the MTO210 is an excellent choice for making DC resistance measurements on all types of magnetic windings including, for example, those in motors and generators. It is also suitable for performing low-resistance measurements on non-inductive components, such as connections, busbar joints, contacts and even control circuits.

Easily portable and with direct digital readout for two measurement channels, the MTO210 features internal data storage, allowing test results to be downloaded for subsequent recall, printing, and analysis. It is also fully compatible with the Megger’s versatile PowerDB data recording and analysis software package.

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