Canadian Manufacturing

Winding resistance testing in power transformers

Protect against costly repairs and long periods of downtime by using accurate and reliable test equipment and methods

—Sponsored article by Megger

Transformers are very reliable devices and can provide service for a long time if regularly maintained and serviced. They’re also the single most costly item in any substation. To protect against failures that can result in costly repairs and long periods of downtime, ensure you use accurate and reliable test equipment and methods.

Below are a few common questions related to winding resistance testing in power transformers:

Is it necessary to use a dedicated transformer ohmmeter to make accurate winding resistance measurements on power transformers?

The use of an instrument that has been designed with transformer resistance testing in mind is strongly recommended, as it will give more dependable results faster and more safely, especially with large transformers.

It will also include provision for demagnetizing the transformer core after testing has been completed. This is important since a transformer returned to service with a magnetized core could result in a large and potentially damaging inrush current. It may not be necessary to use a dedicated transformer ohmmeter though.

A new type of multifunction tester has recently reached the market which provides an extensive range of tests for transformers, including not only winding resistance tests but also, for example, turns ratio tests, leakage reactance measurement, and tan delta measurement.

These new testers can also perform basic measurements on circuit breakers, protection relays and many other items of equipment used in power distribution networks. For many users they’re a better and more useful investment than a dedicated single-function transformer ohmmeter.

When measuring winding resistance in a transformer, how do I decide on the right test current to use?

With DC resistance measurements, the objective is always to try to saturate the transformer core as this reduces the effective inductance of the winding and allows the test current to stabilize faster. Saturation typically occurs when the test current is around one percent of the rated current for the winding.

It is, however, usually advantageous to use a somewhat higher test current than this to minimize the effect of noise on the measurements. If the test current is too low, successive measurements might give inconsistent results. Even so, test currents of more than 10 percent of the rated current should be avoided, as they’re likely to lead to erroneous results because of heating of the winding. In most cases, the optimum test current is between one percent and 10 percent of the rated current.

When should winding resistance measurements be carried out?

Winding resistance tests are usually carried out on new transformers after they’re delivered to site and before being put into service as a way of detecting transit damage. Such tests will also provide baseline results for comparison with measurements made later in the life of the transformer.

Winding resistance tests can also be performed as part of scheduled maintenance as an aid to detecting incipient faults. Finally, winding resistance tests are invaluable when fault finding transformers, as many transformer faults or problems will cause a change in the DC resistance measured from the bushings.

What types of fault can be found with winding resistance tests?

Faults that can be detected using resistance tests include shorted and open turns, defective internal connections, and problems with tap-changer contacts, including open circuits during on-load tap changer operations. Information derived from the resistance tests usefully complements information from other transformer diagnostic tests, such as frequency response analysis (FRA), dielectric frequency response (DFR) and power factor measurements. Winding resistance measurements can also help in the determination of winding losses.

Megger provides testing solutions in the most critical maintenance areas including cable fault locating, protective relay testing, and power quality testing. Megger’s product offering spans 30 distinct product groups with over 1,000 specific products. To learn more visit

To view additional articles from Megger, visit the Electrical Testing Success Centre.

—This article is an edited version of the original that ran in Megger’s Electrical Tester newsletter.

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