Transformer Winding Resistance Measurement: Field Challenges

Transformer Winding Resistance Measurement: Field Challenges

Article Cover

Megger’s article “Transformer Winding Resistance Measurement: Field Challenges” was presented at the 2020 NETA PowerTest Conference in Chicago, Il. This article takes an in-depth look at the lesser known facts associated with the DC winding resistance (WR) measurements, diving deeper into topics such as selection of the correct test current, and the importance of compliance voltage during the test. Phenomena as core saturation, current stabilization, the influence of winding inductance on readings, and the effect of temperature, are also explained.

Article Brief:

DC winding resistance measurements (WRM) performed on distribution and power transformers are used to diagnose internal issues such as shorted turns, burnt or open windings, poor connections, and problems with On-load tap changers (OLTC) and De-energized tap changers (DETC). Industry acceptance criteria recommends that winding resistance between phases should be within 2% of each other. Additionally, comparison may also be made with original factory measured data, where differences within 5% is considered satisfactory. Results should be temperature corrected when measuring against factory readings or historical measurements. If readings obtained are outside of industry standards, complementary tests are recommended to assess winding condition. Although performing a WRM is relatively straight forward, there exist several challenges that one can encounter while testing in the field. The following section discusses these field challenges and solutions to support winding resistance testing.

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Authors Daniel Carreno and Dinesh Chhajer