A DC winding resistance measurement is a commonly used diagnostic test performed on distribution and power transformers.This test can be used to detect internal problems such as shorted, burnt or open windings, broken strands, bad connections, and On-Load issues Tap Changers (OLTC) and De-Energized Tap Changers (DETC). Although this test looks simple, in practice it exhibits several technical complexities that can pose difficulties for the field technicians to obtain accurate measurements.
This article will cover some of the lesser known facts related to the DC winding resistance (WR) measurements, delve deeper into topics such as the selection of the correct test current and the importance of compliance voltage during the test. Phenomena such as core saturation, current stabilization, the influence of winding inductance on measured values and the effect of temperature are also explained. Various test techniques and safety procedures are also discussed. In addition, this document will also use field test results to explain the problems and important factors mentioned above.
At the end, the guide shows real-life cases of problem solving.