Introduction

A DC winding resistance measurement is a common diagnostic test performed on distribution and power transformers. This test can be used to detect internal issues such as shorted turns, burnt or open windings, broken strands, poor connections, and problems associated with On-Load Tap Changers (OLTC) and De-Energized Tap Changers (DETC). Although this test looks simple, in practice, it presents several technical complexities that can cause trouble for field technicians to obtain accurate readings. This paper covers some of the less 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. Different test techniques and safety procedures will also be covered. Additionally, this paper will also make use of field test results to explain the issues and important factors highlighted above. At the conclusion, paper presents field cases on how to troubleshoot situations when the readings obtained are outside the NETA and IEEE C57.152 defined limits.

DCarreno_DChhajer_Transformer Winding Resistance Measurement Field Challenges.pdf