Principle of Measuring DC Resistance and Analysis of On-site Different Kinds of Faults

Abstract: The measurement of DC resistance can reflect the quality of joint welding; whether there is turn-to-turn short-circuit; whether the tap changer is good and whether the winding has broken strands. The principle of DC resistance measurement, common measuring methods and a general principle to deal with test results of exceeding the limit are presented in the paper. Then analysis based on different kinds of defects in field is made. In mot cases, if turn-to-turn short-circuit occurred, the DC resistance would decrease; if the phase is poorly welded or has broken strands, the DC resistance would increase. If the test results are abnormal, comprehensive analysis should be made. The combination with other electrical tests can facilitate the reliable judgment of transformer defect. 

Key words: DC resistance, power transformer, high voltage test, winding     

Introduction

The DC resistance test of transformer winding is essential after handover and maintenance. The measurement of DC resistance can reflect the quality of joint welding, whether there is turn-to-turn short-circuit and each tap changer is contacted well. According to DL/T 596-1996 Preventive Test Code for Electric Equipment, imbalance rate of above 1.6MV.A transformer resistance shall not be more than 2% and imbalance rate of line resistance shall be less than 1%. The power transformer winding can be regarded as an equivalent circuit composed of winding inductance L and resistance R in series, which is shown in Fig.1. The winding inductance is very high while DC resistance is low. Besides, the transformer capacity is proportional to voltage stage and ratio of inductance to resistance.