Interpretation of Low DC Current Measurements of a Transformer´s Winding Resistance and the Effect of the Tapchanger´s Contacts

Low current (<;10A) dc measurements of the transformer windings show a uniform decrease in the resistance as the tapchanger selector switch is advanced from the first tap to the last tap. When the resistance data for a newly manufactured transformer is plotted vs. the odd and even tap positions on its primary windings, the data lies with some small, but acceptable variation, on the same straight line with a slope of -3.9 for each of the three phases. After 20 months in operation there is a considerable change in the measured resistance data. The data for the odd and even tap positions each still lie with more variation on a line with the same slope (-3.9), but they lie on quite different lines. In Phase-1 the difference between the data for the odd and even tap positions is about 14.2mΩ. The other two phases also show a difference, but are somewhat less than that on Phase-1. The analysis of this data shows that these differences do not imply that there is an internal problem with the transformer windings or with the internal bolted connections to those windings. Tarnish film formation on the Cu and W-Cu contacts in a transformer oil environment is discussed. These resistive films grow thicker with time and their growth on the tapchanger´s selector switch and its diverter switch contacts adequately account for the difference seen in the resistance measurement between the new transformer and after it had been in operation for 20 months. A discussion of the rupture of these resistive films by the combination of high contact forces and the transformer´s normal load current is also presented.