Rules to estimate the expected values of zero-sequence impedances in 3-phase core-type transformers

Simple rules to estimate the expected values of zero-sequence impedance measurements for 3-phase core-type transformers are shown. The proposed rules are based on the analysis of magnetic circuits. Due to this fact, a conceptual description about zero-sequence impedances of these transformers is included. The rules were verified with measurements on fifteen transformers, and these measurements were used for refining the rules. In case of magnetizing zero-sequence impedances, the presence or not of magnetic shunts on tank walls defines the details of these rules. In case of short-circuit zero-sequence impedances, only the positive-sequence transformer impedances are necessary to apply the proposed rules. These rules are important in order to reduce the probability of errors during the tests. Some rules related to the resistive part of the impedances are also presented.