On-site Low Voltage Determination of Zero Sequence Impedances for Power Transformers

Accurate zero-sequence transformer data is important for correct fault analysis and protection settings. Utilities have been forced to approximate zero-sequence impedances for many three-legged core power transformers in service today that did not undergo testing for zero-sequence data at the factory before site installation. Where approximation of the zero-sequence impedances from positive sequence data is of insufficient accuracy, on-site tests have been proposed. However, these usually involve measurements from the high-voltage windings, which are either expensive with high test voltages or do not offer sufficient current resolution with low test voltages. This paper proposes new methods for carrying out on-site zero-sequence testing entirely from the lowest voltage winding. Affordable equipment can then be used for the test with minimal switchyard test preparations. Two test configurations are described; a One-Open-Phase test and a Two-Open-Phase test. To validate the method, three typical transformer types are considered; a station auxiliary transformer (SAT), a generation step-up transformer (GSU), and a transmission transformer. The proposed method was tested on actual SAT and GSU units, and simulated for a transmission transformer. All test results validated the method and offered high accuracy.