Validation of Three-Winding Transformer Short-Circuit Impedances Using External Fault Relay Data

Accurate transformer impedances are essential for ensuring valid short-circuit models. The positive- and zero-sequence impedance of a transformer are measured in the factory by the manufacturer, and the results are published in the factory acceptance test report per IEEE standards. When the factory test report is missing or unavailable, validating existing transformer models requires significant resources including time, specialized testing equipment and requires the transformer to be taken out of service. It is possible to calculate the short-circuit impedances of in-service three-winding transformers using relay data captured during external faults. This paper presents a method to calculate the following short-circuit impedances of three-winding power transformers: the primary-to-secondary positive-sequence impedance (Z_PS1 ), the primary winding zero-sequence impedance (Z_P0), the secondary winding zero-sequence impedance (Z_S0), and the tertiary winding zero-sequence impedance (Z_T0). Due to the number of unknown variables, the analysis requires the relay event data from two different external faults to solve for the impedances—one relay event for an external fault on the high-voltage system and a second relay event for an external fault on the low-voltage system. The required relay data includes three-phase voltages and currents from both the primary and secondary of the three-winding transformer. Additionally, this paper will also compare the factory test report results with actual field measurements.