Transformer test to calculate Z0 for interconnected windings transformers using symmetrical sequence components

It is important that protection engineers of electric utility systems know the fault current levels before setting relays for a specific protection zone. In addition, information about transformer impedances needs to be available for fault current calculations. A manufacturer who constructs power transformers provides a test report that includes information about the positive- and zero-sequence impedance. This impedance is determined by tests as described by the IEEE/ANSI Standard C57.12.90. The standard covers a variety of transformer connections such as wye-delta, delta-wye, and some three-winding transformers, but it excludes transformers with interconnected windings like wye-delta-zigzag. To date, no reference is given as to where this information can be located.